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Thoughts on the '50-Ohm' Direct-Fed Dipole

from James M. 'Murf' Murphy, WV4R on September 28, 2008
View comments about this article!

Thoughts on the “50-Ohm” Direct-Fed Dipole

As I look back on my personal history of antennas from 1959 to today, I recall my first dipole epiphany.

I knew, from an old copy of the ARRL handbook, a factoid of dipoles being 75-ohms at the feed point when properly tuned & tweaked for the desired operating frequency. But who wants to buy 75-ohm coax when all the modern radios are designed for 50-ohm impedance? Not everyone has a built-in tuner, eh?

For many years I marveled at some of my lucky dipole erections without a clue as to how they worked so well. When pulled up straight and tight, I could never get the SWR better than about 1:2 with 50-ohm coax feed. Every once in a while I would see from my trusty ole SWR meter almost 1:1 with direct 50-ohm coax feed. I would simply chalk it up to location and the lucky length of random coax.

Then, in the 1970’s I came upon a wondrous factoid in an old ARRL Handbook. There it was in a graphic depiction… ALL the correct coax lengths for feed lines for all the H.F. bands for mono-band and multi-band installs! Improvements were immediately realized, especially when operating multiple bands with only one dipole due to location restrictions. Don’t bother trying to find this graph in new publications because they deleted it sometime after 1977. Of course IF you have a random length of coax, then you really do need something to ‘choke’ off the reflected power (SWR) you most probably will have on your coax. Of course you could get lucky because random length might just be the correct length for your install, eh?

I found I could further improve dipole performance with either a simple 19-foot coil of coax at the feed point for 160m. Thank you Collins Radio Company for the tip I found in your antenna designs showing the R.F. choke balun for 160-10m antennas. Please note the higher the resonant frequency the smaller the minimum coax choke windings required. For example, on 20m only 8-turns in a 5-inch coil is more than adequate according the Force-12 antenna manuals and personal discussions with Tom, President of Force-12.

A further improvement in this simple R.F. Coaxial Choke is to wind it around a large coffee can or PVC pipe. After years of just making a simple rope-like coils held together with plastic tape, I discovered it is considerably more efficient to make the coils parallel to each other. Pay attention that your coil diameter is sized according to manufacturer specifications on how tight a coil you can make in their coax without affecting impedance and/or migration of the dielectric. Note: do not use ‘foam’ dielectric because over time impedance will change as the center migrates toward the shield. Hard dielectric rules in making a choke balun.

Did I mention I steal knowledge and expertise from anywhere I can find it?

Further multiple band improvement is gained by constructing the “FAN DIPOLE” which is simply cutting multiple dipoles for each desired band & connecting them together at the feed point, fanning them apart, a.k.a. the ‘fan’. There are two schools of thought on constructing this fan dipole. One thought is to erect them all parallel to each other(longest one on top to support the rest) & separated an inch or more depending on the design of the wire used (twin lead, multi-conductor, plain wire, etc.). One cheap and good separator is lengths of small diameter PVC pipe drilled for the wire to pass through and cable-tied to prevent slippage. Oh, you are the classic cheap ham… well TV twin- lead will make Two-band operations. A real good thing to remember is to cut each dipole for the LOWEST operating frequency. Better to be a foot too long than an inch too short, eh? Next best thing to remember is to prune and tweak your dipoles starting from the longest one to the shortest one… checking All bands each time you prune any band because there is a ‘coupling factor’ alive and well in this multi/fan design that must not be ignored.

Also you can build or buy a 1:1 balun using ferrite doughnuts around coax with connectors on both ends or the fancy ones using Teflon wire wrapped around a super ferrite doughnut for a no-fuss no-muss install. So, somewhere between a couple bucks and a couple hundred bucks you can have yourself an R.F. choke balun.

I thank NT5W for the birthday present of a Mighty Fine Junk (MFJ) Antenna Analyzer which helped me SEE how the critical “INSIDE ANGLE” of the dipole allows the determined dipole builder to Direct-Match a 75-ohm dipole to 50-ohm coax. After many hands-on experiments I found the ballpark solution to be about a “150-degree” inside angle. To put it another way… a classic “DROOPY DIPOLE” as I have heard it called & how I describe my dipoles to this day. Think of it as an inverted V with each leg drooping down 15-degrees thus making the magic 30-degree inside angle droop. I found the advantage of the droopy dipole over the classic 90-degree inside angle inverted V to be the resonance and the maximum power point to be fairly the same with the droopy dipole over the classic inverted “V” and hands-on proof that you can match a 75-ohm classic dipole to 50-ohm. Oh yes, inverted “V” dipoles are more omni-directional… just my personal observations.

Oh yes… if you desire to maximize your signal to a given point, the dipole appears to work best when erected at least ½ wavelength above the true reflected ground, or as I call it, the wet water table, at your location. In other words, if you were to dig a hole in the ground under the feed point of your dipole, the point where you start getting a wet shovel is the true reflective ground. To put it another way, if your mast/tower is 35-feet above ground and your wet ground is 7-foot below the ground, your total height is the magical minimum height for 5/8 wavelength on 20m… plus or minus your “Kentucky Windage”. Now if you are suspicious of the “magical 42-feet” minimum height, please read the books of the late/great William Orr/W6OR and experience your own epiphany.

There does not appear to be any improvement above 1-wavelength, however, I have heard a number of stations describing their dipole install as high as 2 wavelengths… and… on 75m that is quite a marvel of commitment to the hammy radio hobby, eh?

Benefits of proper height are concentrating all your signal to a 45-degree takeoff angle with real nulls of the ends of up to 3-4 “S-units”. Concentrating all signal at this 45- degree takeoff is just like the classic maximum trajectory of a cannon ball (45-degrees) which will give you the best reports according to the real propagation skip on your frequency at the time. However, like a cannon ball, or a heavy round stone thrown into a pond at 45 degrees there is one plunk. If you want DX you gotta git a flat stone and throw it at the flattest takeoff angle to git the “skip” and the DX… thus the proof of the odd-multiple wavelengths above ground to achieve flatter takeoff angles and thus greater DX skip with your dipole.

So, IF you want some real DX to spice up your life, you can increase your height above your true reflective ground to a minimum 5/8 wavelength and/or higher odd multiples to give you minor lobe/lobes with a lower takeoff angle and some advantage for DX.

I know some skeptics are wondering HOW I discovered this factoid. The key word here is “empirical testing/hands-on installs”. I like 17m for my testing for a number of reasons. It is a narrow band so antennae can be constructed in many restricted locations and be broadly resonant across the whole band with No tuner. The magical height more easily achieved in restricted zoning situations. A rotatable dipole install is possible readily showing those hams living in Missouri, the “Show Me” state, the fantastic nulls. Let’s face it, for many non-technical amateur radio operators/short wave listeners, about the only thing you can tune/tweak nowadays is your antennae, eh?

Recently, I constructed a homebrew 17-meter dipole made out of aluminum tubing scraps from many wounded beams. I used my trusty ole Craftsman pop-riveter to provide the third best way of securing the telescopic elements. I then mounted this dipole on my 42- foot tower, fed it with 50-ohm coax a simple R.F. coax choke balun wound from the same continuous piece of random coax and rotated it. Voila… Empirical Evidence!

Please note:
The late great William Orr, W6OR books, in my personal opinion, are my best plain talk & authoritative sources nowadays for hammy radio operators. Any of his books far surpass all others I have read. He writes like he is simply talking to you and he explains the most complicated things in the most understanding ways. Once you have read Orr you will become a believer and never go back to plain vanilla.

73 es God bless, murf/WV4R hammy radio since 1959.

Member Comments:
This article has expired. No more comments may be added.
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N7BUI on September 28, 2008 Mail this to a friend!
Nice article James. Some interesting facts that I didn't know about.
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W2BLC on September 28, 2008 Mail this to a friend!
Good practical advice without all the high math, graphs, heavy theory. Nice read - and, yes, Orr's writings are well worth reading.
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by KB2DHG on September 28, 2008 Mail this to a friend!
Now this is the kind of post I like to see on Ehams. Bravo...I can tell you from my own experence, HOME BREWING DIPOLES are the very besT.
I just built a home brew full size G5RV and the performance is WONDERFUL. YOU DON'T NEED TO BUY A COMERCIAL ANTENNA!
nice article a good read.
Thank you
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N3JBH on September 28, 2008 Mail this to a friend!
Here is some information I took from both the ARRL Antenna Book and from the
www.n0ew.org Web site and then modified to come up with. I hope it will help.
Jeff N3JBH

All choke are made from RG/8 coax .405Dia. stuff. All windings must be beside each other BUT !!! Never over lapping each other
For mono band antenna's

3,8 Mhz 22 feet made from 8 turns on 8 1/2 inch diameter
7,2 Mhz 22 feet made from 10 turns on a 7 inch diameter
14 Mhz 10 feet made from 4 turns on a 7 1/2 inch diameter
21 Mhz 7 feet made from 7 turns on a 3 inch diameter
28 Mhz 6 feet made from 6 turns on a 3 inch diameter

For multiband use antenna's

80 to 10 meter's 10 feet made from 7 turns on 4 1/2 inch diameter
80 to 30 meters 18 feet made from 10 turns on 6 inch diameter
40 to 10 meters 12 feet made from 8 turns on 5 inch diameter
20 to 10 meters 8 feet made from 6 turns on a 4 inch diameter

 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by NN2X on September 28, 2008 Mail this to a friend!
Outstanding, I went through some of the same experiences..

Very informative.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N3OX on September 28, 2008 Mail this to a friend!
I agree, a good half-wave dipole high above ground is sure one of the best antennas out there when you factor together price, performance, and easy construction.

There are pretty much no pitfalls.

In my current situation, it's hard to put up wire dipoles; just not much to hang anything from. I also decided I needed a little more performance on 17m and 20m and I built a modest beam (not that much better than a rotatable dipole though ;-) ) But, even with the fancy stuff I build with matching networks and relays and switches, sometimes I just string up a simple dipole where I can:

http://n3ox.net/projects/sixtyvert/antennafarm_lg.jpg

That 15m dipole was one that was originally thrown together in a big hurry to land a brand new country: a 15m CW contact with ZL8R. At the time, about two years ago, I felt like 15m was on the way out and didn't bother with a 15m antenna in my original antenna plans at my current house. ZL8R came up and I realized that was a mistake ;-) Tuning a mismatched coax fed antenna with my tuner was not even coming close to cutting it... dipole did the trick.

I also think a half-wave dipole is a basic measurement tool... it should be there in your toolbox just like your antenna analyzer. You build a half-wave dipole and you know it's within about 0.2dB of working like any other half wave dipole, and that lets you compare your other creations in the real world. If it doesn't work better than a half wave dipole that you can put up instead, well, the decision kind of makes itself, doesn't it?

73,
Dan




 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K1CJS on September 28, 2008 Mail this to a friend!
Nice article with some good information. This is the kind of writing that should be on the site, informative and factual--not opinionated and slanted.

This one is a print and keep!
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by AC5UP on September 28, 2008 Mail this to a friend!
Minor correction: Bill Orr's call was W6SAI and that call is currently held by the William Orr Legacy ARC in Tuscaloosa, AL.

For a vintage example of his work, you can download the 15th edition of the Radio Handbook (1959) here:

http://www.pmillett.com/Books/orr_radio.pdf

This is a large PDF (61 megs) so those of you on a dial-up connection may want to make this an overnight kinda' download. Considering the vintage I suspect the book is now within the public domain... If not, use your best judgment. Here's the full list of goodies available from the same site:

http://www.pmillett.com/tecnical_books_online.htm

Is it just me, or does the 813 have a bit of sex appeal?

;)


 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K3AN on September 28, 2008 Mail this to a friend!
"Then, in the 1970’s I came upon a wondrous factoid in an old ARRL Handbook. There it was in a graphic depiction… ALL the correct coax lengths for feed lines for all the H.F. bands for mono-band and multi-band installs! Improvements were immediately realized, especially when operating multiple bands with only one dipole due to location restrictions. Don’t bother trying to find this graph in new publications because they deleted it sometime after 1977."

WV4R, I have several ARRL handbooks from the 60's and 70s and I have been unable to find any kind of table or graph that depicts the "correct" coax lengths for feedlines for the various bands. What edition (year) and what page is this info located? And why in the world would ARRL delete such useful information from their publications?

BTW, in looking through these old handbooks, I found a number of diagrams and pictorials for various types of practical, coax-fed HF antennas. The illustrations usually state "any length" of coaxial feedline.

 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K7AAT on September 28, 2008 Mail this to a friend!

My deep thanks to AC5UP for that URL ! It is a fantastic resource for a lot of old but very useful information.

Ed K7AAT
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by AA4PB on September 28, 2008 Mail this to a friend!
Dipoles are close to 75 ohms only when they are mounted high and in the clear. At the height most people mount dipoles (20-30 feet) the impedance drops closer to 50 ohms. Drooping the ends down closer to the ground inverted-V fashion helps to lower the impedance. In no case will changing the length of the feed line change the match between the dipole and the feed line. If there is a mismatch resulting in standing waves on the feed line then changing its length can change the impedance presented to the transmitter. That can change the transmitter's ability to couple power into it and it can change a tuner's ability to find a suitable match but it won't change the match between the antenna and the feed line, the SWR on the feed line, nor the feed line loss resulting from the SWR.

The best length to use for a feed line that is a reasonable match for the antenna is the shortest length that will reach between the antenna and the transmitter.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K1BXI on September 28, 2008 Mail this to a friend!
The I and R on a feed line repeat every 1/2 wave in length, I guess this may be what the author is referring to. The 1/2 wave lengths (electrically) will let you measure what ever it is up at the feed point without actually being there. Although no matter the length, the SWR is the same at any point, so other than for the ease of measuring what's going on at the feed point, the only correct length would be just long enough to reach the rig.

John.....K1BXI

PS.......AA4PB beat me to it, I have to learn to type faster!
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by AB7E on September 28, 2008 Mail this to a friend!
"Of course IF you have a random length of coax, then you really do need something to ‘choke’ off the reflected power (SWR) you most probably will have on your coax."

A feedline choke won't improve the SWR on the line.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W4LGH on September 28, 2008 Mail this to a friend!
50-Ohm direct feed dipoles are the way to go. Have said it on here for years. Fan dipoles work very well, and dropping the ends ,making an inverted V lowers the fed point impedance closer to 50ohms. No tuner is needed, just switch bands and keep on getting up.

Trapped Dipoles, if the traps are made right is another great way to go multi-band without the use of tuners, and again, a direct 50-ohm feed.

Always use the best coax you can afford, for the lowest loss possible. I came across a deal on 250' of LMR-400 and have used it on all my HF dipoles, virtually no loss at all, the conveniences of coax use, direct burial, easily hidden etc.

I have never been a fan of a tuner in the shack, as I think of it as a bandaid. It really doesn't FIX the problem. If you make your antennas right, they aren't needed and it make operating multi-band more streamline!

I do have 2 single band dipoles, one cut for 40M and one cut for 20M. They usually give a tad better signal, both transmit and receive, and are great for marginal signals.

A lot of people will argue with my antenna theories on here, but everyone that knows me personally, will tell you have have super strong signals , just about all the time.

There is no substitute for as good antenna!

73 de W4LGH - Alan
http://www.w4lgh.com
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by WA8MEA on September 28, 2008 Mail this to a friend!
I found I could further improve dipole performance with either a simple 19-foot coil of coax at the feed point for 160m.
-----------------------------------------------------
Wow! Thanks!

Just getting ready to put up a new 160 meter dipole. This one has been up for more than 13 years. Time for new wire and coax.

Glad to see this little tip before I started the project!

73, Bill - WA8MEA
http://HamRadioFun.com
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K1BXI on September 28, 2008 Mail this to a friend!
"Of course IF you have a random length of coax, then you really do need something to ‘choke’ off the reflected power (SWR) you most probably will have on your coax."

"A feed line choke won't improve the SWR on the line."

Correct....it is the common mode current on the outside of the coax, not reflected power that effects the reading of the common SWR bridge.

John.....K1BXI
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W4VR on September 28, 2008 Mail this to a friend!
Murf: no wonder I have not heard you on 17 meters for a long, long time...you're using a rotary dipole. Ron, W4VR
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N6AJR on September 28, 2008 Mail this to a friend!
I always thought the perfect length for a piece of coax was long enough to go from the antenna to the radio??

Now where did I leave my fan dipole.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N6AJR on September 28, 2008 Mail this to a friend!
I always thought the perfect length for a piece of coax was long enough to go from the antenna to the radio??

Now where did I leave my fan dipole.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by KG6WLS on September 28, 2008 Mail this to a friend!
AC5UP,

Thanks for the link!! That's a keeper. :-) Only took 35 minutes to load from my cable modem. Must be a slow day. ;-)

73
KG6WLS
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W7ETA on September 28, 2008 Mail this to a friend!
Great prose.
TKS for the article.
73
Bob
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W5WSS on September 28, 2008 Mail this to a friend!
I have been enjoying the arrl antenna handbooks over the years. Although I too have not found specific reference to feed line lengths for direct feeding a dipole in the publications I do recall some mention of coaxial cable feed line lenths to avoid with respect to rf traveling back down towards the transmitter. Anyway I enjoyed the article...The late LB Cebik was kind enough to perform extensive modeling using eznec etc, and his findings are very informative not only for center fed wire doublets but for many interesting antennas. Thanks LB for your contributions to our hobby! One of his antennas that I like is the three 88' doublets in a triangle full global coverage no rotating needed and very good multi band performance..check that one out! 73 Bob
 
RE: Mythbusting Coaxial Chokes  
by KQ6XA on September 29, 2008 Mail this to a friend!
Sorry to rain on your parade, but coaxial chokes made by winding coax on a tubular form, or looping it, cannot be depended upon to provide reliable isolation.

Coaxial chokes made by this method use REACTANCE to achieve isolation. In any antenna situation, it is well known that reactance in series with a wire can be affected by reactance in other parts of the wire. That makes this type of coaxial choke especially sensitive to feedline length or antenna configuration.

The use of ferrite devices is the preferred method of achieving reliable coaxial choke isolation.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K9MHZ on September 29, 2008 Mail this to a friend!
>>>KB2DHG on September 28, 2008 .
I just built a home brew full size G5RV and the performance is WONDERFUL.<<<<


Oh man, this is almost like igniting a code/no-code debate. Glad it's working out for you!

Cheers,

Brad
K9MHZ
 
RE: Mythbusting Coaxial Chokes  
by AA5TB on September 29, 2008 Mail this to a friend!
by KQ6XA on September 29, 2008

"The use of ferrite devices is the preferred method of achieving reliable coaxial choke isolation."

I always wondered...
What is the net effect to the signal now that the RF that was going to be radiated from the coax via common mode currents is now being converted into heat within the ferrite device? ;-)

Just another wrench to throw in the works and another reason everybody probably has different results.

Steve - AA5TB
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W5HLP on September 29, 2008 Mail this to a friend!
Murf,

Thank - excellent excellent article and data. Even though as Murf knows, I haven't been "hamming" very long, I can second the effective height using the wet water table depth. In my "past life" in a galaxy far far away, for field commo setups we sometimes poked a ground rod down and measured the depth of significant moisture, then use that as our "extra height" in antenna height calculations (when performance was critcal and when we needed to be stealthy . . err . . . discrete). One interesting thing to note, the more rapid the "gradiant" of moisture, i.e. the more rapid the change from dry to wet, the more effective this "extra height" seemed to be. Purely anecdotal, purely memory, mostly fallible.

Thanks again Murf,

73
Herman
W5HLP
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W5WSS on September 29, 2008 Mail this to a friend!
In 1984 I designed and built a Log-yagi for 10m. The surge impedence was very close to 52 ohms resistive lending itself to coaxial feed. The log cell is balanced,(because they are split elements) meaning I needed to tend to a balanced to un-balanced issue. I resolved by building an air wound rf choke 10 turns 10" in diameter was sufficient for 10m. No rf traveled down the outside of the rg-214 and no bumps in the standing wave were observed. Note* the sides of the coaxial cable must not cross over each other or the choke will not work correctly* otherwise an air wound rf choke works reliably.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K1BXI on September 29, 2008 Mail this to a friend!
"What is the net effect to the signal now that the RF that was going to be radiated from the coax via common mode currents is now being converted into heat within the ferrite device?"

Given a proper design for the frequency in use It will allow the rf to flow into the dipole leg and not that 3rd leg of the outside of the shield by virtue of the high impedance of the choke. So done right, the choke stays cool and the rf that was radiated by the coax now goes into the low impedance dipole leg.

John.....K1BXI
 
RE: Mythbusting Coaxial Chokes  
by WA1RNE on September 29, 2008 Mail this to a friend!
by KQ6XA on September 29, 2008

Sorry to rain on your parade, but coaxial chokes made by winding coax on a tubular form, or looping it, cannot be depended upon to provide reliable isolation.

Coaxial chokes made by this method use REACTANCE to achieve isolation. In any antenna situation, it is well known that reactance in series with a wire can be affected by reactance in other parts of the wire. That makes this type of coaxial choke especially sensitive to feedline length or antenna configuration.

The use of ferrite devices is the preferred method of achieving reliable coaxial choke isolation.


>>> A coaxial choke is as much a current balun as the other designs that use ferrites around the outside of a coaxial cable or a parallel transmission line wound on a core.

If designed correctly for the frequency of operation, a coaxial choke can effectively block common mode currents from flowing along the outer shield of a coaxial line - with no sensitivity to feedline length. Antenna configuration can be a limiting factor, i.e. coaxial chokes for 160 and 80 meters can become very large and impractical. On the other hand, at 12 meters and higher, their effectiveness decreases dramatically due to interwinding capacitance, especially scramble-wound types.

BTW, the impedance created by any common mode choke is primarily inductive reactance and a small resistance - otherwise they wouldn't work.

There are lots of good articles on the web and several books on the subject that explain how common mode chokes work and how to build them. This article is not the place to brush-up on the subject.


...WA1RNE
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by WW5AA on September 29, 2008 Mail this to a friend!
"As I look back on my personal history of antennas from 1959 to today, I recall my first dipole epiphany."

In 1970 I had a parallel feed line epiphany. No more coax except for the balun.

Good work!

73 de Lindy

 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by G3LBS on September 29, 2008 Mail this to a friend!
Why did you need the balun - why not a true differential link tuner like Johnson Matchbox?
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by K7AAT on September 29, 2008 Mail this to a friend!

"Why did you need the balun - why not a true differential link tuner like Johnson Matchbox?"

A properly located balun AT THE FEEDPONT of the antenna will prevent RF from getting back into the shack,. I doubt most people would want to locate their tuner outside in the weather at the feedpoint.

Ed K7AAT

 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by N3OX on September 29, 2008 Mail this to a friend!
"A properly located balun AT THE FEEDPONT of the antenna will prevent RF from getting back into the shack,."

The link coupled tuner's common mode impedance is going to basically be set by the capacitive reactance between the coils. That's an extremely high impedance in most cases.

This is an excellent common mode current choke, and pretty much enforces a zero-common-mode-current-at-the-shack condition. If your antenna+feedline system is physically and electrically symmetric /balanced with respect to the environment, this translates to basically not allowing common mode current to flow.

The dangerous spot on an antenna for exciting common mode currents isn't the feedpoint, per se, but it's anywhere where the electrical system goes from balanced with respect to the surroundings to unbalanced. So you run balanced line back to the shack and what you need is a choke right at the tuner, and a link coupled unit should work great for that.

73
Dan


 
Opps i goofed sorry  
by N3JBH on September 29, 2008 Mail this to a friend!
Dear folks it has been brought to my attention that i made a mistake on the formula's i made The one for 28 Mhz should been a 4 inch coil. Many thanks to N3DRH for pointing it out to me.


3,8 Mhz 22 feet made from 8 turns on 8 1/2 inch diameter
7,2 Mhz 22 feet made from 10 turns on a 7 inch diameter
14 Mhz 10 feet made from 4 turns on a 7 1/2 inch diameter
21 Mhz 7 feet made from 7 turns on a 3 inch diameter
28 Mhz 6 feet made from 6 turns on a 3 inch diameter

I do not get how 6 turns on a 3 inch diameter form requires 6 feet of cable?

What am I missing

N3DRH
John Ossi


Here is some information I took from both the ARRL Antenna Book and from the
www.n0ew.org Web site and then modified to come up with. I hope it will help.
Jeff N3JBH

All choke are made from RG/8 coax .405Dia. stuff. All windings must be beside each other BUT !!! Never over lapping each other
For mono band antenna's

3,8 Mhz 22 feet made from 8 turns on 8 1/2 inch diameter
7,2 Mhz 22 feet made from 10 turns on a 7 inch diameter
14 Mhz 10 feet made from 4 turns on a 7 1/2 inch diameter
21 Mhz 7 feet made from 7 turns on a 3 inch diameter
28 Mhz 6 feet made from 6 turns on a 3 inch diameter

For multiband use antenna's

80 to 10 meter's 10 feet made from 7 turns on 4 1/2 inch diameter
80 to 30 meters 18 feet made from 10 turns on 6 inch diameter
40 to 10 meters 12 feet made from 8 turns on 5 inch diameter
20 to 10 meters 8 feet made from 6 turns on a 4 inch diameter



 
I give up  
by WB2WIK on September 29, 2008 Mail this to a friend!
and I give up.

But how about those Dodgers?
 
RE: I give up  
by W5WSS on September 29, 2008 Mail this to a friend!
Wb2wik/6 hello.

Now to the 10 meter case where someone suggested that we construct an rf choke using a 3" form? I do not think it is wise to use such a tight radius with coaxial cable. A broader radius can be chosen to ensure no migration of the center conductor towards the sheild...especially when any of the lower frequency rf chokes would cover 10m too. I encountered no problems on 10m with a larger and longer rf choke.


There are coaxial lengths that detune the cable from rf traveling down the outside of the sheilding....I read a chart that listed them for the ham bands in a Scelbi publication back in the early 80's.


N3ox's posting is also correct. The main point being that balance is restored. I neither endorse the use nor discourage the use of coaxial cable as a transmission line but merely want to point out some of the effective precautions that we often times over look when coupling it to a balanced load such as a dipole. usually closer to 75 ohms = 1:5:1 a 52 ohm dipole? that is an oxy moron because a dipole by definition is such because of current balance while the 2-elements are linear not drooping which technically changes the definition to an inverted v to a lessor degree! Regards 73
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by KW6LA on September 30, 2008 Mail this to a friend!


Murf,

Great article from a real Ham Fan. I very much enjoy the meat en tater types written here on E-Ham and loath the comments like

It's only a hobby ! ! What the hell does that mean……… I guess it was only your Bank last week ! ! ! For most of us here on

this web site , we come for hands on experience and real world results to help engineer our own stations. I sure like the humble

statement ( I steal knowledge and expertise from anywhere I can find it? I could not agree you more. When I got my Extra

some 17 years ago I knew it was only a license to learn. I have limited experience with the dipole, however found that coupling

over a roof, the apex, wave / height over ground, patterns , ground reflections, and the feed lines have great influences on how the

effected radiated power performs. So many times over the air, I have asked why the other station has chose to run their 20 meter

dipole @ 20 feet. If its done in a pinch----- game /on , but a lot of hams have no idea what it has done to the radiated pattern and

loss of ( Gain ) I am grateful for hams here that pour hours of personal time into a well written well intended articles. Helping

other Hams learn this very technical hobby, is essential for it survival. Thank you my friend for the info and get your rotator fixed.

I miss having dialog with you on SSB.



God Bless !

KW6LA / T2 Commercial.
 
RE: Thoughts on the '50-Ohm' Direct-Fed Dipole  
by W4LGH on September 30, 2008 Mail this to a friend!
~~"I do not get how 6 turns on a 3 inch diameter form requires 6 feet of cable? What am I missing"~~
N3DRH John Ossi

============================

Don't think you are missing anything. 6 turns on a 3" coil form is 56.88" which is 15.12" shy of 6 feet!! However it is probably close enough to work.

73 de W4LGH - Alan
http://www.w4lgh.com




 
RE: I give up  
by WB2WIK on September 30, 2008 Mail this to a friend!
>RE: I give up Reply
by W5WSS on September 29, 2008 Mail this to a friend!
Now to the 10 meter case where someone suggested that we construct an rf choke using a 3" form? I do not think it is wise to use such a tight radius with coaxial cable. A broader radius can be chosen to ensure no migration of the center conductor towards the sheild...especially when any of the lower frequency rf chokes would cover 10m too. I encountered no problems on 10m with a larger and longer rf choke.<

::But did you measure its common mode impedance, or just "have no problems?" There are many types of coax that can be wound on a 3" diameter form just fine. I usually use RG400A/U PTFE coax which can handle 3kW with VSWR>10, and winds just perfectly on even a 2" diameter form with zero conductor migration (the Teflon dielectric is much too hard for that).

The migration issue mostly occurs with cellular PE ("foam") dielectric cables and it's simply as misapplication of that construction.

WB2WIK/6
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by WV4R on September 30, 2008 Mail this to a friend!
Thoughts on the '50-Ohm' Direct-Fed Dipole
COMMENTS_1

Many thanks for All the COMMENTS. I have received many personal Email comments as well. I am humbled and flattered.

Since it is difficult to edit a published article please let me try to add some stuff as a result of all the Emails and Comments I have received.

I did not publish a formal “Bibliography” because this article is quite the collage from so many places and from the Mind of Murphy as well.

First off… I truly mourn the passing of William Orr/W5SAI who I believe was one great asset for Hammy Radio Operators. He collaborated with others as you can see from credits. I had temporarily ‘misplaced’ his texts due to a move and was trying to recall his call sign from memory… it IS the second thing that goes. Thank you William Orr/W6SAI wherever you are.

Atlee/NT5W and I have been hammy radio buds since 1959 and were poking around his hammy radio library one day and discovered the “GRAPH” in one of his old ARRL Antenna Handbooks… 1977, pg111 for those inquiring minds who need to know. I suppose the the ‘graph’ was deleted in newer versions because it did not consider All the different types of coax. For MY purposes the Magic SEVEN lengths good for 80m-10m are: 27.5’, 39.5’, 58’, 76’, 95.5’, 109’, 145’. Of course for monoband performance there are specific lengths graphed.

My thoughts in this article considered COST vs PERFORMANCE. It is true greater efficiency in commercial BALUNS may be achieved… at a COST of somewhere between $30 to $300 and more! The advantages of the “Coax R.F. Choke” are cost, fewer connectors since you can wind a coil around the appropriately sized container, tape it together and remove the ‘innards’ and have ONE CONTINUOUS run of coax with no insertion breaks (connector loss, weather-proofing, reliability, COST). Hey hay it may not be the best but we all can wind a coil around a plastic coffee can, right? And… it IS fund to ‘do it yourself’ saving money, eh? Let’s face it… not all of us can correctly install a PL-259 on a piece of coax… the Human Factor.

As for my “COPOLE” homebrew monoband design… the idea is for the average hammy radio guy to Clamp it on the end of an existing dipole from 20m – 2m and get Instant Gratification on Receive and Transmit. A simple weekend project with stuff in the junk box. Since it will change impedence to about 100-ohms, some kinda matching is Required for Nirvana… the holy grail… the elusive SWR of 1:1, eh? I have many responses and all are positive.

To the Nay Sayers… the lower the frequency the beeger the choke, certainly vice versa. This article is a one-shot attempt, In-The-Blind, to be ‘generally true’ since I got No idea what the readers might have in mind for their particular LOCATION and USE, eh?

My simple goal… Easy Reading you can take to the bank, without a lot of Tech. Stuff & Gobblygoock most of us are not interested in and cannot quite git around anyway.

73 es GOD bless, murf/WV4R.
 
RE: I give up  
by W5WSS on September 30, 2008 Mail this to a friend!
wb2wik/6 hello. Your point is well taken and normally over looked and No I did not perform that measurement, however I did perform the most important changes specific to my situation. The rf choke I made using RG-214 polyethylene dielectric was effective because after I installed it remedied rf traveling back down the outside of the sheilding towards the transceiver. The six element log-yagi which was a 52 ohm 10 meter mono bander did not need matching but rather only proper measures to couple a coaxial 52 ohm feedline to a balanced feed point. The choke I constructed was within the RG-214 length that was to be assigned the antenna to transceiver link. so the air wound coils were made at the near the feed point.when the antenna, coax, and transceiver related correctly the log-yagi at it's final position at 30 meters high There existed only one standing wave inherent in this design So in my case choking was not stringent to a monoband choke design.(which is why I chose a radius that would be less prone to migration in an outdoor high temperature climate Miami,FL) one observation I made was that with the same feedline connected and minus the coiled rf choke a measure of rf was seeing the sheilding as common mode path via a third circuit. in other words I could see that the choke was indeed effective. Without the choke rf was getting into my audio as opposed to with it installed an unknown measure of un-wanted rf was removed. I am certain that positive changes in the log-yagi pattern resulted when a radiating feedline was removed from the equation....this was in 1983 before eznec etc. I really liked knowing that the rf choke was superior to a 1983 class balun and less prone to alot of lightning strokes which did occur with no damage to anything except the 1/4 inch hole in the apex log cell element.I am certain I would have had to repeatedly replace a balun too often. We have worked a few times on 20m and look to talk to you again soon regards and 73 Bob
 
RE: Nice Thread  
by AL7GA on October 1, 2008 Mail this to a friend!
What a great exchange of ideas and opinions! And no one getting "preachy". Thanks to everyone for your input, and E-Ham for the space.
 
Poor Orv, K6UEY!  
by W6WBJ on October 2, 2008 Mail this to a friend!
In the early '60s, Bill Orr (W6SAI) and Orv Dalton (K6UEY) were both on the OSCAR I Committee. 'SAI was one of the head honchos on the Committee, and he had relegated poor 'UEY to the status of go-fer. One evening the Committee had a meeting planned when the QST photographer called and said he'd like to take a picture of the group for the cover of the magazine, so everybody should wear a suit or sport coat and tie that night. Well, the word got out to everybody except Bill Orr. When Orr showed up for the meeting and discovered the group was going to appear on the cover of QST, he made Orv loan him his tie! So Orv was the only one in the picture without a tie! He was so angry about it that he almost spit! And he has never gotten over it! That is the principal reason why he is still so mean!
 
RE: I give up  
by W5WSS on October 2, 2008 Mail this to a friend!
Hello, an 1:1 coaxial balun (effecting an excellent common mode attenuation)and negligible incident signal loss with excellent choking reactance for 10m-20m can be made by winding 6 turns of rg-213 on a 4" form. The location is best at the antenna terminals. This 1:1 rating means the choke is not a matching transformer, And is designed to be used with a balanced feed point. I agree with Steve wb2wik/6 that todays coaxial cables can better withstand a tighter winding radius. The performance of this design is one that reaches well beyond the latest and best ferrite compound techniques, with a non saturation heating issue at full legal limit. Remember this is a 1:1 rf choke design only. The ideal example would be such as 52 ohms balanced surge impedence(the antenna)coupling to 52 ohms un-balanced (the coax).

Do not bunch ( means overlapping in the coiling) and be sure the coax is side by side as a single layer whether you use air wound or wind around the 4" form. and make sure that the coils remain in a single layer.

The best construction technique is to allow for the extra winding length in the coaxial run that links the antenna to the equipment and wind where the finished rf choke does not need any additional connectors or a barrel splice etc.= free!
For a 40-30m version use 12 turns on a 4" form.

Good home brewing! 73 Bob W5WSS
 
Thoughts on the '50-Ohm' Direct-Fed Dipole  
by KL7AJ on October 5, 2008 Mail this to a friend!
It's amazing what one can accomplish on ham radio without knowing anything...at least when there's some propagation. I used a raw coax fed dipole for years before I knew what a balun OR an SWR meter was, and worked the world.

A little feedline radiation probably helped. :)

Eric
 
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