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Author Topic: Flat dipole vs. Inverted V  (Read 3762 times)
KE7NVY
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Posts: 46




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« on: September 30, 2007, 09:52:38 PM »

I'm just starting to set up my first HF station.  I'm trying to keep the cash outlay to a minimum, so I'm going with a dipole of some flavor.  I have enough room for a 10, 15, or 20 meter dipole, or possibly a fan arrangement.  I could go to 40m but I'd have to put up a special mast in the center of the yard to support that, so anything lower in frequency than 20 meters is for sometime in the future.

The tree I'll be using as the center point has convenient mounting points at about 18' and about 25'.  I did some trigonometry based on end point heights that are relatively easy to achieve, and found that if I use the higher center point, the center angle of the inverted V would be about 114 degrees, a little less than the minimum 120 degrees that I've seen recommended.  If I use the 18' center point, that angle works out to about 141 degrees.

So, would you recommend that I use the lower center height and a somewhat flatter antenna, or go for more height at the cost of a steeper V?  A third option might be to go for a true flat top, but one of the end points would be hard to get that high up (the other is a 6' corner fence post; I can extend that upward by clamping PVC pipe to it).  For a flat top, the 18' height may be do-able, but the 25' would be tough to do on my budget.

Jeff KE7NVY
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K7LRB
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« Reply #1 on: September 30, 2007, 11:31:35 PM »

Hello Jeff,

Please do your research and don't take the following as the "final word". I have been a ham for about 47 years. When I was a kid I was looking for basically the same thing as you, wire antenna, inexpensive, easy to construct and easy to "tune".

I certainly need to "recheck" this, but I seem to remember that a flat dipole has an impedance of 75 ohms in the middle, hence the need for a 2:1 balun. I also seem to remember that an inverted vee has an impedance of 50 ohms at the apex IF the legs are 90 degrees apart. Please bear in mind, this is all from memory from a long time ago, however, it should be easy to check out and I am sure subsequent posters will either confirm or correct me on this.

Here is the bottom line (from MY experience): Don't get hung up on the angle between the two legs. From my experience any angle in the 90 degree plus range will be fine. "Trim" the ends for resonance and your antenna will work fine for the band for which you have it cut. My typical antenna setup consists of several inverted vees, all fed from the same point with 50 ohm coax (Whatever happened to 52 ohms?). I try to have each "vee" go in a different direction (if that even matters). Anyway, the trimming of this "fan inverted vee" is not nearly as hard as some make it out to be. It just seems to me that 50 ohm output from TX to 50 ohm feedline to 50 ohm antenna makes for a pretty good deal. No tuner required and I "presume" all the power that leaves the TX goes out the antenna, un-encumbered by matching devices, baluns, etc (the "losses" in a 100 ft run of good coax is not enough to sneeze at). I had such a setup in Mississippi with 4 inverted vees, 160, 80, 40 and 30. I used to work all over Europe on 160m with 50 watts. Please read on.

Now, I have read and heard much "bashing" about the venerable inverted vee, even in an ARRL publication (I forget what article, been a while!). They described the inverted vee as "radiates equally poorly in all directions". I will certainly not argue that an inverted vee will perform like a multi-element beam on a 100 foot tower, if you can (or WANT to) put a beam on a 100 foot tower, knock yourself out. However, when I moved to this QTH I decided to try some different wire antennas, first a 40m Windom, then the famous G5RV. I was very disappointed with both those and of course each was a mono band antenna (the G5RV is a 20m antenna folks!). Anyway, I once again put up multiple inverted vees and immediately experienced much improvement.

Virtues of the inverted vee: 50 ohms (or close enuff) at the middle; easy to tune (trim) since typically the ends are close to the ground; coax fed; no matching device needed; INexpensive; easy to install and probably a few things I have not thought of. Oh, I did think of one more; if you use a pole to support the apex, the vees act as guy wires, takes a hurricane to knock one down!

Couple of points: You CAN use an antenna coupler for other, "non-resonant" bands, in fact, dependent upon which bands you have as resonant, several of the bands  which you do not have installed are harmonics of ones you do and are not that far off "resonance", all that is needed is a little "tweaking" via the coupler.

Also, let's say you put up a 20m inverted vee and you are able to get the apex up pretty high (which you should) but now you cannot reach the ends for "trimming" (each "leg" is only about 16'). No problem, simply cut the antenna for a wave and a half (each leg about 48'), trim it to resonance and you're in biz (you'll also get the benefit of broader bandwidth)!

The only change I made from all my previous setups is the addition of the "Ugly Balun", http://www.hamuniverse.com/balun.html. This seems to eliminate ANY RF back into the shack when operating on "non-resonant" bands, a "sometimes" problem I had with previous installations.

An inverted vee is a quick, easy, inexpensive antenna and once the solar cycle comes back, DX will be like shooting fish in a barrel!

Again, I am not interested in a flame war. If you have constructive comments I am certainly open to suggestions, corrections, etc. I am simply going from memory and what has worked for me for many, many years.

Hope this helps a little Jeff!

73,
de Larry
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KB9CRY
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« Reply #2 on: October 01, 2007, 01:42:17 AM »

Higher is better
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AC5E
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Posts: 3585




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« Reply #3 on: October 01, 2007, 03:51:30 AM »

Yep, higher is better. And flatter is better. Impedance varies with height above ground. And there are a lot of hams using much less than optimal antennas who think they have a world beating antenna system. And you will get a whole lot of free advice.

With propagation what it is right now, I would at least try to put up a 40 Meter antenna. 40 is open to somewhere most of the time - but I spend a lot of time tuning a dead band, looking for 20M DX. Hopefully the sun will wake up from its snooze and propagation will pick up - but even a few weeks is a long wait.

A 40/20M antenna would probably give you the most pleasure right now, unless you want to operate CW. If so, I would throw 30M into the mix.

For 40/20 ONLY, it's hard to beat a regular G5RV. 20 was what it was made for, and it works pretty well on 40. But you have to find a way to hang 102 feet of wire, and get it more than 40 feet high at the feedpoint. Which can be a problem.

But by all means do your homework. Just remember, "There's no such thing as a free lunch." Like any other activity, there are plenty of people who promise the world and deliver a spadeful of dirt.

73  Pete Allen  AC5E

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WA3SKN
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« Reply #4 on: October 01, 2007, 04:26:44 AM »

OK Jeff...
For HF, higher is better, and flatter is better!
You will want the angle of the inverted Vee to be at least 90 degrees. Lower than that will effect the amount of RF radiating from the antenna.
With the bands the way they are, you will want 20 meters and 40 meters operational.  You can expand from there.
You will probably want to start with dipoles and inverted Vees, but do consider the Loop.
And, consider the 450 ohm open-wire feedline and tuner, instead of coax... you will be glad you did!

-Mike.
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N4JTE
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Posts: 1158




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« Reply #5 on: October 01, 2007, 05:33:54 AM »

Hello Jeff, welcome to wacky world of wire antennas.
Just a couple of comments to add to the excellant replies you have gotten so far. You do not mention if you will be using an antenna tuner in your setup which may dictate what antenna would be best for you.
If you are connecting straight to the antenna with coax I would suggest the inverted vee, as it's impedance can be adjusted by the angle and the heights and lengths of the ends. If you have a tuner I would try  to get the 20 meter  antenna as high and flat as possible, at 18ft you would be about 1/4wl high which is not bad and will surprise you with it's stateside capabilities. Impedance will depend on height, hence the need for a tuner or matching system.
As said before, 40 meters is the band of choice for most activites these days so if you can find room for 66ft of wire in either configuration you will find a more consistant chance for contacts any time of the day.
Good luck, keep us posted.
Bob N4JTE




Keep up the experiments.
Bob N4JTE
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KE3WD
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Posts: 5689




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« Reply #6 on: October 01, 2007, 08:39:17 AM »

Flattop may arguably be "better" if only one parameter is being talked about.  

If you only have room for the inverted vee, by all means put it up there with the apex as high as possible and damn the angle.  


A good antenna matchbox is a worthwhile investment here, something manual with roller inducter and two air variable capacitors is my recommendation.  


If you can use a balanced feedline, so much the better IMO.  


KE3WD
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KT8K
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« Reply #7 on: October 01, 2007, 09:27:31 AM »

75 ohms at the feedpoint (and its 1.5:1 SWR) is not a problem.  Low loss feedline is always a good idea, so go with twin lead, ladder line, window line, etc. if you can.  Going higher, flatter, and with a lower loss feedline will all be to the good.  I would *really* want to have 40m at this point in the solar cycle, especially since 20m is sometimes surprisingly dead at these low sunspot numbers.

High SWR is only a problem when the mismatch is significant and the feedline is lossy.  I run up to 6:1 at times (band segments I rarely visit) and still make contacts with my QRP station.  I plan to put up a doublet with an autotuner at or near the feedpoint sometime soon.  That eliminates almost all feedline losses, a major cause of poor antenna system performance.
Best rx & 73 de kt8k - Tim
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KT8K
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« Reply #8 on: October 01, 2007, 09:33:52 AM »

Well, I forgot to mention that high SWR is also a problem if you have a rig that "folds back" power at 2 or 3:1 SWR.  In that case you really need a tuner of some kind.  But for most of us, the rig will push the power into the antenna system almost regardless of SWR, and the power will either be radiated or soaked up in feedline resistance/loss.  If the feedline is a minimal-loss type like open wire line, then antenna system efficiency (ratio of radiated power to power absorbed by resistance) will be high in spite of the SWR.

Don't let SWR rule your radio life.  Read up on it, and you will see that it is a lot less of a problem than many hams think.  Antenna *system* efficiency is the key.
73 de kt8k - Tim
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AA4PB
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« Reply #9 on: October 01, 2007, 09:38:45 AM »

A flat dipole impedance is around 75 ohms IF it is mounted high (at least 1/2 wavelength) and in the clear. The impedance drops when you bring it closer to the Earth. You'll probably find that the impedance at your height is somewhat lower than 75 ohms. If you drop the ends it should be even closer to 50 ohms.

In either case, no need for a 2:1 balun because even at 75 ohms the SWR will only be 1.5:1 which is quite acceptable. Adding a 2:1 balun to a 75 ohm antenna would only drop the SWR to 1.33:1 - not a very significant change.
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N6AJR
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« Reply #10 on: October 01, 2007, 12:28:26 PM »

Bye the way, none of this is mine, Its just stuff I pulled from here and there. thanks to all the folks with the great sites. this is My take on the multi-band, single feed wire dipole ( but fan dipole is easier to type.  a true fan dipole is also called similar to a cage dipole and has 3 or 4 wires of different lengths on the same band , like a 64 foot , 66 foot and 68 foot  double leg wires on the same feed to give wider bandwidth, but bottom line, they are cheap, easy to build, and they work..
 
 
http://www.ku4ay.net/dipole.html
http://www.qsl.net/kd7rem/antdipole.htm
http://www.hamuniverse.com/multidipole.html
http://www.angelfire.com/nb/ni4l/ni4ldipole.html
http://www.arrl.org/tis/info/pdf/9611073.pdf
http://www.geocities.com/n2uhc/2banddipole.html
http://www.qsl.net/na4it/fandipole.html

 
 
 
by N3JBH on October 12, 2006    
FAN DIPOLE OR MULTIPLE BAND DIPOLE SPECS:
(Each leg is shown in length so you will need two legs.)

10 METERS = 8'4"
12 METERS = 9'5"
15 METERS = 11'1"
17 METERS = 12'10"
20 METERS = 16'8"
30 METERS = 23'2"
40 METERS = 32'9"
60 METERS = 43'7"
75 METERS = 60'9"
80 METERS = 65'6"
160 METERS = 123'5"
137 KHZ = 1708'1"  
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KE7NVY
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Posts: 46




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« Reply #11 on: October 01, 2007, 01:28:51 PM »

Thanks for the replies.  Some additional details/clarifications, and a couple more questions:

This is one of those "I've been wanting to try this hobby for years, but I don't want to spend a lot of money until I have a chance to try it out for a while" situations.  Given that, I'm trying to avoid buying a tuner or putting up any special masts.  I'm looking at this setup as very temporary; If I wind up having fun with it I can always upgrade my system later, and if not I can bail out without taking too much of a hit to the pocketbook.

I have plenty of space for a flat dipole, at least for 20 meters and higher.  30 would just fit, although with a pretty short cord on one end.  The issue with a flat dipole is getting the ends up high enough.  Based on the replies so far, I think I'll go with the higher center point.  I already have all the pieces needed to build the "Ugly Balun" that Larry recommended.

I'm curious now about the balanced feed line, though.  If I went with twinlead or ladder line instead of coax, would I then have to use a tuner, or a different balun?  A balanced feed line won't radiate RF into the shack, right?  That is, if I run a balanced feed line into the house (whether or not I had to pick up a relatively inexpensive manual tuner), would I have to worry about zapping myself?  About how much difference would there be in power loss between coax (RG-58) and say 450 ohm line, assuming about a 50' run between the feedpoint and the radio?

Jeff KE7NVY
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KE6VG
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Posts: 297




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« Reply #12 on: October 01, 2007, 09:11:47 PM »

I put up both a 40 meter dipole feed with 300 ohm twinlead and a coax feed fan dipole for 5 or so bands. If you put up the fan dipoles you will have instant bandswitching between bands. You will also have about 1/2 S-unit stronger signal. I guess the tuner introduces some loss with the twinlead feed dipole. Both have advantages. The twinlead dipole just goes up with no pruning and works. And, most people wouldn't notice 1/2 S unit anyway. The fan dipole requires you to trim each dipole until you get a good match on that band. An MFJ-259b or similar antenna analyzer works wonders, otherwise you might go crazy. On either antenna, if you need, you can bend the 40 meter elements as a Z or L to make it fit your lot with little loss.
While I am waiting for my shack to be completed, I am using several of those 32' windsock poles for temporary antennas. You can easily put a dipole up at 32' in minutes with one of these. I have a portable dipole for 10-40 meters with little slide switches at each band length. I lower the antenna, flip the switch to shorten or lenghten the resonance for that band and put it back up in seconds. It will handle 100 watts easily and cost $3 for the switches and $6 for 100 ft of 22 gauge hook-up wire. If I short the coax at the bottom of this portable dipole and feed it against a couple of radials on the ground you can use it as a T loaded vertical on 80 meters. Much more efficient than even a Butternut HF2V on 80 meters. I also use two windsock poles 66ft apart to support a 40 meter half-square. You can't get a low enough radiation angle for good dx with a dipole at 32' high on 40m. You can with a half-square.
Use you imagination. My antennas would even be considered stealth. I just drop them to the ground when I am not using them.
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ONAIR
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« Reply #13 on: October 01, 2007, 10:26:29 PM »

   For 20 through 10 meters, I have just 3 words for you...   Bazooka Coaxial Dipole!!
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WA3SKN
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« Reply #14 on: October 02, 2007, 11:54:30 AM »

Jeff...
RG58 does have more loss than open-wire, but at HF frequencies and with 50 feet, there is no real problem.
I recommend the 450 ohm open-wire (not twinlead) and a tuner for multiband operations.  It is low loss, low cost, and works well with a tuner... you do not need to match impedances like coax.
You should not have radiation problems as long as  your antenna is balanced, another reason I recommend the Loop.  And, the Loop works well at fairly low heights as well.
Twinlead does not work well when wet, and does not handle any power well.
Consider a 40 meter dipole with the ends hanging down, sometimes called a "bi-square", and fed with the open-wire.  This would cover the 40 and 20 meter bands OK, and possibley more.
Even a small manual tuner is usefull, but get one with a balun... so it is easy to use the open-wire lines.

-Mike.
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