Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Three wire folded dipole antenna tips  (Read 7246 times)
AA4HA
Member

Posts: 1424




Ignore
« on: March 10, 2010, 08:49:34 AM »

Hi, I am collecting all of the components to put together a three wire folded dipole that is capable of working up to 160 M. I need some guidance on wire spacing, length and the need for terminating resistors.

Here is where I am so far;
I have plenty of land and will run the folded dipole between two trees at a height of around 40'. The antenna will be fed with 600 ohm open ladder line down to a 12:1 Balun at ground level. At this point I will attach the coax shield to a grounding system consisting of buried radials. The balun coax will be installed below-grade in burial rated coax, approximately 100' to the house where I will install, surge protection, more grounding an a SG-239 SmartTuner. The antenna element conductors will be made of bare 1/16" stainless steel rope (wire). My element spacers will be made from 2" fiberglass antenna mast sections, drilled to pass the 1/16" wire rope through the sections, then the two end spacers will be capped off and filled with fiberglass resin for stiffening and strength.

The application is for 24/7 ALE operation off of a Harris RF-350K at 100 watts. I need to be able to support band-switching from 10 to 160 Meters. If this antenna works out well I will put up another for general usage.

I understand that the lack of height will mess up the takeoff angle in the lower bands and make the antenna more NVIS-like. That is not my concern.

Ok, now for my questions;
1. Having gone over the specs from several three-wire folded dipole antenna makers (Codan, Array Solutions, etc...) I really did not like what happens to the SWR when the antenna gets up to 160 meters so I will make the antenna longer (like for 200 meters). What I want to avoid are the terminating resistors that are used on some of the antenna designs. Resistors just seem to be a way to transfer antenna power into ice-melting duty. This seems to be common when the antenna is fed on the outside two elements with the terminating resistors on the center element at the middle of the antenna (Array Solutions AS-BB-175). Are the resistors necessary or is the antenna naturally at a 600 ohm impedance? It seems with the resistor design they use a 16:1 balun.

2. How critical is the spacing between elements? I was planning on using some mil-surplus fiberglass mast sections for a 18" spacing between elements (36" total).

3. Is there an advantage either way of feeding from the center element and folding back the dipole at each end to an opposite leg? Or what are the advantages (if any) of feeding from the outside elements and tying them back to the center element and into terminating resistors?

4. Does this antenna design just use the standard dipole formulae?

I know that the three wire folded dipole is a classic antenna design but I have been searching for days on design details and have not come up with any good examples. BTW, I have most of the ARRL antenna books and this antenna design seems be mentioned only in passing.

Tisha Hayes
KJ4SHJ (AA4HA)
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #1 on: March 10, 2010, 09:12:52 AM »


The application is for 24/7 ALE operation off of a Harris RF-350K at 100 watts. I need to be able to support band-switching from 10 to 160 Meters. If this antenna works out well I will put up another for general usage.
<snip>
Quote
Are the resistors necessary or is the antenna naturally at a 600 ohm impedance?

The problem with dipole antennas, whether they're folded or not, is that the impedance is a strong function of frequency.  A three wire folded dipole is only near 600 ohms resistive on the band it's cut for, and a similar impedance on odd harmonic frequencies.

Resistive termination helps flatten the impedance excursions but wastes power.

It's extremely difficult to get a low SWR from an efficient antenna system across the whole 1.8-30MHz range without one of these:

1) switching matching networks, like using an autotuner (sounds like that's in the works anyway)
2) building a huge broadband antenna like a discone or conical monopole
3) adding loss

All of those have disadvantages.  #1 has some appeal, and probably will work OK with your SG-239, but you need to figure out if that will really switch fast enough and if you go that route, you might want to reconsider your antenna type.  On the even harmonics of the operating frequency, a folded dipole has a *very low* impedance, which makes for a huge mismatch to the 600 ohm line and can make the impedance the tuner needs to match even more extreme.    If you're using the SG-239 autotuner, a plain 160m dipole or maybe slightly longer or shorter doublet fed with open wire line to your tuner might have less overall system loss.   The devil's in the details though.

 #2, none of us have space for.  #3 has the obvious disadvantage :-)

Now, if you're running an ALE system that uses the ham bands or any other set of narrow fixed allocations, a multiply resonant antenna like a bunch of parallel dipoles or a good trap dipole would probably be the best system, but would be really finicky to set up.    This is the big appeal of the resistively loaded antennas... they don't require a day of painful, tricky initial tuning.   It also wouldn't work well if you have full frequency agility... you sacrifice broadbandedness for good SWR in a bunch of narrow bands when you build parallel antennas or trap antennas.

Using the autotuner with some antenna that has no resistors could work, but I think the folded dipole wouldn't be great.

A *cage* dipole, or other multiple wire antenna where you put all the wires in parallel at the feedpoint is good to help limit the impedance excursions.... it's like running a dipole made of huge, fat pipe, which has less reactance change with frequency.  So that aspect of the folded dipole would be good for bandwidth broadening and limiting the nasty loads your tuner has to handle.

73
Dan

« Last Edit: March 10, 2010, 09:18:25 AM by Dan » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W0FM
Member

Posts: 2055




Ignore
« Reply #2 on: March 10, 2010, 09:35:41 AM »

Hi Tisha,

The SG239 Smart Tuner is typically designed to be used to feed the open ladder line directly, not the coax.  Unless I read it wrong, it appears that your design is to have the SG-239 in (or near) the shack and feeding the coax that runs out to the ladder line.  I think that a little more research on the Smart Tuner will indicate that it sould be at the bottom of the ladder line and the coax running from the shack will connect directly to the Smart Tuner at the junction of the coax and the ladder line.

Just my 2 cents.

good luck with the project.
73 de Terry, WØFM
Logged
W5DXP
Member

Posts: 3579


WWW

Ignore
« Reply #3 on: March 10, 2010, 09:50:01 AM »

I am collecting all of the components to put together a three wire folded dipole that is capable of working up to 160 M.

Let's be sure what kind of "three wire folded dipole" we are talking about. The standard three-wire folded dipole is 468/f feet long fed with open-wire line. A shorter dipole that has been folded for linear loading is a different antenna. Are we talking about a three-wire dipole that is 468/f feet overall or a dipole that has been folded back on itself in order to shorten the antenna to a length less than 468/f?

In any case, that configuration depends on an SWR of ~1:1 on the parallel feedline. Don't expect that same configuration to work on 160m, plus 80m, plus 40m. EZNEC says that a 40m folded dipole used on 20m has a feedpoint impedance of 6-j160 ohms causing the tuned feeder to become a transmission line transformer where the impedance seen by the balun depends upon the length of the parallel transmission line.
--
73, Cecil, w5dxp.com
« Last Edit: March 10, 2010, 10:57:27 AM by Cecil A. Moore » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
Member

Posts: 13234




Ignore
« Reply #4 on: March 10, 2010, 10:42:39 AM »

The late W4RNL has some extensive work on folded dipoles on his website here:

http://www.cebik.com/content/a10/wire/wbmw.html

Yes, you have to register (free) for access, but it is well worth it for the wealth of
information he has available.


That said, a folded dipole might not be the best antenna for your needs unless you
are planning to use resistive loading for a wide bandwidth.  The conventional folded
dipole doesn't work very well on even harmonics of the design frequency.  It presents
a 600 ohm impedance only on the design frequency (where it is about the same length
as an ordinary half wave dipole).  Without the terminating resistors the SWR will jump
all over the place as you change frequency.  Depending on your ALE system, your
tuner may or may not be able to keep the antenna tuned properly each time the system
changes frequency.

The resistive-loaded antennas (there are several types) are popular for this service
because there is no antenna tuning required when you change frequencies.  The downside
is the loss in the resistor:  for the common 90' folded dipole this can be 10dB on 80m and
20dB on 160m.  The Oregon Office of Emergency Services changed to a diamond-shaped
loop (also with terminating resistor) instead of the folded dipole because they said it had
better performance on the frequencies of interest.  Unfortunately, critical information such
as antenna efficiency vs. frequency are not always provided by the manufacturers.

Even if you can make your antenna tuner work fast enough, you still have to consider the
radiation pattern of the antenna vs. the desired coverage area.  It is difficult to find a
single wire antenna that will give good NVIS radiation from 160m through 40m.  For example,
an OCFD designed for 80m will also work on 40, 20 and 10m with relatively low SWR, but
the 40m radiation pattern will have an overhead null.  The same thing happens when you
design the antenna for 160m and use it on 80.  A straight center-fed doublet will have a
useful NVIS pattern on 160m and 80m, but if it is 1/2 wave on 160m then the pattern on 40m
will have an overhead null.  You may be able to shorten it a bit, trading efficiency on 160m
for NVIS coverage on 40m, to some extent.  There are some solutions to this problem with
a bit of creativity:  I've found a couple designs that should give reasonable NVIS patterns
from 160m to 40m (you don't need them any higher than that) but they require some number
of bends and/or multiple wires.

If the system is just intended to cover the ham bands, then starting with several half wave
dipoles on a common coax feedpoint is difficult to beat in terms of performance, especially for
the lower bands.  That gives you good NVIS radiation without needing a tuner.  That isn't always
a good solution for commercial/military ALE systems, however, because they have a wider range
of frequencies to chose among.

While Dan mentioned wideband antennas such as the discone, and how much room they take, there
are some reasonable options that might work for you to give low SWR over a wide range of
frequencies.  One of the simplest is a true fan dipole:  for 40m through 10m, cut 10 wires each 40'
long.  Connect 5 wires to each side of a 4 : 1 balun and spread the wires on each side out into
a fan with perhaps 20 to 25 degrees of angle.  (This is from memory, as I don't have my notes
handy.)  If you have limited end supports, you can support the antenna from the middle wire in
each fan and let the outer wires angle downwards.  This should give an SWR less than 2 : 1 or
so over the entire range from 7 to 30 MHz.  Obviously it could be made larger to include 80 and
160m, but you start to run into the problems with radiation patterns at some point.  If you are
only using ham frequencies, one of these in addition to inverted vee dipoles for 80 and 160m
might give you the coverage you need.
Logged
AA4HA
Member

Posts: 1424




Ignore
« Reply #5 on: March 10, 2010, 04:12:48 PM »

Wow, quick responses...

I will scattershot my responses to your questions;

Yes, this would be for a full size 468/f at 160 M (~246' at 1.9 MHz), give or take differences for the element diameter and material (stainless steel). It did not seem to make much sense to create a three wire folded to save space when that is not an issue (90 acres in the woods).

The tuner details were a bit thin on the output connections. It appeared that one leg was attached to ground and that led me to think of it as a unbalanced connection. If it is a balanced output then all the better unless it would be easier on the tuner if I did an impedance transformation through a bal-bun (balanced to balanced??). I want to avoid coax line except where it runs to the house. I can always put the tuner out right at ground level below the midpoint of the antenna and have ladder-line running up to the midpoint of the three-wire dipole.

From what I could parse out of the details on the termination resistors is that they really came into play at frequencies lower than antenna resonance. For an electrically short antenna (let's say, cut for 80 meters) then it seems to me that the resistors would try to eat up the power when running at '160.

The SC tuner (this is from memory so I may not be right on) appears to have a 10-20 mSec tuning time once it has been on that frequency once before. It pulls it out of memory and flips in the right capacitance/inductance to make a match. I could always program in a "transmitter attack time" of >20 mSec to allow the tuner to do it's thing before passing data to the transceiver. Relays are a different can of worms to me, I hate the darned thing and the RF-350K has a ton of them to do filter selection during bandchanges (that is a different kettle of fish).

I did not detail it out in my long-ish question but I understand that antenna impedance is a function of frequency and swings each way as a frequency sweeps through the resonance points of an antenna. Are the resistors acting like "Q spoilers" to give a broader bandwidth response for the antenna? I want the antenna to be able to support MARS operations that are off the beaten ham frequency path. (once I get this thing up and the gear all hooked up I will pursue participation in MARS).

Losing 10 dB is painful to me. I have spent the last 25 years of my career trying to account for every dB on microwave systems. I hate to give up dB's as I really understand what 10 dB means for EIRP and receiver performance. If the resistors are the best choice then I will need to bite the bullet on that one. I had to when I strung up all of the terminated beverages for my receivers.

Are there any thoughts on the 2-3 different schools on configuring the feedpoint and if feeding the center element with the ladder line and then at the end of the center elements letting the long outside leg zig back down the entire length of the antenna? Are there advantages to feeding from the two outside elements and tying to the load resistors on the middle element?

There was quite a bit of appeal of the caged dipole to me. By extension I can see where a caged dipole could end up becoming a biconical. The feed point impedance was also interesting that it could be brought to 50 ohms. I did not want to put 500 pounds of wire up in the air and making the spacers for a cage appear to be a challenge (again, element spacing is a question).

When I put up antennas I hate to pull them down to either snip and clip wire, to replace insulators, redo connections that have become diodes or to fix a lack of weatherproofing. "In the business" that is a second trip on-site and paying some tower climbing company $500/hour to fix something that should have been done right the first time.

If the opinion is to go with the resistors for the bandwidth advantages (the more I think about it, the more sense that makes) I can bite the bullet and buy two 300 ohm non inductive resistors. The other thing that gave me heartburn with the resistors is that if I do decide to add an amplifier later (yea, I know I could not pass a kilowatt through that tuner) then the resistors need to be big enough to dissipate the heat without becoming a Fourth of July firework.

I wish I could find an EZNEC representation of a three wire folded dipole that I could play around with in the program to work out soil conductivity quality, height above ground, wire spacing, wire diameter/materials, element spacing, etc... I learned on the original NEC program back in the 80's but it has been a while since I have done any antenna modeling from scratch.
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AA4HA
Member

Posts: 1424




Ignore
« Reply #6 on: March 10, 2010, 04:34:44 PM »

Wow, great info on antennas, thanks for the URL. I had read some of his work before but I did not see this article.

thanks
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WX7G
Member

Posts: 6035




Ignore
« Reply #7 on: March 10, 2010, 05:25:45 PM »

A folded dipole, whether two wire or three wire, will not work at even harmonics of the fundamental. That is because each half of the dipole is a shorted half wavelength t-line and presents a 0 ohm load.

The terminated wideband dipole, built with two or three wires, presents a 900 ohm input impedance over a 10:1 (or more) frequency range. The radiation efficiency varies from about 25% to 75% depending on the band. Where the antenna becomes less than 1/2 wavelength the efficiency drops quickly.

These antennas are quite easy to simulate using NEC-2.

I have built these antennas and they will provide a VSWR of 3:1 or less over a broad frequency range.

What you want is wideband and a good match. This can be done and is done for large military dipoles using a fan-like arrangement. If you want to stick with a simple dipole as you propose the VSWR on 450 ohm ladder line will run 10:1 on the amateur bands. A 9:1 balun that is up to the task will allow coax to be used. Note that the coax will have a 10:1 VSWR.

I can run the numbers for you but my guess is that the efficiency of the terminated wideband dipole is comparable to a dipole that is connected via a 9:1 balun to 100' of RG-213 coax.

However, if you move the tuner to the balun, thereby running a 1:1 VSWR on the coax, the 260' dipole fed with ladder line can be efficient. A folded arrangement is not needed nor desired.

The radials are not needed for RF performance. They will however provide a lightning ground.
Logged
W5DXP
Member

Posts: 3579


WWW

Ignore
« Reply #8 on: March 10, 2010, 05:42:29 PM »

I wish I could find an EZNEC representation of a three wire folded dipole ...

Here's a standard 3-wire folded dipole at 70 feet that is resonant on 1.871 MHz with a feedpoint impedance of 575 ohms. The feedpoint impedance on 3.8 MHz is 6.2-j135 ohms. The feedpoint impedance on 7.2 MHz is 41-j665 ohms. All geometry and segmentation checks are OK.

http://www.w5dxp.com/160mFD.EZ
--
73, Cecil, w5dxp.com
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0ZN
Member

Posts: 1547




Ignore
« Reply #9 on: March 10, 2010, 06:02:14 PM »

Hi,

You are doing some good home work and it will pay off.

To kick in my two cents:  keep in mind that if that antenna is a half wave on 160 it will present an EXTREMELY high feedpoint impedance on the 2nd harmonic (80 M). It will probably give an astronomically high SWR because you are attempting to feed what are essentially 1/4 wave long closed stubs which present many thousand ohms Z with 450 ohm line; a BAD mismatch!! i.e. the impedance ratio between the line and the load will be VERY high on those frequencies. Same would basically apply on any even harmonic.

Folded dipoles are very good antennas, broadbanded, DC shorted, etc, but they make POOR multiband antennas due to the extremely high feedpoint Z on even harmonics.

I have built and used a number of FD's over the years and spacing does not seem very critical from what I have seen as long as it is "reasonable" i.e. 4 to 12 inch between conductors on 160 should be fine...with the wider spacing probably the better choice.

FYI: give some serious consideration/thought to mechanical support; that antenna will be relatively heavy for the span involved and tensil loading will fairly high. If you get ice in your area it will be very high with ice loading!

73,  K0ZN
Logged
K0ZN
Member

Posts: 1547




Ignore
« Reply #10 on: March 10, 2010, 06:08:48 PM »

FOLLOW UP....

I just saw Cecil's EZNEC analysis.... I am amazed that the feedpoint Z is that low on 80 M. Seems kind of contrary to theory, but I am not going to argue with EZNEC.

I have had 80 M and 40 M Folded Dipoles (two wire) fed with 450 ohm ladderline and they are not at all "happy" and would not work on the 2nd harmonic.  

  "Your results may vary....."

73, K0ZN
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #11 on: March 10, 2010, 06:18:26 PM »

I just saw Cecil's EZNEC analysis.... I am amazed that the feedpoint Z is that low on 80 M. Seems kind of contrary to theory

It's inverted from the familiar even-half-waves situation.  With a simple doublet you have a very high feedpoint impedance at what some people call the "anti-resonances", but with a folded dipole it's very low instead. 

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5DXP
Member

Posts: 3579


WWW

Ignore
« Reply #12 on: March 10, 2010, 07:11:17 PM »

I just saw Cecil's EZNEC analysis.... I am amazed that the feedpoint Z is that low on 80 M. Seems kind of contrary to theory, but I am not going to argue with EZNEC.

If it were a simple wire dipole, each element would represent a 1/2WL wire open-circuited at the end. Therefore the feedpoint impedance of a 1WL wire dipole is high since the open-circuit is mirrored at the feedpoint.

Since it is a folded dipole, each element represents a 1/2WL section short-circuited at the end. Therefore the feedpoint impedance of a 1WL folded dipole is low since the short-circuit is mirrored at the feedpoint.

That's why folded dipoles don't make good all-band antennas.
--
73, Cecil, w5dxp.com
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0ZN
Member

Posts: 1547




Ignore
« Reply #13 on: March 10, 2010, 08:06:03 PM »

W5DXP:
"Since it is a folded dipole, each element represents a 1/2WL section short-circuited at the end. Therefore the feedpoint impedance of a 1WL folded dipole is low since the short-circuit is mirrored at the feedpoint."

THAT makes sense!  Thanks, guys. Sometimes the obvious is inobvious!

I just know from several Folded Dipoles I have had that Folded Dipoles (which are excellent antennas on their fundamental frequency) sure do not radiate very  well on the even harmonics. Theory and the real world agree on this one!

73,  K0ZN
Logged
WB6BYU
Member

Posts: 13234




Ignore
« Reply #14 on: March 11, 2010, 07:10:09 AM »

If you can get the autotuner to work in that application, here is a possible antenna you can
use with it that has a reasonable radiation pattern over a wide range of frequencies.  It is
a variant of the original single-wire fed Windom,  but bent to improve the NVIS performance
on 80 and 40m.

From the center point, run one wire out in a convenient direction for 150'.  Run at second
wire at a right angle to the first out 100'.  Connect the wires together, and to a vertical
wire that drops down to the autotuner at ground level.  You'll need some radials, especially
for optimum efficiency in the 2 to 3 MHz range. 

This isn't perfect, but is one of the simpler designs that gives a suitable radiation pattern on
most frequencies of interest.  There may be a high impedance point near the bottom of 80m
(depending on the length of the vertical wire) that some autotuners have difficulty matching
so you may want to tweak the dimensions a bit to either move it down lower or up into the
3.6 - 4 MHz SSB portion where no ALE systems should be operating.  You'll have to verify the
operation on your specific list of frequencies.

Any time you use the autotuner for wide-range use like this, you'll want it at the antenna
feedpoint rather than in the shack.    That's what they are designed for, as it avoids the
losses in a long length of coax operated at high SWR.

On the other hand, you certainly do NOT want to use it with a resistive loaded antenna - the
whole reason for resistive loading is to provide a good impedance match across a wide range
of frequencies (with a resulting trade-off in efficiency).  If you are going to use the tuner
anyway, find an antenna that will give the desired performance over frequency and let the
tuner do the matching rather that wasting power in resistors.
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!