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Author Topic: Length of counterpoise on end fed half wave  (Read 8870 times)
N8TI
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Posts: 115




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« Reply #15 on: January 02, 2012, 09:51:13 AM »

Well, for a few years I used a 66 foot end fed with a 33 foot counterpoise. Worked great on 40 meters and had massive rf feedback on 20.  Now I think I know why   I think it will be worth it to prepare a couple different Zepps and take a minute or two more to throw them up and bring along a two or three position antenna switch.

Thanks a lot for taking the time to explain the basics on the zepp.

Joe
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WB6BYU
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Posts: 13010




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« Reply #16 on: January 02, 2012, 10:01:15 AM »

Quote from: N8TI

What I am trying to do is figure out what lenght my end fed half wave should be and the lenght of the counterpoise (or the ladder line) should be for a portable operation where I am sitting at a picnic table out in the park working from 5 to 30 watts with battery power.  Given that you really can't know how your antenna is working when you just set up outside for some portable work, at least until you have been there a while, I wanted to at least get the theory correct. I understand that the tuner will load up practically anything, so I don't want to rely on just getting a good SWR match.




First, there is no length that your antenna or feedline should be.  There are many
combinations of antenna and feedline lengths that will work, depending on how you plan to
string up your antenna, the distances and directions you want to work, the type of tuner you
are using, etc.  The best we can do is to offer suggestions, but you have to make the
final determination of what works best in your situation.


Obviously, if you are using a half-wave end-fed, then it should be half a wavelength long,
though the same matching system will work for multiples of that.  In this regard, a half-wave
wire for 40m can also be used on 20m.  If you are using a base tuner attached to the wire
then a parallel-tuned circuit can be arranged to match both bands by just adjusting a single
variable capacitor.  In that case, adding a quarter wave radial for each band to the tuner
should give a reasonably good system, and you may be able to get by with shorter radials,
possibly just one for 20m, or even a metal object such as a pack frame, but you'll have to
see how that works with your rig.

If you are using a Zepp feed, then a half-wave wire for 40m will work, but the quarter wave
feedline becomes a half wave on 20m.  Is this a problem?  Well, I've used several Zepp
antennas fed with 50' of 300 ohm twinlead (because that was the only feedline available.)
This is close to a quarter wave on 80m (when corrected for velocity factor) and roughly
a multiple of a half wavelength on other bands.  This worked reasonably well running QRP
with a parallel-tuned antenna tuner, but the high impedance might not be a good choice
at higher powers when using a relay-switched auto-tuner.  As you suggest, I don't really
have a good comparison to say whether the antenna was working or not, but I was able
to make a number of contacts from Alaska with my Argonaut.

So there are a number of factors to consider:

(1) the impedance to be matched.  Most relay-switched auto-tuners don't like high-impedance
loads due to the voltages that can be developed across the relays.  An end-fed half-wave
antenna has a high input impedance.  I've never had a problem matching one with a manual
tuner, and it isn't difficult to make a purpose-built tuner for one band (and perhaps multiple
bands).  Using a quarter wave of feedline will bring this down to a much more reasonable
impedance - you might not even need a tuner, but this only works for specific feedline
lengths.  (And limiting yourself to 17' of feedline doesn't let you get the antenna up very
high on 20m.)  If your tuner can handle it, you can use other lengths to get your antenna
up higher, though the impedance will vary over a much wider range, and the common
mode currents may not be as well behaved.

(2) the radiation pattern.  While a 40m half-wave wire will also have a high feedpoint
impedance on 20m, the radiation patterns are different:  maximum radiation on 20m will
be between 15 and 60 degrees to the direction of the wire.  If the wire is vertical, then
maximum radiation will be at about 35 degrees above the horizon.  You'll still make
some contacts, but a half wave wire will work better for most likely vertical angles.
When the wires are horizontal there will be a 4-lobed pattern on 20m which should
work well in the directions of the lobes and less well towards the nulls.  It makes a good
general purpose antenna, but might not work in a specific direction of interest unless
you align it properly when you string it up.


(3) this brings us back to the important question - what is it about the end-fed half-wave
antenna that lead you to choose it in the first place?  If you are thinking of using it
horizontally with a Zepp feed, then center-feed with twinlead to a balanced tuner would
probably be a better choice.  Then the maximum radiation is in the same direction for both
40m and 20m, and it can be used on other bands even when the wire is not a multiple of
a half wavelength.  You may still have a high impedance load with some combination of
antenna and feedline lengths, but you can simply choose a non-resonant combination that
allows your tuner to work on all bands of interest.

If you are planning to run the wire directly to a tuner at the rig (or a fixed tuner of some
sort connected to the rig via coax) as is often done for end-fed half-wave antennas then
you run into the problem of the radiation pattern of the full wave wire on 20m, besides
having to get the wire over a branch more than 60' off the ground.  For 40m it makes
sense to slope the wire or run it over a branch and back down towards the ground, since
even high angle radiation is still useful during the day, but this isn't as effective on 20m
since the average height above ground of the maximum current points is fairly low.


It does make a difference where you are located, however.  Over poor ground (very dry
or rocky) then horizontal polarization is likely to work better than vertical.   But if you
are in a salt marsh or overlooking the ocean, vertical polarization may be much better.
One of best portable antennas I've ever used was a sloping Zepp-fed long wire over
salt water for one Field Day in Alaska.


Having done a lot of portable operation over the years, I've gravitated to a set of dipoles
on a common feedpoint.  It gives me more flexibility in how I install the antenna and I can
optimize the configuration for whatever combination of bands I want.  Best of all, once
I tuned up the antennas (>30 years ago now) I haven't used an antenna tuner with them.
Well, except for the cases where I've combined the wires in various other configurations
to suite the circumstances.  Generally I string up the feedpoint as high as I reasonably
can and tie the wire ends off to convenient branches, etc. in an inverted vee configuration.
This also has the advantage that it requires only a single high point to install the antenna,
which could be a portable mast in an area that doesn't have trees.


So there is no "one right answer".  There are lots of options, all of which will work to some
extent, though they will work differently.  Fortunately all of these antennas are cheap and
easy to build, so you can try two or three different ones and see which works best for your
preferences and style of operation.
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AA4PB
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Posts: 12667




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« Reply #17 on: January 02, 2012, 10:28:11 AM »

What I would do for your portable application is to use something a little shorter or longer than 1/2 wavelengh antenna wire connected directly to the tuner sitting on the picnic table or perhaps directly on the ground. I'd connect one or two random length wires to the tuner chassis and lay them on the ground. That way the counterpoise radials won't be resonant, they will just couple to the Earth. The longer the radials (up to about 1/4 wavelenght) the better and more of them is better - but just do what is reasonable for your location.

The closer the antenna is to 1/2 wavelength the higher the feed impedance and the lower the loss in a less than ideal counterpoise. The issue is that many tuners won't match such a high impedance so you shorten (or lengthen) the antenna a little to lower the feed impedance enough that your tuner can match it.

MFJ has a low power manual tuner designed to be used with a single wire feed and Elecraft has a nice automatic tuner designed for that type of antenna.
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N8TI
Member

Posts: 115




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« Reply #18 on: January 04, 2012, 08:01:31 AM »

Thanks everyone for the help.  I can see that the largest problem I had was not understanding that the counterpoise or, if using a End Fed Zepp, the feedline should not be a 1/2 wave. I simply did not understand the whole concept there.  One common theme in all of your answers and advice was to stay away from a 1/2 wave counterpoise/feedline when using a end fed wire/Zepp.

I have a couple of painter poles that go up about 15 feet or so, and was going to use one or two of them as supports with some guy wires attached to some spare paving blocks.  Alternatively, I was going to throw the end fed antenna wire over the picnic shelter.  I figure that I would operate under picnic shelters for that reason alone.  The State of Michigan is kind enough to provide picnic shelters at most of its parks and in the middle of winter I don't think I will be disturbing many family reunions that usually populate the shelters.

I recall seeing in either QST or CQ a while back how a guy had a drum or wheel type setup where he wound, in his shack, his ladder line and then used clips to cut in and out various lengths of ladder line feedline for his Zepp or dipole.  I thought it was interesting that he had the setup right in his shack and that he thought it was important to adjust his feedline. Now I can see why.  Given that there is not supposed to be much radiation from a zepp feedline until relatively close to the antenna, having the feedline in the shack probably did not give him trouble.

I have a loop that never worked.  The thing is about 600 feet long and is fed with ladder line.  I could not get it to work on any band. But, I never measured the ladder line feedline.  Perhaps I should when I take it down and it may reveal something.  Perhaps I should start over with that loop or turn it into a Zepp or giant dipole.

I have ten acres with my lot line about 1000 feet, and where my antennas start I could go a good 850 straight along the lot line. I was thinking of some sort of long wire.  I would be pointing due west.  It might be interesting to try something like that starting off with the wire from the loop and adding to it.

Thanks a lot for all the help.  I want to be out in the field probably this Saturday morning  January 7, 2012, between 10:00 a.m. and noon EST. I will be working around 7.030 and 14.060 and 10.106.  Look for me calling CQ and we will see what your advice has wrought.

Joe N9TI
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N8TI
Member

Posts: 115




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« Reply #19 on: January 07, 2012, 11:00:52 AM »

Thought I would let you guys know how my portable operation turned out.

I got to the park and used a 66 foot long wire put up into a tree and back down for an end fed antenna. The end of the wire went into my pl 259 connector on my LDG auto. I connected it via a banana clip into the tuner. I then used a 16.5 foot counterpoise connected by an alligator clip to the ground of the tuner.

It worked perfectly on 20 meters and 40 meters. It loaded up with about a 1.3:1 on 30 meters.

I mostly worked 20 meters with it. A full wave antenna there. So, would I have better going with just a half wave, or would the full wave, 66 feet on 20 meters, work just as well or better?  Would the ohmes at the end of a full way be the same as a half wave?  It should, right?  Had the counterpoise at 1/4 wave on 20 meters.

Anyway, it worked really well. Heard QRP stations throughout the US and talked back to them.

Joe
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W5DXP
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Posts: 3540


WWW

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« Reply #20 on: January 07, 2012, 11:09:16 AM »

So, would I have better going with just a half wave, or would the full wave, 66 feet on 20 meters, work just as well or better?

It depends upon whether you prefer the 1/2WL radiation pattern or the 1WL radiation pattern (higher gain, smaller beamwidth). Do you know which you prefer?
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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.
N8TI
Member

Posts: 115




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« Reply #21 on: January 07, 2012, 12:37:13 PM »

Well, I guess it would depend on the way my antenna was orientated. Would the gain be on the sides of the inverted v or toward where the end of the antenna faced?

Joe
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W5DXP
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Posts: 3540


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« Reply #22 on: January 07, 2012, 07:12:57 PM »

Would the gain be on the sides of the inverted v or toward where the end of the antenna faced?

An inv-V decreases the difference but for a dipole, any length between a small percentage of a wavelength and 1.25WL, the radiation is broadside (normal) to the wire. Above the EDZ 1.25WL, the pattern becomes cloverleaf, i.e. multi-lobed.
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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.
K1DA
Member

Posts: 473




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« Reply #23 on: January 11, 2012, 06:20:03 AM »

I spent a long time getting an end fed wire antenna to work properly  and  I agree with AA4PB.  With a counterpoise,  ATU at the antenna, and a current choke right at the ATU input SO 239 I reduced the RF in my shack to a very low level and at the same time, signal reports got better.  I guess we know where all that RF which used to bang my shack field strength meter to the pin is now going. 
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