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Author Topic: Random wire antennas and counterpoises  (Read 6111 times)
K8AXW
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Posts: 6310




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« on: May 24, 2017, 09:51:46 AM »

Something I've never done but is in my bucket list, hopefully to be done before I punch out.

I want to go to a park, set my QRP gear up on a table, run a single wire to a tree limb and lay out a counterpoise.  Then see what I can work.

My questions are, do I make the antenna itself a specific length according to band/frequency or can I just string out what I can and used my tuner to match it to my radio?

How long do I make the counterpoise with the correct above information at hand?

This subject has been asked and answered on this forum before but I can't find it now that I need to learn it.
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WD4ELG
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Posts: 164




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« Reply #1 on: May 24, 2017, 10:59:17 AM »

Without getting into a religious debate on counterpoise or not, I shall simply provide the following articles for your review:

http://www.w8ji.com/end-fed_1_2_wave_matching_system_end%20feed.htm

http://hamuniverse.com/randomwireantennalengths.html

http://www.aa5tb.com/efha.html

http://udel.edu/~mm/ham/randomWire/

http://vk6ysf.com/longwire_antenna.htm

http://www.eham.net/ehamforum/smf/index.php/topic,79968.0.html
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WB6BYU
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Posts: 17049




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« Reply #2 on: May 24, 2017, 02:47:23 PM »

Quote from: K8AXW

My questions are, do I make the antenna itself a specific length according to band/frequency or can I just string out what I can and used my tuner to match it to my radio?



Well, if you want to string up a random wire, then it would be a random length, right?  If it
were a specific length, then it wouldn't be a random wire.


And sometimes you string up whatever piece of wire you can find, or whatever fits between two
trees, and use your tuner to make it work.  Usually however it makes sense to pay a bit of
attention to choosing a good wire length based on the bands you want to cover, the desired
radiation pattern, available supports, the capabilities of your tuner, etc.

There are some practical limits:  when the wire is too short (like less than 0.1 wavelength) the
efficiency will often suffer, though you still may make some contacts.  On the other hand, if
the wire is too long it may not be as simple to string it up.  For HF work I would recommend
not more than a half wave on 80m.  If the wire is going to be vertical, then radiation efficiency
drops when it is more than 3/4 wavelengths or so, though it might only be down 1 or 2 dB at
a length of 1.25 wavelengths.

How much of a counterpoise / ground system you need (for good efficiency) depends on the
feedpoint impedance of the antenna:  with a high impedance antenna, a bit of ground loss
resistance doesn't make much difference in overall efficiency.  20 ohms of ground loss in series
with 1000 ohms of antenna resistance is insignificant.  But for low impedance antennas, it
becomes much more important:  20 ohms of ground loss in series with a 12 ohm short vertical
is less than 40% efficiency.

That's why end-fed half-wave (or multiples thereof) are popular, because they aren't as
dependent on a good ground system for efficiency.  But they are also hard on many auto-tuners
due to the high impedance, which means that voltages will be higher for the same power level.

Personally, I like end-fed half-wave wires, and I haven't had any problems matching with a manual
tuner.  But that's not the only possible solution.

If you use a quarter wave (or 3/4 wave) wire, you may be able to plug it straight into the antenna
jack on the radio (a banana plug works well with a SO-239 jack) and use it without a tuner, though
a good ground system will contribute to higher efficiency.

Intermediate lengths typically are less dependent on the ground system for efficiency, while
not presenting as radical of a load impedance to the tuner.  The popular "recommended lengths"
tables you find on the internet are based on a premise that you can't match an end-fed half-wave
wire, which is clearly false in my experience, so I'd ignore them and base my choice on other
criteria, such as distance between supports, or how much wire I happened to have left on a spool.
There's no "one right answer" - do whatever you want.


Then there's the question of radiation pattern:  wires up to about 5/8 wavelength will have
maximum radiation broadside to the wire. At 1 wavelength the pattern has broken up into lobes
that are roughly 45 degrees to the direction of the wire, off both ends (but a bit stronger off
the far end than off the fed end.)   As the wire gets longer, there are more lobes and nulls in
the pattern, and the strongest lobes are closer to the direction of the wire.  If the wire is angled
to run up to a tree branch, that may lower the angle of radiation, but if the wire is too long and
angled too steeply you can end up dumping much of your power into the dirt.  Generally it is best
to keep the wire as horizontal as possible when the wire length is over 1 wavelength or so.


That still gives you a lot of options.  Say you want to cover 40 through 15m:  a quarter wave
40m wire would be a reasonable minimum length (about 30') but longer would be better.  That's
3/4 on 15m, and you probably don't want to go much more than twice that unless you can make
the wire mostly flat.  (For example, run it straight up and over a tree branch, then across to
another tree.)  So something like 40 to 70' might be a good range, depending on the relative
priorities of good performance on the highest and lowest bands.  That range includes a half wave
on 40m (around 65' or so), which is a popular choice, but 43' or 50' or other values would also
work.



Quote

How long do I make the counterpoise with the correct above information at hand?



A common answer is "one quarter wavelength".

That's a good answer when the wire is elevated off the ground, especially if your antenna is rather
longer than a quarter wave.  But if your wire is laying on the ground, it will primarily serve as a
capacitor to Earth rather than a resonant length of wire.  (The dielectric constant of the dirt will
shorten the length required for resonance, and there is some evidence that 1/8 wavelength may
be a better estimate for resonance for radials on the ground, but it depends on a lot of factors.)

With a low impedance antenna, such as a quarter wave wire, ideally you would have several radials
to lower the ground loss resistance, and the number is more important than the exact length as
far as efficiency is concerned.  With a high impedance antenna (such as an end-fed half-wave wire)
many stations manage without a counterpoise / ground system, or with a minimal length.  Anywhere
from "none at all" to "a few feet" to "a quarter wavelength" will probably work.


And don't get too carried away about your exact wire lengths.  I remember an article in QST some
years ago where someone was discovering how well a half wave wire worked when fed against a quarter
wave counterpoise.  So first he built one for 40m and calculated a counterpoise length of 32' 8".  Then
he tried 20m and cut a radiator wire of 32' 4".   Why didn't he just use the 40m counterpoise wire for
the 20m radiator???
  That level of precision is totally out of place in the real world of stringing wires
over tree branches and making it work with a tuner.

So see what lengths you have in the junkbox, or what's left on a spool, or what you can salvage by
joining all the pieces in a scrap of 26-pair telephone wire, or whatever you plan to use.  And then see
what is left for the counterpoise.  If you can get something longer than about 3/8 wavelength on the
lowest band you plan to use, you should have good enough efficiency and make plenty of contacts.

You can even run an experiment:  take several different wire lengths and see if it makes any difference
which one you connect to the ground side of the tuner.

Don't overthink it.  Just go string up a wire and have fun.
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W5AGK
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Posts: 188




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« Reply #3 on: May 24, 2017, 07:32:52 PM »

Here's a non-technical answer.  I go to the river bottom and string up a 57 ft wire about 12 ft high in the pecan trees.  I lay a 30 ft wire on the ground for a counterpoise.  Connect it with a 9:1 Unun and about 16 ft of RG174 coax.

Here are my SWR's according to my MFJ 259B:
6M    1.5:1
10M  1.1:1 to 1.5:1
12M  2.4:1
15M  2.4:1 to 2.5:1
20M  2.4:1 across the band
40M  4.4:1 to 4.6:1
80M  3.3:1 to 3.7:1

My 9:1 Unun consists of 10 turns of #26 wire bifilar on 3 FT82-43 cores.

I have tried antenna lengths from 30 ft to 64 ft.  57 ft works best.

I have tried counterpoise wire of 35, 40, 45 and 50 ft and 30 ft works best.

Just my 2 cents worth.

George
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K8AXW
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« Reply #4 on: May 24, 2017, 09:30:13 PM »

Thank you both for your answers.  I should have been more specific with my question.

I have a DSW-20-II (20m CW 3.5w) that is quite small and a new ZM-2-ATU that I have never used that I want to take outdoors a couple times as per my original post.

What is really creating a problem for me is that I have 2 or 3 antennas which I have no idea how long they are although I would guess they close to 100ft each.  They are very flexible stranded wire antennas wound on varnished wood and have brown cloth insulation on them, which make them pretty invisible. They also have a small white insulator on one end which is connected to about 50ft of cotton cord.  Overall size when wound on the wood is about half again as large as an Altoids can.

Each came from WWII spy transmitter/receiver/PS/ATU unit in a single suitcase.  I simply can't bring myself to start cutting any of them up to "experiment."  That's why I asked if someone has a particular length to at least start with. 

This goes for the counterpoise as well.

Perhaps the best thing to do is simply string one out and have at it. (Each one came with spare tubes, key and crystals but no counterpoise.

I can bring myself to cut on to a specific size and keep it with the rig and ATU but the last thing I want is a pile of wire pieces with nothing to show for it.
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KL7CW
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Posts: 256




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« Reply #5 on: May 24, 2017, 09:50:15 PM »

I will give you an example of a simple "park bench" antenna which I have used for most of my portable QRP operations for 13 years.  It consists of a 26 or 27 ft piece of wire thrown over a branch.  I have set it up as a vertical, sloped wire, or even up more or less vertical, then the top portion stretched out more or less horizontally, or even sloped downward if necessary. For a counterpoise I usually just lay one or two 17 ft pieces of wire on the ground, or even just use a clip lead to clip onto a metal picnic table, guard rail, chain link fence, car body, or whatever is around.  Usually the limited range built in antenna tuner on my KX1 tunes this just fine on 40, 30, and 20 meters, and I have made 100's of QSO's usually running only about 2 watts. Many good QSO's (even rag chews) of 1000 miles or more are common, with occasional DX possible.  This antenna is great since it is simple to set up.  Is it the best antenna performance wise??....NO.  If I am going to operate in a contest or for prolonged operation, a linked dipole set up as an inverted V or possibly an EFHW with a PROPER matching device will probably be significantly better.  However when I travel around the lower 48 states, the UK, or Europe, this 26 ft wire with one or two counterpoise wires is usually the only antenna I carry.  If I am interested mostly in 40 meters, sometimes I have used a longer wire for a radiator....sometimes 34 feet, or some of us consider something like 41 feet even better.  Once again no claim this is a great antenna, but it is good enough to yield many nice QSO's, and it should work much better than most small expensive commercial antennas.
             Rick  KL7CW  Palmer, Alaska     QRP portable for about 63 years now.  (CW)  
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K8AXW
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Posts: 6310




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« Reply #6 on: May 24, 2017, 10:09:40 PM »

I SAID "BOTH" WHEN I MEANT ALL 

ELG:  I am in the process of reading the links provided.  TY.
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K8AXW
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« Reply #7 on: May 24, 2017, 10:12:37 PM »

Rick:  Straight to the point!  Just what I'm looking for.  Many thanks.
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WB6BYU
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« Reply #8 on: May 24, 2017, 10:15:16 PM »

Then just use one for the antenna and another as the counterpoise and see if
your tuner matches it.  Or just unroll the counterpoise wire to a convenient
length.

No need to cut the wires.
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K8AXW
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Posts: 6310




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« Reply #9 on: May 25, 2017, 09:55:49 AM »

BYU:  That brings up a question that has bothered me for some time.  (I have more questions/problems than I have time to address)

If I unwind, say 50ft  of a hundred foot wire and leave the remaining 50ft wound up on the piece of wood. what kind, if any, problem(s) can I expect?
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WB6BYU
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« Reply #10 on: May 25, 2017, 12:33:21 PM »

Quote from: K8AXW

If I unwind, say 50ft  of a hundred foot wire and leave the remaining 50ft wound up on the piece of wood. what kind, if any, problem(s) can I expect?



I wouldn't expect any problems.  Try it and see.

What might happen if you have a problem is that there is some arcing between turns in the wound wire.
That (likely) will cause the SWR to jump.  So, when you are using it, keep an eye on the SWR meter for a
sudden change in reflected power.

There have been several antennas over the years that relied on rolling up the unused wire at the end of a
dipole, or a similar method of adjusting the length.  The "Yo-Yo Antenna" used a couple of "camping clothesline"
wind-up spools to hold the wire, while one of the military antennas used bare stranded wire that was wound
on a board (assuming that the turns would be shorted together.)  In either case there will be some amount of
capacity loading due to the "lump" of wire at the end, so a resonant antenna will be slightly shorter than the
standard formula length.

While it may be harder to predict the resonant length of such an antenna using insulated wire, it would be
easy enough to just adjust the length until it is resonant, or to use it as is with your tuner and not worry
about the exact resonant length.



Meanwhile, rather than throwing a wire over a tree branch, I've found it easier to throw a rope
over the branch and use it to pull up the wire.  That makes it much less likely that your wire will get
tangled and/or stuck in the tree.  A spool of Mason's Twine from the local hardware store works quite well.
I generally use a weighted tennis ball as a throwing weight because it seems more benign to other users
of the park than some of other common methods.  In fact I have several "throwing buckets", each with a
ball and 100' or more of good throwing rope in a bucket or a plastic dishpan.  The line is tied to the handle
then gently flaked into the bucket starting at that end:  this way it almost always pays out cleanly without
tangling on itself or nearby branches / grass / etc.  I let the ball swing underhand then fling it up over the
branch, which is much easier than trying to throw the ball overhand up at a high angle.  It takes a bit of
practice, but most folks get the hang of it in about 5 tries.
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KL7CW
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« Reply #11 on: May 25, 2017, 04:33:28 PM »

BYU's ideas are good ideas on how to get your wire up into a tree.  I used to use various types of twine, however I always felt bad if a piece of twine or rope got stuck in a tree.  It looked awful and probably will still be up there 100 years from now.  When backpacking or traveling I often just use a piece of (cotton ?) kite string and often just tie it to a half full water bottle.  It throws OK and if it gets tangled I can just break it.  I usually use the string to pull up my antenna wire.  For car camping I often carry a 31 ft telescoping pole which I can bungie to something, or I often use it just lay an antenna wire or twine over a convenient branch near the picnic table.  Probably the easiest way to get a line up moderately high without too much risk of a tangle is to buy a arborist kit.  There is a beanbag looking weight and some really slippery line which seems to drop easily through the branches.  In theory you should be able to go quite high with practice, and I have found that something like 25 or 30 feet is rather easy.  There are many other methods for getting wires up high in the trees, however if you can get your 26 foot wire over say a 15 or 20 foot branch this will be plenty high to give you many happy QSO's.  I used to spend an hour or more on set up, but more recently have enjoyed a 5 minute set up time, and enjoyed more operating time.
       KL7CW
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K8AXW
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« Reply #12 on: May 25, 2017, 07:43:21 PM »

BYU and CW:  Thank you both for the great info.  I've bee a ham for 62 years and although I've done everything from HF CW (QRP & QRO)to working the birds when they were first launched (Remember AO-10?) but have never operated portable HF!

At my age, packing up a lot of stuff to go out and play radio seems to be a lot of trouble but as I said, "it's in my to-do bucket."

Thank you all for helping an old timer with the basics.



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KJ4DHI
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« Reply #13 on: June 26, 2017, 04:51:11 PM »

I have been exclusively cw/qrp on 20. I want to be on 40 also. My antenna is a 33ft longwire. I am told that even with a tuner it will not work on 40. Yes, no, maybe so?
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WB6BYU
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« Reply #14 on: June 26, 2017, 05:05:09 PM »

Why wouldn't it work on 40m?  It is about 1/4 wavelength, so will have a low feedpoint impedance.  An antenna
tuner designed for half wave wires probably won't match it, but try putting a banana plug on the end and sticking
it into the SO-239 on your rig (along with some sort of counterpoise.)

My first 75m SSB antenna was a quarter wave wire stuck in the center pin of the output jack of my SWR meter.
It does mean that the counterpoise characteristics are more critical to good efficiency than they are for a
half wave wire (I used a water faucet ground, which certainly wasn't ideal) but I still made contacts.
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