more about QRP / Backpack portable Random Wires

Discussion in 'QRP Corner' started by KK4NSF, Dec 14, 2015.

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  1. KK4NSF

    KK4NSF Ham Member QRZ Page

    OK.... after our earlier discussion about QRP Field Antennas, I spent some time studying Random Wires, Tuners, and Counterpoises. There is a lot of good material online on QRZ and other sites. Unfortunately there is also a large amount of contradictary information out there. To help cut through the contraditions, I did some tests this afternoon.

    Before I list the results, I want to define what I tested: I tested non- quarter or half wave, end fed antennas. These were NOT actually random length wires, but were wires cut to the lengths recommended by AB3AP (Mike) in his web-site: He wrote a computer program to calculate the best lengths for multi-band end-fed wires, with a tuner and a counterpoise.

    Also... we need to define the word "counterpoise" since this word has been defined in several diffent ways by Amateur Radio and engineers through the years. For our purposes, the word "counterpoise" is meant to mean a single insulated wire, attached to the ground terminal of the tuner, and then laid directly on the ground.

    The bands I tested were 40m, 30m,20m,15m and 10m.

    If you look at the AB3AB chart, you see which antenna lengths work on which bands.... and I picked 4 lengths that look like they'd work on all of the bands of interest, plus one of the NOT recommended lengths just to see how it worked. These are 33', 40' , 60', 70', and 85'.

    see the chart here:

    Since I could not find any consistent data on counterpoise length, I just "winged it" and used 17' and 34.5' counterpoises.... since those lengths worked just fine on my min-Dxpedition a few weeks ago, and I happened to already have wires that long.

    My mast was a fiberglass pole 25' tall, the tuner was an MFJ 971 and radio was my Icom 703.

    The technique was simple: First I set the antenna up at 85' long and tuned it up on each band with each counterpoise. Then I cut the 85' wire to 70' and did it all over again.... then to 60'.. then 40'. and finally to 33'. It took ALL day, but wasn't really hard work. Now I'm eating very good Ham Sammich and typing up my results. These are as follows:

    Results w 17' Counterpoise (SWR)

    length---> 85 70 60 40 33
    40m 1 1.5 1.75 2 inf
    30m 1.3 1.5 1.2 1 2.5
    20m 1 1.5 1 1.75 1
    15m 2 1 1.5 1.2 5+
    10m 1.5 1 1.2 1.2 3

    Results w 34.5' Counterpoise (SWR)

    length---> 85 70 60 40 33
    40m 1 1 1.2 1.2 3
    30m 1.2 1.2 1.5 2 inf
    20m 1 1 1 1.2 ~5
    15m 1.5 2 2 2 inf
    10m 1.5 1.2 1 1 3

    (sorry about the chart not lining up very well. It looked a lot better on my screen)

    What these results tell me is that the lengths shown on the AB3AB web site do indeed produce usable results, even when used with relatively small counterpoises. Essentially, the lengths predicted to work well did tune up to workable SWRs, while the one predicted to be not so good did not. This experiment did not take things like ground conductivity into account, nor did it explore what sort of patterns might have been produced..... but it does give me a good idea of what lengths of "random wire" work.

    For my "backpack portable" QRP Station, I think I'm going to use a 70' wire, withthe 34.5 counterpoise, it tunes to less than 1.5 on all of the bands tested..... but Ill also bring other lengths too, just in case.

    So... there you have it. how i spent my afternoon. Maybe this will help you when trying to figure out how to configure your own random wire sytem.

    KX4O, AA9SD and W4SEX like this.
  2. W7CJD

    W7CJD Ham Member QRZ Page

    I like this.

    This shows just any "random" length of wire can get "inf" for SWR.

    I use a counterpoise 1/4 wave length or less of the operating frequency.

    This is counterpoise as RF ground.

    Other "definitions" seem to be about radials, elevated or ground, or, about "the other half of a dipole".

    I started with a MFJ-931 Counterpoise, and so, for me, a counterpoise is a RF ground.

    I have used this chart, for "random" wire lengths to avoid and for useful lengths recommended by Baluns Designs. Lengths for 9-1 ununs.pdf

    I added it, here, because I am so "dyslexic" I find that other chart difficult.

    image.jpeg image.jpeg
    N8AFT, K6FNI and W4SEX like this.
  3. AI6KX

    AI6KX Ham Member QRZ Page

    Nice work, Dave. That adds a lot of good data to the confusing info about counterpoises. I use a 40' wire on the higher HF bands, with a 10' length of coax to the T-network tuner. I tried a 17' counterpoise but noticed that when I disconnected it I was still able to get the same low SWR without retuning. (The MFJ documentation that W7CJD posted hints at this possibility. Furthermore, the received signals and antenna noise was louder without the external counterpoise, so there might be some interaction there. I'll try a comparison with a short coax line and the external CP versus the longer line without.

    Steve in Okinawa JS6TMW
    KB0TT likes this.
  4. K8JD

    K8JD Ham Member QRZ Page

    About my Portable antennas
    I do not waste my limited time in the field messing with counterpoises and tuners to make a random antenna work ! Been there-Done that. Forget it !!!
    I have come up with a relaiable antenna that requires no tuner or messing around.
    I have single band QRP radios for 80, 40, 30 and 17M. I usually only take the 40 and 30M ones.
    I built a coulple of two band fan dipoles that I can hoist into trees and tie off in 15 minutes !
    They were tuned and trimmed in the woods behind my house at the usual height that I can get to with a few weighted rope tosses.
    I roll them up and put them in my carry bag with the QRP radios, battery, logbook note book, and My J38. I Don't even bother to take the small SWR meter anymore, they are that reliable.
    After I finish my camping trip I untie and pull the wire down and roll it back up and I am ready to head home.
    AJ6KZ and AI6KX like this.
  5. AI6KX

    AI6KX Ham Member QRZ Page

    Yeah, my "A" portable antenna is a 3-band parallel dipole, but sometimes there aren't 2 trees! I'm a "belt and suspenders" ham.
  6. KK4NSF

    KK4NSF Ham Member QRZ Page

    yes, i found that too on some bands.... but not on others. By using the right wire and counterpoise though, I found that I can maintain a very low SWR across the entire spread of bands I use, which is very handy.
  7. W7CJD

    W7CJD Ham Member QRZ Page

    I am not familiar with the term "parallel dipole".

    How is it configured?

    Coax dipole?

    Balanced line doublet?

    Is your "parallel dipole" a "fan dipole"?
  8. AI6KX

    AI6KX Ham Member QRZ Page

    Yep. Almost the same thing. The wires are parallel, and do not fan out. I have used various spacing from as little as an inch to as much as 5 inches with various antennas and they all work. My portable antenna uses 1-inch spacing and is half-wavelengths on 20, 17, and 15, and is center fed with about 45 feet of RG-174. Like K8JD I tuned the wires when it was hanging it on my roof from 2 poles.

    I use only one hanging cord from each end, attached to all 3 wires. First time I put it up in the field I had some problems with the parallel wires twisting. I added a few fishing swivels, and that lets me untwist them easily before raising it.

    Last edited: Dec 14, 2015
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    Dave, Nice work. You have just proven my signature line. :)

    AI6KX likes this.
  10. KK4NSF

    KK4NSF Ham Member QRZ Page

    it's a great quote!

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