Random wire counterpoise problem

Discussion in 'General Technical Questions and Answers' started by WB4C, Feb 5, 2008.

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  1. WB4C

    WB4C Subscriber QRZ Page

    I recently deployed a temporary simple indoor random wire antenna for 40 meters and just monitoring stuff. I gave it a quick try without a counterpoise and made a good QSO with it. I ran a simple counterpoise on the floor and attached it to my tuner. Worked better.
    The addition of the counterpoise extinguished the radiation from another indoor antenna I use on 80 meters (magnetic loop). Even on RX it is apparent something has gone wrong. There is NO change in the loop's resonance or SWR.
    The loop is connected to the direct/bypass coax 2 port, the random wire is connected to the random wire port and the counterpoise is connected to the tuners ground lug. If I disconnect the counterpoise, the loop comes back alive.
    Everthing in the shack is connected to AC ground which in turn is connected to earth ground.
    I've done a bunch of experiments grounding the loop at its feedpoint; nothing good or bad happens; even tried some ground screen experiments with no big deal changes.
    Shouldn't an antenna that stops radiating because of a change made in a system have its SWR changed as well??
    I disconnected the random wire from the tuner and the effect still existed.
    Any comments or suggestions about all this?
    The counterpoise is not in close proximity to the loop.

    73, Jim
  2. K7UNZ

    K7UNZ Ham Member QRZ Page

    Counterpoise addition

    Hi Jim.....you didn't mention what tuner you're using, but you might want to check the tuner's manual.

    A lot of dual port tuners use "coax 2" in common with the single wire/balanced line connectors, and this could be the situation you have. Usually there are jumpers on the wire connection terminals that determine the use of the connection for either a coax or wire/balanced line input.

    Just a suggestion.....

    73, Jim/k7unz
  3. WB4C

    WB4C Subscriber QRZ Page

    Wheew...that would mess things up!
    I'm using a MFJ 949E. I also dusted off a much older version of the same tuner and tried it...same stuff happened. Kinda strange problem.
  4. KR2D

    KR2D Ham Member QRZ Page

    For those that don't know, the MFJ 949E has a separate circuit for single wire/balanced line, it doesn't share Coax 2. The Balanced and single wire circuits are shared, and you have to do something with a jumper to use one or the other (forget the details, but it's in the manual).

    Not sure why adding the counterpoise caused trouble with your loop. Remember that with a counterpoise attached to the ground screw, it is connect via the tuner's chasis to the shield on your coax cables. How long is the counterpoise and where is it resonant? That may provide a clue.
  5. AG3Y

    AG3Y Guest

    "The addition of the counterpoise extinguished the radiation from another indoor antenna I use on 80 meters (magnetic loop)."

    "Extinguished" That is a very strong word. You mean to say that putting a counterpoise on your antenna tuner that is nowhere close to the 80 meter antenna makes the 80 meter antenna NOT radiate ? IMPOSSIBLE ! You are saying that you can pump 100 watts ( an assumption, I realize ) into your 80 meter loop, and NOTHING comes out ? ? ?

    Please be more specific. Do you hear stations, but they are just several S-units down in strength ? Does tieing the counterpoise to a different point help or hurt the results ( try the back of your rig, is there a thumbscrew marked with a ground symbol, or words ) ?

    Are you sure you are hooking up the two antennas to the proper outputs of your tuner, and that any needed jumpers are in the proper places ? I know it has been mentioned in an earlier post, but it doesn't hurt to re-emphasize the point.

    The biggest unanswered question, how MUCH does the loop STOP RADIATING, and how did you determine that ! Do you have a field strength meter? Some of those old CB style meters have a little jack on the case for plugging a small whip in to, just for that purpose. Beg or borrow one from someone and do some sniffing around with it. If the fields of the counterpoise and the loop are interacting, you should be able to detect that with a FSM .

    Good luck! Jim
  6. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    That's a very interesting problem. If you disconnect the parasitic (ground) wire from the system but leave it right where it is does the loop perform normally?
  7. K8JD

    K8JD Ham Member QRZ Page

    I wonder if a random wire antenna needs a random length counterpoise?
    Is your counterpoise anywhere near a quarterwave length ? (34 FT) Anything less will geve bizarre results...
    Last edited: Feb 6, 2008
  8. WB4C

    WB4C Subscriber QRZ Page

    Sorry I could't keep up with the thread!!
    Well here goes.
    Extinguish was a very poor choice! When I wrote that I must have been thinking in terms of watts not Db!
    Yes, I've got a FS meter; my neighbor gave it to me("get to know your neighbors").
    I trust the relative readings I get when using the mag loop. I certainly don't when working on my vertical and looks like the horizontal is very quirky. Steve, WB2WIK/6 and some others pretty much explained it on a thread I started over a year ago about Field Strength meters. But the mag loop stuff always tracks.
    My random wire is about 47 feet long as the crow flys; about 10 feet of turns getting out of the radio room. I went for straight line length when I tacked it up. Didn't look at the charts to see if it was a poor choice! The counterpoise is 50 feet of coax run out of the radio room in the opposite direction from the random wire. The counterpoise is bent around in a semi square.

    The 80 meter mag loop dives down 7 Db when the counterpoise is connected to my tuner. This reading I trust.

    The 40 meter random wire dives up 2 Db when the counterpoise is connected. This reading can change up or down by changing the way the counterpoise runs. I don't trust the figures. It looks darn good on the meter Ha!

    When I remove the counterpoise from the tuner I just drop it on the floor.

    I've had a bunch of stuff connected to the tuner before and never saw the loop degrade. The last wire antenna I had was run all over the place and did not have a counterpoise.

    When I add the counterpoise my random wire SWR comes up and can be tuned back down. The SWR on the loop does not change and to the best ability to measure, the resonance does not change.

    Conditions on 80 and 40 seem to be TERRIBLE tonight. As soon as I can I'll see if I can find a reliable person on 80 CW who can clearly copy me and give me a before and after S meter reading. Tried it the other night and had to trick the other guy into it; he clearly did not copy a thing I sent with the counterpoise on.

    If anybody out there can think of things to move around or try, I'm open to suggestions. I won't move the loop...the thing is 18 feet long and is made out of 3/4 inch copper pipe.

    Just to clarify: thats 7 Db and 2 Db power.
    Last edited: Feb 6, 2008
  9. WB4C

    WB4C Subscriber QRZ Page

    I confirmed on 80 last night that the loop does take a 1 "S" unit dive and sounds weak.

    I added another 50 feet to the existing 50 foot counterpoise(used for the 40 meter random wire) and the effect on the mag loop went away. So somewhere between 50 and 100 feet the loop recovers; probably 67 feet is the spot.

    If I elevate the 100 foot counterpoise off the floor the negative effect on the loop comes back.

    Strange that all this happens on the rig side of the transmission line(50 feet) to the mag loop. I can hang anything at the feedpoint with no effect.

    I'm wondering if an experienced antenna guy would go out of his way to run a 1/4 wave counterpoise to a random end fed horizontal wire or run one as long as he could or maybe one as long as the horizontal wire??
    Would he run multiple counterpoise wires if space allowed??

    As K8JD pointed out, a short < 1/4 wave counterpoise might not be a good choice. Hmmm

    Last edited: Feb 7, 2008
  10. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    OK, what's happening is that one side of your loop is grounded at the tuner as it should be. When your counterpoise wire is connected it becomes part of the loop (on the ground side) and creates an imbalance which will throw the loop for a loop..:p If you look inside the tuner you will see that one side of the balanced port and the single wire port are connected together. The other balanced port is connected to ground. With both antennas connected they will directly impact one another.

    You will need to come up with a way to disconnect it from the tuner when operating with the loop antenna. A simple toggle switch can work here. as for the position of the coax affecting the random wire, that's no surprise. Moving the counterpoise can move the current node on the wire to some extent. If you had a large fixed radial system that wouldn't happen. I would arrange the coax for best performance of the end fed and leave it like that.

    Also, try disconnecting the loop ( both sides) when you use the random wire and see what effect it has on performance of the wire. As long as the loop is connected to the tuner it becomes part of the system as does the random wire.

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