Counterpoise and RF Grounding a mobile antenna

Nashua Area Radio Society Topics In All Forums Elmering Forum Counterpoise and RF Grounding a mobile antenna

This topic contains 8 replies, has 4 voices, and was last updated by  Tom Mahon 3 months, 1 week ago.

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  • #29943

    Jay Cohen
    Participant

    Hello all!

    I’m a very new member to the club, and just was licensed in December so I’m a newbie all around.  As part of my first station, I purchased a Yeasu 857D in addition to a Yeasu ATAS120A mobile antenna mounted on a tripod, connected to a duplexer to allow both HF and UHF/VHF connections to the multi-band antenna at the same time.  The idea is to have a somewhat portable station I can take with me to Boy Scout camping weekends. I also have 75 feet of coax to connect things together.

    I’m having a bear of a time though getting the system to tune on anything HF.  I’ve contacted the retailer for advice, as well as Yeasu, and both are suggesting I need a counterpoise. Here’s where the confusion sets in.

    I have read that the correct length for a counterpoise would be 25% of the wavelength of the lowest frequency to be used, in effect to ‘mirror’ the 1/4 wave antenna on the tripod to create the other half of the antenna.  If I do that math ( 300 / 7.125 (for 40M) = 135.8 and divided by 4 again I get 33 or so feet.)  So, in my mind, this means four 33 foot counterpoises attached to antenna ground at the base of the tripod mount.

    However, when I talked to Yeasu, their engineer suggested 4 16′ counterpoises.  Which translates to 1/8 a wavelength or so.  Unless I’m misunderstanding something along the line here.

    Can anyone shed some light on the numbers for me?

    Many thanks!

    Jay

    KU1DU

    Jay, KU1DU

    #29945
    Fred Kemmerer
    Fred Kemmerer
    Keymaster

    Hi Jay,

    FB on the portable HF setup you are building for demos with the Scouts!

    Your initial thought was probably the right one – the counterpoise wires should be 1/4 wavelength on the lowest frequency that you will be using. You should try for 3 – 4 wires if you can handle the space requirement, 2 minimum. It will be best to keep them slightly elevated if you can find a way to do that to limit ground losses but they will also work if placed on the ground.

    It is probably best to try to setup the entire system in see what it wants in terms of radials. The radiator on your antenna is pretty short for 40m and it looks like it uses a coil/base loading to overcome this. There are scenarios where shorter radials and a greater use of the base loading effect might give a better match. Only way to know if this is the case is to try it.

    I hope this helps. Happy to talk with you live some more about all of this. Looks like we may be getting together for lunch sometime soon!

    - Fred (AB1OC)
    President, Nashua Area Radio Society

    ab1oc@arrl.net
    Visit us on the web at - https://www.n1fd.org

    #29947

    Jay Cohen
    Participant

    Thanks Fred!  I’ll be replying to your email soon.

    From the research I’ve been doing, length DOES matter here until I hit around 32-40 counterpoise wires, when longer than 1/4 wavelength seems to work better.   Originally I had tried 8 counterpoises at 18 feet, and while I was able to hear even Barcelona Spain, when I dropped to 10M for the club Net, I had tuning issues again.  I’m wondering if 2 tuned wires per band might be the right answer here – which I hope to figure out later.

    Is there anyone else using this antenna out there – NOT mounted on a vehicle?

    Thanks!

    Jay

    KU1DU

    Jay, KU1DU

    #29949

    Patrick Morrison
    Participant

    Hi Jay,

    First the “Disclaimer”!!!! I am no pro at this, Like you im new to the Ham Nation.

    THat said, i run a similar setup with an FT-817 connected to an MFJ-1622 apartment antenna through an LDJ tuner. This too has a coil to offset the short radial, and uses a counterpoise. Your calculations are right at 33’for 40 meters. Im using a single wire streched out in my apartment on the floor. I average about a 1.5/1 SWR with this configuration. I also have heard that more radials will help but have not experimented yet. So far my punny 5 watts has gotten my out to Michcan, Erie Pa. and the Carolinas. My problem is getting stepped on by the 100 watters????.

    Ill be curious to hear what you come up with as I will be doing SOTA expeditions this summer and am thinking of more radials.

    Patrick KC1HDN

    Patrick, KC1HDN

    #29957
    Fred Kemmerer
    Fred Kemmerer
    Keymaster

    Hi Jay,

    The basic choice on the radial situation is one of elevated radials vs. radials on the ground. If you choose elevated radials, then cutting 2 minimum for each band would be the way to go. This can obviously get out of hand if more than 2-3 bands are involved.

    With radials on the ground, you will typically want a certain number that are all equal length. In this scenario, you are trying to make the ground behave as a conducting surface. The best overall length depends up how many radials you can lay down. See –

    http://www.w0btu.com/Optimum_number_of_ground_radials_vs_radial_length.html

    If you only want to do one band, I’d suggest 3-4 elevated radials cut to the right length(1/4 WL)  for the band you are on.

    If you want a multiband setup, I’d go with perhaps 4-8 radials on the ground cut to 1/4 wavelength or less for the lowest band that you want to operate on. Note that the table in the link above suggests shorter ground radials for low numbers. My experience is that this is good advice if you are in the range of 8-32 radials. Setups with small numbers of ground radials might need longer ones to get the antenna to tune up properly (but never more than 1/4 WL unless you have more than 90).

    Given your goal to setup portable on multiple bands, I think I’d recommend the second scenario (4-8 ground radials) as the practical one for you. If you also plan to setup your antenna at your Home QTH, you could either do either elevated radials or radials on the ground with say 16-32 buried radials centered at the location where you plan to setup your antenna for operation from your home qth.

    There are obviously lots of choices between these two. Also, performance for any such installation will depend upon the conductivity of the ground at the actual site where the antenna is installed.

    Note that in Patrick’s situation, his radial system is essentially an elevated one and radials of 1/4 WL are needed.

    - Fred (AB1OC)
    President, Nashua Area Radio Society

    ab1oc@arrl.net
    Visit us on the web at - https://www.n1fd.org

    #29963

    Jay Cohen
    Participant

    Fred – I actually came across that article in my research!  The ‘relative’ question is how elevated is elevated?  Meaning, if I hang three 1/4 wave counterpoises  angled down from the antenna who’s as high up (6 or 7 feet for the bottom antenna connection) as I can get it on the stand I have and terminate them about a foot from the ground, which makes the downward angle around 30 degrees of slope gives me between 40 and 50 ohms of impedance which ‘should’ give me a decent swr.

    But, and there’s always a but, if the impedance numbers are only true if the antenna is at least 1/4 wave above the earth, I still might be better off with the 4-8 ground radials at 33.5′.

    The other school of thought though is do I extend the ‘whip’ on top of this 1/4 wave antenna by about 33.5 feet and call it a day…  A 1/2 wave antenna technically shouldn’t need radials, and with the automated screwdriver adjustment of the atas120a, that should be easy to tune at 40M, 20M and potentially 10M as well.  I do happen to have a pretty tall oak tree on that side of the house…  Hmmmm.  2M and 70cm work perfectly already, so no big deal there.

    Does the club ‘rent’ SWR meters for those, who like me, are still in the ‘tinkering’ process?

    Thanks!

    J

    KU1DU

    Jay, KU1DU

    #29966

    Tom Mahon
    Participant

    Jay,
    I have an MFJ-259 you are welcome to borrow. I can vouch for the advice of the other replies. I have an MFJ screwdriver antenna that I have used portable with 8 radials off a DX Engineering radial plate. I cut mine for a little over 22″ and was hitting central Europe with my Kenwood TS-140s and 100 watts when I used it on the Cape. Sea water was a great help, I’m sure. I also have 20 radials installed in my yard for an LDG s9 31 foot collapsible antenna. 4 of the radials are the suggested 28’10” the others are around 22’8″. Used another DX engineering radial plate. I used the calculations for the shorter ones from one of the articles attached to an earlier reply. Had good results with the set up. As has been stated, it’s a matter of try this, then that until you get either frustrated or good results. For me this is the “fun” (i.e, satisfaction) in amateur radio.

    73
    Tom
    AB1NS

    #29992

    Jay Cohen
    Participant

    Howdy~

    First, Fred’s talk last night was quite good on HF portable and mobile rigs.  There’s a lot of similarities to what I’m trying to do, and I found it very valuable. Thanks!

    Tom, I’d love to take you up on the offer to borrow the meter.

    I did run across one more article that I’ve re-read a few times now, and I think I’m convinced that 3 distinct sets of elevated radials might be in store for me.  Elevation at a few feet off the ground can be accomplished with 4 wooden sticks and a tie-off of the radial.  At 160M and 80M, they’ve found in the article (link provided at the bottom) that a length of LESS than 60 degrees of the WL or more than 120 degrees capped at 135 degrees  is optimal, and four of them actually performed better than more ground based ones.  My hypothesis is that this can be accomplished as well on the 10M, 20M and 40M bands using less wire, and within 14% efficiency of the many radials/counterpoise approach.

    With this approach, on 10M and even 20M I should be able to get pretty close to the ’45 degree angles’ as the 1/8th radial lengths should be about 4 and 8 feet.  With an antenna about 7 feet off the ground on a tripod, that would be pretty easy.

    Add that to a Yeasu 100W transmitter, and I think I’m in business.

    Thanks!

    Jay
    KU1DU

     

    https://www.nonstopsystems.com/radio/pdf-ant/article-ant-5IU-97.pdf

     

    Jay, KU1DU

    #29994

    Tom Mahon
    Participant

    contact me at [email protected] for analyzer.

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