Portable RV Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G3YRO, Jun 10, 2021.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    I've just bought a cheap old small Motorhome (what you Americans call an RV)

    Most of the time we'll be on campsites with no room to string out wire antennas, so I've designed a portable vertical antenna to use on 160, 80 & 40m using my telescopic 10m fibreglass mast.

    The fibreglass pole will slot into the top of a speaker tripod stand.

    So there will be 23'6" of wire hanging down from the top of the mast, going to a 2" diameter loading coil.

    The whole coil - 120uH for 160m . . . just 24uH of it for 80m, and no coil for 40m.

    Then about 9'6" of wire from the bottom of the coil down to the ground, the feedpoint.

    I won't be able to run any proper radials out, so will just attach four 20ft radials to the outer of the coax, which I will spread out on the ground as best as possible (and may be on tarmac/concrete, so no earth spike)

    I realise this will be very much a compromise . . . but I wonder if any of you experts on Eznec would kindly model it for me, to see what it shows ?

    (For the rig, I've decided I'll take my spare FT101E, as it has a Pi-Tank, so it should tune this OK, and runs off 12V DC or 240V AC when I'm hooked up to the Mains on a campsite.)

    Incidentally . . . I guess the actual Motorhome will act as part of the "Ground" . . . it's 17ft long, 7ft wide, and 8ft tall, and the base of the antenna will typically be a couple of feet from the front corner of it.

    Roger G3YRO
     
    Last edited: Jun 10, 2021
    WB5THT, KC3RN and AK5B like this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Sounds like a good plan, Roger; all antennas are a compromise in one way or another, anyway. Try to camp/park by the sea if you can; a free 6db of low-angle enhancement awaits you there.

    I expect @WA7ARK to be along shortly to run all the numbers for you once he sees your thread...

    73,

    Jeff
     
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Actually, if I'm on a Campsite where I can plug the vehicle into the mains, there will effectively be a lot more radial wires - all the Earth wires running from parking space to parking space !

    So I guess that will improve the efficiency of the Vertical a bit (albeit with a distorted radiation pattern)

    Roger G3YRO
     
    K0UO likes this.
  4. AK5B

    AK5B Ham Member QRZ Page

    Actually, a partial radial field does not alter the radiation pattern significantly in any one direction or the other. As others have often pointed out, if it did we would all be using verticals with one radial pointed in the desired direction instead of beams.

    https://rudys.typepad.com/files/qex-ground-systems-part-7.pdf
     
  5. K8JD

    K8JD Ham Member QRZ Page

    Beware when camping with radios. Many camper units have internal lighting Etc that run from a power supply that often has a switcher regulator that puts out a terrible RFI from long wave up to 40 M !!! I have had local AM stations totally blocked by the camper power unit up to 50 ft from the device. Unpugging my camper from the AC power post kills the QRM !
     
    K3XR likes this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Even if you unplug your own RV's "inverter", the RV parked 10ft to the right and the other one parked 10ft to the left will still have theirs running. If it isn't the "inverter", it will be the electronic ballast for the LED lighting. If it isn't that, it will be the HV igniter for the gas furnace, gas refrigerator, or gas hot water heater. If it isn't that, it is the built-in stupid 5V USB charger for your mobile phone. Or the control panel for the gas refrigerator. Ask me how I know all this....

    I ham from my RV, and use it for FieldDay, but it is parked in the forest with no other RV units or AC commercial power within miles. Even so, I had to modify some of its "features" so that I can ham from it without listening to birdies...
     
    Last edited: Jun 10, 2021
    AG6QR, K3XR, K0UO and 2 others like this.
  7. G3YRO

    G3YRO Ham Member QRZ Page

    Yes I get all that about interference from any noisy Inverters or Switch Mode power supplies !

    But the biggest source of HF Band noise here in Britain is from Broadband VDSL delivered via overhead copper phone wires, which radiate like hell !

    So at least on a Campsite I should get away from all of that.

    Would be great if someone has the time to Model my proposed Antenna.

    Roger G3YRO
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    To model your situation, I would need to know a lot more about your "ground system". How big is the RV.? Does it have a metal chassis? Is any part of the RV cabin made from either a metal skeleton or is it covered with aluminum? How high is the frame above the earth that it is parked on (ie ground clearance)? There is likely more mass (length/width) in the RV frame than there will be in the short wires you can deploy as "radials"

    As I mentioned, I do this from my 22ft Lance RV trailer. Mine is fiberglass/wood (nothing conductive except an aluminum ladder) on a rigid metal frame (chassis) that is 23ft long and 7ft wide, elevated about 12inches above the ground (on rubber tires), so the RV chassis is the largest part of my "ground plane". For 40m, I use a 36ft fiberglass pole to hold up a ~35ft vertical wire fed against the RV frame. I can make that resonate without loading coils, and it works well. I can make contacts all over the Western USA and Western Canada.

    On 40m, signals get about half an S-unit better if I add a 30ft radial wire that runs along the dirt, diametrically opposite to the RV frame.

    This says not much about how all that might work on 80m or 160m, where I fear that my existing "ground plane" would become inadequate... I have worked lots of DX from the RV on 20 and 17m, by using an automatic tuner at the base of the vertical. I have to shorten the 35ft vertical wire on 20m to avoid the "40m monopole anti-resonant on 20m" conundrum.
     
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  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    Just to show what I see with my RV antenna model:
    upload_2021-6-10_17-13-46.png
    Wires 31, 32, and 33 are the optional radials. None present for "frame only". Only wire 32 present if "single radial". All three present for "3 radials". Radials are each 30ft long. Wire 1 is tuned to resonance at 35.31ft

    40metre patterns:

    upload_2021-6-10_17-6-29.png

    upload_2021-6-10_17-9-48.png
    Adding 1 radial (wire 32) goes a long way to making the pattern more symmetrical.

    Average gain for this antenna with one radial is -6.7dB (-0.7dBi). About the best you can do on any vertical on lots of radials is about -5dB (~+1.5dBi), so much better than a mobile whip on a car.
     
    Last edited: Jun 11, 2021
    AK5B likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Using the same vertical monopole (34.6ft tall) and frame (7x25ft @12in agl), no added radials. Feedpoint is between the frame and the monopole. Here is a frequency sweep from 1.8 to 7.3 MHz, skipping non-ham frequencies. There is a lot of information in this tabulation. Feel free to ask what any column means.
    upload_2021-6-11_6-54-4.png

    Here are two networks that would match this antenna at 1.8MHz:
    upload_2021-6-11_6-57-57.png
    see: https://www.analog.com/en/design-ce...-tools/rf-impedance-matching-calculator.html#
    Note that L1 or L2s are effectively the required loading inductance if the loading coil is placed right above the feedpoint. If the loading coil is moved vertically up the monopole (further from the feedpoint), the required inductance increases, but the shunt part (C1 or L2, which are wired across the end of the feeding coax) remain the same.
     
    Last edited: Jun 11, 2021

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