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Dual Antenna Masts

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4SRF, Jul 3, 2020.

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  1. K4SRF

    K4SRF Ham Member QRZ Page

    Is there a "set" distance, or "rule of thumb" for how many feet apart two antenna masts should be when installing them in close proximity?

    I have an existing 33' mast with a UHF/VHF vertical attached. I will be installing on a seperate 33' mast, a HF vertical. I would like these two relatively "next" to each other. I plan moving the UHF/VHF mast since there is a high tree limb that is growing into the antenna, and trimming this limb is near impossible unless I call in a tree service, which I really don't wish to do.

    Both of these masts will be anchored to the eve of the house, extended to the full 33' and guyed accordingly. Would a distance of ten feet apart seem adequate? I have never considered doing what I am about to do, and I have researched this topic online but have found little to no information about it.

    I plan on having a shared grounding system with a total of six 8' copper-clad ground rods and using 6 AWG bare copper wire.

    Are there any concerns I should be made aware of about this setup?
  2. AJ5J

    AJ5J Ham Member QRZ Page

    Mostly depends on how tall your VHF/UHF vertical is and taking into consideration the possible coupling or interaction that could come into play on the HF side. As far as the masts go, whatever works---but the only potential problem is the effect of coupling on the HF band(s) you are using. To be on the safe side I'd say at least 1/8th wave length apart but more will obviously be better.

    How tall are both verticals? No idea why you failed to mention that important bit of information as that will tell us if the scenario is inadvisable or not a problem (if the VHF/UHF are no more than a couple/three feet tall you're good to go a few feet apart).
  3. K4SRF

    K4SRF Ham Member QRZ Page

    I guess I should have mentioned this. The HF is a Comet CHA-250B with a length of 23' 5". The UHF/VHF is a Comet GP-3 with a length of 5' 11". The HF bands most likely be in use will be 10, 20 and 40, and sometimes others depending.
    AJ5J likes this.
  4. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Does the CHA 250 B require a ground plane? If so, what kind of ground plane are you planning?
  5. AJ5J

    AJ5J Ham Member QRZ Page

    Those CHA 250Bs are one of those "radial-less" extended-length dummy load types that rely on the coax shield for a sort of half-baked counterpoise and I don't think the GP-3 will affect it much, if any. I wouldn't envision a problem running the two antennas about ten feet apart---but as always---more distance is always going to be better, even though the difference will be more or less negligible.
    K4SRF and WB5YUZ like this.
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Thanks, Jeff. I read the manual, but wasn't any the wiser for it regarding how the antenna worked.
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Probably not a problem. I've had HF and V/U antennas on my last two cars within feet, and haven't blown up any radios.
  8. K4SRF

    K4SRF Ham Member QRZ Page

    No, this antenna does not require any ground plane.
    WB5YUZ likes this.
  9. K4SRF

    K4SRF Ham Member QRZ Page

    Thanks all. I didn't think it would be a problem, but just wanted to make sure. This is the third time I have moved and/or added antennas in less than a year, (love this wallet not so much,) and I know my neighbors wonder about me.
    AJ5J likes this.
  10. K6BRN

    K6BRN XML Subscriber QRZ Page

    The Comet CHA-250B has a non-resonant vertical element driven by a fairly complex broadband multimode matching transformer. It does not need the coax shield as a counterpoise at all. You may place a common-mode choke/unun close to the antenna feed, if you want. For best performance, it should be up at least 33 feet, per manufacturers recommendation. Running over 100 watts to this antenna (and sometimes less) can cause the matching transformer to overheat and SWR to rise. Go too far and the transformer fails - permanently.

    But it works decently as a 60M antenna, which few other store-bought antennas do, likely because 60M is so quiet to begin with. On other bands, it's relatively poor, but as a limited space compromise antenna, it goes up easily and works well enough to make contacts.

    Regarding proximity of one antenna to another, generally a separation of at least one wavelength on the lowest frequency of interest is nice to have and will help both with coupling and antenna pattern distortion/SWR changes. But on 80M this would be about 250 feet and not practical for many stations. As the two antennas grow closer, inductive and capacitive coupling dominates (near-field effects) and significant POWER can be coupled from one antenna to the other, especially if they are parallel. If the transcerivers connected to the two antennas are running at vastly different frequencies, like 1.8 MHz and 147 MHz, the broad, passive front-end filters before any active circuitry in the receivers will usually bypass the unwanted RF energy without harm. But if (WAY) too close, a receiver may at best be desensed and at worst be damaged. Near field coupling goes up exponentially with proximity - so increasing distance between antennas by mere feet can make a difference, if they are very close to begin with.

    To summarize, get the antennas as far apart as possible. Different polarizations - one antenna vertical and the other horizontal - will help, especially in near-field conditions, as the coupling elements will be perpendicular rather than parallel to each other. Try NOT to overlap the clear-sky operating area of the antennas - i.e. a rotatable dipole with a wire antenna directly under or over it.

    As you describe it, since the two antennas will be operating on very widely separated frequencies, there will be a lot of forgiveness in proximity operation. Unless VERY, VERY close, pattern distortion should be negligible and SWR changes on the CHA-250B will be as well, due to its bizarre matching transformer that could probably couple a brick to your radio with low SWR.

    Receiver overload protectors CAN be purchased and placed in-line with a standalone receiver to protect it from damage by a nearby transmitter. But this does NOT work on a TRANCEIVER as transmissions will also be clamped and the overload protector will be destroyed. Receiver overload protectors are usually diode based rather than filter based and their non-linear clamping action generates enormous PIMS noise in the protected receiver when the nearby transmitter is operational. VERY annoying.

    BTW: I have a variety of antennas at QTH#1, including the CHA-250B and a MyAntennas EFHW-8010-2K end fed wire antenna. The wire EFHW-8010-2K is way, way more effective at 23 feet than the CHA-250B is at 30+ feet ... except on 60M. If you ever do purchase an EFHW-8010 antenna, ONLY purchase the highest powered version (-2K) as it is well known that the power ratings provided by the maker are very optimistic. In reality, on 80M the EFHW-8010-2K can handle about 320 watts in the FT8 mode, before overheating - and significanly more on higher frequency bands. But 2,000 watts is just not in the cards.

    My comments are based on quite a bit of direct experience with these antennas and others (7 in all) on a compact 50 x 150 foot lot. Ideal separation distance is not something I can achieve, either.

    Hope this helps!

    Brian - K6BRN
    Last edited: Jul 3, 2020
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