> transmit and the directional beverages on receive? Since the vertical
> is situated perpendicular to the beverages, there should not be much
> mutual coupling, I think. If there was, it could be detuned through
> additional switching.
Mutual coupling is not the issue. The radials provide the other
terminal for the feedline to "push" against. Unless the feedline is
radiating, you will have exactly the same current into the ground
system as you have in the form of common mode current exciting
the vertical. That is the "push against" effect of the radials.
Elevated radials handle this effect OK.
The second effect is losses in soil, and the radials should shield
the soil from the antenna and distribute the field evenly around the
antenna. Elevated radials don't help much here, they normally
aren't dense enough. The field is concentrated near the radials, and
other areas are directly exposed to the antenna's induction fields.
This is the lossy part of small elevated systems close to earth.
If you stood back and saw 4 amperes in the vertical, the total
current into the ground system would be four amperes. You would
have a ground impedance of 500 ohms divided by four. Not an easy
thing to feed and likely very poor efficiency. While that number
does not translate directly into loss, it would certainly be much
worse than the normal 5 dB or so of loss expected with four
elevated radials. You'd also need termination resistors able to
dissipate very high power, unless you plan on running QRP. They
have to be big because they will soak up power, along with the
ground under the Beverages.
> During transmit, they are all connected together through switching and
> are basically an elevated radial system. During receive, the
> appropriate "Radial/Beverage" is selected and the others de-selected.
It also would likely not work on receive, unless you detuned the
vertical. You'd need a ground system under the vertical for the
Beverages, and if you leave it connected while transmitting loss will
increase. (With a small elevated system, loss increases with any
RF ground path at the antenna base...including the path back
through the coax.)
You'd wind up with more loss than a conventional small elevated
system on transmit, and likely poor receiving to boot. Better to
keep the Beverages away from the vertical, and use a better ground.
73, Tom W8JI
W8JI@contesting.com
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