On 6/5 Roger Sheridan wrote (in part) --
>> We replaced the existing welding rods with copper tape in the fin close
out,
>> were unable to obtain ferrite toroids. I therefore made a "balun" from a
loop
>> RG-58 cable...It worked for us, just don't ask me why!
The (excellent) text accompanying RST Engineering's (www.rst-engr.com) foil
tape antenna kits explains all. The bandwidth of an antenna is a function of
its aspect ratio. Copper tape exhibits the same bandwidth as a round wire or
tube of equivalent diameter, so a welding rod would be excessively "peaky."
An RG-58 coax balun similarly is "peakier" in bandwith than one made from
toroidal cores. In either case (using a half-wave dipole verses a
quarterwave rod with ground plane), a balun is needed, and your post hints
the welding rod setup lacked one. As a sidenote, Bob Archer says toroids are
"lossy." Of course, he sells pricey composite aircraft antennas, but his
PC-board foil baluns work well too, users say.
As to optimizing radiation pattern, RST says the ends of the VHF dipole must
be at least 24 inches from any metal part that is 12 inches or larger in size.
>> ...clearly the antenna is required to handle much greater power for
transmitting.
Yes, maybe 7,000 times greater at 1 mv of received RF. But so as not to
mislead those solving problems in this murky area, I'll add that the
equivalent utility of an antenna for receiving and transmitting is cited in
texts on antenna theory as the Law of Reciprocity. Further, the transistor
output stage of a 14-volt comm radio can pump only a fraction of an amp of
equivalent DC into the antenna. In fact the antenna actually sees even less,
since it radiates the energy (variably) along its length, and talking into
the mile reduces it further. Copper tape can more than handle it, and ohmic
losses in general are negligible compared to other sources of loss in
antennas.
(removing shirt pocket protector and propeller beanie now)
Regards,
Fred Fillinger, A063
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