On 11/29/19 12:00 AM, Herbert Schoenbohm wrote:
The perfect example of your point Jim was the infamous Maxcom antenna that
claimed 1:2 to 1 from 2-30 Mhz! Actually, when you hooked your rig up that
was very close. Further examination of the potted matching block, I think
by the ARRL labs, by an X-ray device, reveals a series of toaster elements
inside. They refused to run any more ads in QST but 73 continued for a
while. Essentially it was a dummy load that you hung in the air between two
wires and fed with coax. The U.S. government had something close to this
called the T4FD that they put on embassies and military installations all
over the world. It worked a bit better as the non-inductive load was
placed in the top center of a wide-spaced folded dipole, This was also
supposed to cover 2-30 Mhz with a reasonable SWR curve.
The T2FD is a well understood and widely used antenna, lossy as it is.
In the design application, they don't have a "transmit power limit" like
hams do. Nor are they contesting trying to break a pile-up or working
the weak ones as the band opens/closes.
They're also typically using ALE, and if you've got a decent propagation
path, if the antenna has 6dB of loss, it's not a big deal.
The typical commercial installation often uses the stainless steel
version (which is lossier) for environmental ruggedness.
One could do essentially the same thing with a dipole fed through a
3-6dB pad, but then you'd have to figure out how to dissipate 700 watts
in a small place. Much easier to put the big dissipator up in the air
where it can be more easily cooled.
Where there is a power limit of one sort or another, or the other end of
the link is poor, the commercial/government users tend to use big LPDAs.
Both T2FD and LPDA have the advantage of "no moving parts", unlike an
auto-tuner.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|