[TowerTalk] JK C3S WARC Modifications

[jimlux]

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.