[Elecraft] Attic Antenna

[N2EY at aol.com]

In a message dated 10/27/06 6:38:11 PM Eastern Daylight Time, ebjr at i-55.com 
writes:


> WARNING!!!!!!!!!
> This same scheme (scam?) happened quite a few years ago.  Don't remember the 
> name
> of the "device", but is was a "black box" type "proprietary" antenna 
> coupler.

You may be thinking of the Maxx-Com Matcher.

> The price was rather high  ($300-400) and it was guaranteed "flat SWR" 
> between 3-30 Mhz.

Less than 2:1 over the range. And it did!
> 
> SO-239 input and two standoff insulators with wing nuts (output).  Was said
> to match a random wire or a balanced line dipole!  Someone finally
> "x-rayed" one and it turned out to be a dummy load actually.  It was
> enclosed in a cast aluminum "Bud box" and potted with some sort
> of opaque epoxy if you took the lid off!

"Someone" was the ARRL Lab. They X-rayed the thing and then dissolved the 
epoxy (a long and messy process), and found what was inside. 

There was a toroidal transformer, some noninductive resistors, and some 
circuit boards with ICs on them. The circuit boards were obviously scrapped boards 
from some digital system or other, and performed no electrical function at 
all. They simply looked good on the X-ray.


> They didn't last too long, 

Not true.

Maxx-Com is still around:

http://www.maxx-com.com/InstallationInfo.html

with a bunch of products.

They claim 80% or better efficiency, which is absolute bunk on most 
frequencies. However, if the wire length is right, it may be possible for the system to 
be almost that efficient. (See below).


but long enough for the manufacturer to
> 
> make a bundle!  It was not guaranteed to radiate with high efficiency
> but was guaranteed to have an input VSWR no greater than 2:1!
> BEWARE of ANY "MAGIC DEVICE" that makes these claims.
> More than likely it is a large pile of bovine fecal matter that they
> are feeding you!
> 
> No, I didn't buy one, but in some "unusual installations" it was
> tried on some commercial marine limited coast station installations.
> The results were very mediocre no matter what 'antenna' was tried.
> Low SWR....BUT!   Not much radiation!

Here's how such a system *could* work on a few frequencies:

Suppose the matcher consists of a  "T" or "pi" attenuator. The SWR of such a 
system will not be worse than 2:1 if the attenuator is designed right. IIRC, a 
3 dB attenuator will do the job.

Suppose an antenna with an impedance of 50 ohms is connected to the other 
end. The attenuator loss will be minimum, and the SWR 1:1.  

Such a system could be as efficient as 50% (!)

Of course if the attenuator is removed, the efficiency is 100%.

The big problem is that as you move away from those few perfect frequencies, 
the losses grow enormously and the system efficiency goes way down.

Of course if you feed 100 watts into an antenna system with 5% efficiency, 
the result is the same as feeding 5 watts to an antenna system with 100% 
efficiency.

73 de Jim, N2EY


73 de Jim, N2EY