In a message dated 96-07-01 08:58:40 EDT, you write:
> If you check with the experts, or do some modelling, you'll find that the
XA comes out >4-8 dB better in the forward gain department. 4 dB is 60% of
the power.
Tony --
Intertesting post. Not being an engineer has its disadvantages but I
understand that the TOTAL gain of antennas such as the ones you're citing
would be in the 4-6dB range. How can there be a 4-8 dB ADVANTAGE? Wouldn't
that put the total gain between 8-10 dB which is impossible theoretical gain
for a short or even medium boom length?
> To equate the two antennas' performance would
>require that the XA dissipate almost 1KW in the form of heat. That would
>be 100 watts per trap (there are 10 traps in an XA). I think that much power
>would be enough to melt the plastic end caps out of the traps.
>The traps of the XA are made out of three materials - air, aluminum,
>and polyethylene (?) capacitor caps. Air is very low loss, the aluminum
>tubing is of reasonable diameter so skin effect losses are minimal, and
>the capacitor end caps constitute such a minute percentage of the capacitors
>that they can't dissipate 100 watts each. The
>upshot is that while there is merit in the efficiency arguement, there
>isn't 4 dB of merit. I'd be willing to accept that antennas with lumped
>traps like a TH7 or A4 etc would have higher losses. But the XA uses
>linear loading and air-capacitors - you can't get much lower loss than that.
>
It talking to Lew McCoy at Dayton this year, he told me of an
unpublished test of a KT34XA that he ran when they first came out. He put
2kw keydown into the XA for 30 minutes. He had a guy on the tower and Lew
asked him to touch the elements to see if they had warmed up. They were so
HOT that the poor guy got burns on his hands (no RF, remember - just residual
heat) that they had to take him to the emergency room of the local hospital.
I think that your observation of the heat dissipated by the XA is probably
accurate. That heat, as you know, is RF that is staying in the antenna as
an indication of inefficiency. Also, each connection in an antenna results
in ohmic losses. The XA has dozens of connections and a lot of cumulative
ohmic loss. While the XA is a known high-performance tribander (I even
recommended it in my talk at the contest forum at Dayton this year), do not
discount the alledged performance advantage of the Force 12 designs. BTW,
your modelling of the XA as monobanders does not address the traps in the
design or the connection ohmic losses. IMO your evaluation is interesting
but may not reflect on-the-air effectiveness. 73, Steve K7LXC
BTW I'm selling my TH7 and getting a C-3.
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