RadioBanter - More on T2FD Antennas - RLM Electronics MN100-1

In article ,
C. J. Clegg wrote:

Do any of you know anything about the RLM Electronics MN100-1 broadband HF
antenna matching network? (see http://www.rlm-electronics.com.)

It looks like the same general idea as the T2FD antenna except the
terminating resistor is in the box with the matching transformer.

Hmmm. Looking at the photo, it's pretty clear that the lefthand
terminal strip is just used to tie together the four resistors which
make up the terminator (it's a series/parallel arrangement). On the
right side, I can't tell whether all four of the terminals on the
strip are soldered together, or whether it's two separate pairs.

They claim that the "four non-inductive balancing resistors" ("balancing"
resistors?) will dissipate no more than 3 watts each with 150 watts of RF
drive, which if true (big "if") would seem to indicate an overall
efficiency somewhat better than a T2FD.

That's definitely not something I would bet upon, for three reasons.

One is that it seems implausible. If the antenna itself is of a
length which causes its impedance to be very high and reactive (and
there _will_ be some frequencies where that's the case), the antenna
itself will not accept much power and will present a high SWR at the
feedpoint. The modest 2.5:1 SWR has to be coming from resistive
loading of some sort, and by definition that sort of loading is
entirely lossy. I don't see how they could get a super-high SWR down
to 2.5:1 by dissipating only 12 watts out of 100.

Another reason is the fact that the manufacturer makes no claims about
the actual efficiency of the antenna system, but instead acknowledges
that it should not be expected to perform as well as a full-size
antenna. (Even if it's only 25% efficient, it'd still be only one
S-unit or so down from the performance of a low dipole, and the claim
of "very respectable performance" wouldn't be outlandish).

The third reason is that the antenna layout diagrams shown on the
"more product information" page suggest that there may be losses in
the system other than those in the resistors themselves. Notice how
all of the diagrams show the termination device mounted on an 8'
ground rod, with one lead of the termination device being securely
bonded to the ground. Based on the internal picture of the device, it
looks as if this ground-rod bonding point is tied to one end of the
termination resistors.

Notice also that all of the antenna configurations pictured are
loops, with a significant part of the loop sitting right on the ground.

I rather strongly suspect that this particular "matching arrangement"
depends to some degree on RF being coupled into the soil, in a lossy
way... these losses are part of what help "tame" the high/reactive
feedpoint Z at those frequencies where the loop is near a high-Z
resonance point and doesn't "want" to accept much power. This would,
if true, be slightly analogous to the way that radials for a vertical
antenna are "de-tuned" by soil losses if not elevated well above the
soil. There might also be a lossy path from the ground rod back to
the building ground in which the transmitter was being operated.

So, the losses in the resistors themselves may be limited to around 12
watts... but I have a feeling that the total loss is probably a good
deal higher than that, and (probably) occurs across the antenna's
whole frequency range.

Gee, maybe there is such a thing as a free lunch. :-)

I try not to bet on that assumption if I can help it :-}

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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