[TowerTalk] Building a "little pistol" contest station and the C-31XR

[Jim Lux]

At 04:03 AM 10/6/2006, Andrew Roos wrote:

>Rather than starting with an "all-band" antenna, I've decided to get the
>best tribander that I can (almost) afford, and add antennas as finances
>permit. I'm getting a Force 12 C-31XR - the ultimate deciding factor was
>its ability to have three separate feedlines.
It's not clear what advantage separate feedlines would give you. Are 
you planning to run signals on all bands at the same time?  I would 
imagine that the isolation isn't all that great between the feeds.

Leaving aside magnetic coupling (i.e. induced current in one feed 
from the other one next to it.. probably the dominant source of 
coupling) just the coupling from  capacitance is fairly hefty..
1 cm diameter rods 20 cm or so apart is about 1.4 pF/meter.  5m for 
the 10m DE is about 7 pF. At 30 MHz the reactance is about 750 
ohms.  So, considering it like a power divider, the victim will see 
about 50/750 times the voltage of the transmitter, or about -44dB 
down converting to power..  If you're putting 100 W into the DE (+50 
dBm) you'll see +6 dBm into the receiver, just from the capacitance.

Realistically, you're probably looking at a coupling more like -20dB, 
so with that same +50dBm into the DE, the victim will see something 
like a watt (+30dBm).

I assume you're planning on having a filter with pretty good out of 
band rejection?  The Dunestar filters claim 40dB rejection out of 
band, so now you're seeing a -10dBm out of band signal into your 
receiver.  That's about 60dB over S9...

If you just want to have multiple receivers, you can just use the 
single feedline, and use a power divider or tuned multicoupler at the 
shack end.  At the antenna end, the "other" driven elements present a 
high impedance in parallel with the "in band" driven element for a 
given signal, and so, don't have a huge effect on the overall match 
(and it could be compensated by adjusting the lengths of the 
elements.... tough to do by cut and try, trivial to do with a 
modeling program and a bit of testing)

On 10, the 20 feed is probably thousands of ohms Z, and the 15 feed 
is somewhat inductive, so the 10 feed gets shortened a bit to 
compensate.  On 15, the 10 feed is capacitive and the 20 feed 
inductive, and they probably cancel. On 20, both the 10 and 15 feeds 
are capacitive, so you make the 20 feed a bit longer.

In any case, this is sort of a classic textbook problem (granted, in 
a textbook, you don't have to worry about all the other parasitic 
elements) that can be solved a variety of ways, so you get a good 
match on all 3 bands.

So, for single band transmit, you have no issues with a single feed line.

Which is why I ask about whether you intend to use 2 radios at once 
(I guess you do, with your SO2R questions), in which case, there 
might be "better" ways to solve the problem.  (two different single 
band antennas stacked and separated by a 10 feet might be cheaper 
than all the filters you'll need with the tightly coupled C31XR)

Others who may have tried this will certainly chime in, but, if it 
were given to me as a design job, I'd be a bit cautious.

Jim, W6RMK