[Premium-Rx] Suggestion for T-R Switching

[Ed Tanton]

Hi Gary... over the years I have kept a lookout for 1/100 A fuses and 
others. For example, I recently found a bunch (probably 50 or 100 as I 
recall) of 63mA sub-miniature axial wire fuses.

Another method: AMECO, in their 2-output self-switching preselector, uses 
just a 10 ohm 1/8 watt resistor in series with the MOSFET. Somehow, back 
when I was using a TR-7 and ALPHA 78, the timing would get screwed up with 
CW above 20-25WPM, and occasionally pop the resistor. Literally. It did 
that a dozen times over a period of several years. By the way, I used the 
preselector full time-but mainly because it was a great way to get 
full-time, dual-frequency, receive; AND because of 40M, 80M, & 160M-where 
it nicely livened up the TR-7 front end w/o causing noticeable problems 
from other signals that were more than,say, a mile away. (I had a pair of 
2-hole Alphas [think: RG-17] relatively nearby: one ~ 1.5 miles, the other 
< 3 miles... and when everybody was using directional antennas-and even 
(usually) just when I was.)

Anyway, the relay would stick, in a similar manner to yours, and blow the 
resistor. Did this maybe a dozen times, relatively safely-if you're not a 
10 ohm 1/8W resistor! But TWICE, over those several years, it blew the 
MOSFET. Never did hurt the "2nd receiver".

I have been working on something I'll call a "receiver-brick" that you plug 
your key into, and it keys your TX (and amp-if you're using one) as well as 
selectably allowing you to mute your receiver (and 2nd receiver-if present) 
or ground your receiver front ends. While AMECO simply used a resistive 
divider for the 2nd receiver, I plan to use a 50 ohm (all around) 2-way 
divider. Got a couple of nice ones from Alltronics (not to be confused with 
ALL Electronics) that INCLUDE 160M-unlike the majority of 2-ways out there. 
Of course, one can roll one's own-but these are too attractively priced to 
bother. Not cheap, but VERY nice, and priced lower than Mini-Circuits' 
offerings (and they don't do 'regular' HF AND 160M. Close (2-30MHz... but 
no cigar.) I'll add a preselector to the 2nd-receiver version.

The plan is to use several methodologies to insure that nothing I can think 
of can get all the way to my 16-output receiving multi-coupler input. In 
order to provide security and protection for possible multiple rigs as well 
as multiple receivers, the plan will to use a keying (reed) relay, as well 
as a passed-through key "bus". Also an RF detect will force the keying bus 
'ON" (= 'KEYED') as well as an input for external key-bus inputs, both 
active-HI, and active-LOW. This isn't nearly as complicated to do as it is 
to say. There is also the opportunity to apply a nice time-constant (if 
necessary) to have the best T/R transitions possible. I don't THINK this 
latter will really be necessary in most cases, since one relay will be 
controlling another in the chain, and hence should provide their own small, 
built-in, timing.

For low-medium power, it will be possible to use a 12V solenoid coil being 
whomped by ~ 48V (with a resistor for current limiting) and the high-power 
Hamlin reeds offered by Digikey (50W switching/150W passing power). For 
higher power, HC-1 Vacuum Relays can be used. Since the high power version 
is actually faster than the low-and since I HAVE the HC-1 vacuum 
relays-I'll just be building that version.

The only things remaining to be done with this design is to check on 
whether I can easily use opto-isolation for everything related to 
switched-busses, and DC-switching; timing checks; and lastly actual testing 
using a germanium diode as the "receiver" to see if I can manage to pop it. 
I figure if I can't, then everything is OK!



73 Ed Tanton N4XY <n4xy at earthlink.net>

Ed Tanton N4XY
189 Pioneer Trail
Marietta, GA 30068-3466

website: http://www.n4xy.com

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