[Moon-net] T/R relays

[Chris Bartram]

Joe

> Does anyone on this reflector have experience using heavy-duty
> open-frame relays for QRO T/R switching at 144 MHz?
>
> Obviously this can be a workable approach at HF, and even at 50 MHz,
> where the resulting "impedance lump" in the signal path exists over just
> a small fraction of a wavelength.  I wouldn't want to try it at 70 cm;
> but what about 2m?
>
> I would be interested to hear if anyone has used such an approach
> effectively.  Is it a foolish idea, and just asking for trouble?

I'm not sure about what you mean by open frame relays, as it's a term which 
seems to have slightly different meanings on the two sides of the Atlantic, 
but more than 25years ago I designed a 2m preamp for my then company, muTek, 
which used relatively low-cost OMRON power relays to switch QRO with low 
losses (from memory ~0.05dB) at low cost. We were only able to test the 
preamp to about 1.2kW at the time, but feedback from customers was that it 
was fine at higher powers. When I last spoke to him GM4JJJ was still using 
one of these amplifiers, which incidentally used so-called 'noiseless' 
negative feedback around a GaAsFET to give an input 3rd order intercept of  
>+10dBm.

Providing it's not radiating too much, the relay will tend to appear as a 
series inductor at VHF, and if the inductance isn't too large it's relatively 
easy to match with a trio of shunt capacitors, one on each port of the relay, 
effectively turning the mismatch into a low-pass filter on both paths. 

The downside was that the isolation wasn't wonderful - about 20dB. We handled 
that by having small 'isolation' relays. At the time these were little 
Panasonic reed relays which we'd already demonstrated would handle 100W on 2 
with something close to 50dB isolation and also around 0.05dB insertion-loss.

I did try the same power relay on 432. Matching was, not surprisingly, more 
difficult, although it was possible. Insertion losses were much higher and 
isolation was terrible!!

Vy 73

Chris
GW4DGU

Vy 73

Chris
GW4DGU

-- 
Christopher Bartram MIET
Christopher Bartram RF Design
Blaenffos
Pen-y-Bont
CARMARTHEN
SA33 6QG

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email:  	chris at chris-bartram.co.uk
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