On Mon, 2007-12-10 at 23:03 -0800, Kevin Purcell wrote:
> I've seen similar criticism in 1990s Technical Topics columns in RadCom.
> The 1N4007 does well on insertion loss, 2nd order IP and 3rd order IP
> but fails on isolation (worst case 35dB at 10m rather than 80dB with
> a RF PIN diode). So a pair of them in series might do the job.
But only if each diode was to get the full reverse voltage. Dividing the
DC voltage between them may not decrease the coupling when the
capacitance goes as the inverse voltage squared. Half voltage, 4x the C,
twice the stray coupling.
Hence my thought that a series open followed by a shunt short is far
more effective, though it complicates the bias scheme. Probably one
series, one shunt, one series, and one shunt would give isolation
limited only by shielding and stray coupling through the bias circuits.
And with the shunts biased off and the series biased on, the insertion
loss would be usably small.
> Another cheap rectifier to try (with better isolation characteristics
> -- high 60s dB) is the BYD11M 0.5A diode in a SOD91 package (about
> the same size as 1N4148 switching diode). Colin Horrabin, G3SBI (i.e.
> Mr H-mode Mixer and higher intercept front-ends) characterized them
> at Daresbury and they came out as the best cheap diode for amateur
> use: similar loss, 2OIP, 3OIP numbers to the 1N4007 and better
> isolation (smaller junction?).
Or smaller package C.
> Works with similar bias to the 1N4007 (5 to 10mA).
> BTW, the FT-102 was a classic case of adding relays for performance
> and getting reliability issues in return. There was even a noticeable
> difference between smokers and non-smokers radios in failure rates.
> The solution is to "DC wet" the relays (a technique used in Strowger
> telephone exchanges). Pass a few mA DC through the contacts so they
> clean themselves on breaking. It makes a big difference. I presume TT
> already do this in their high end radios.
Kenwood TS-430 needed that. Some actually got it, some didn't.
> 73 Kevin N7WIM
73, Jerry, K0CQ
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