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.
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?).
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.
73 Kevin N7WIM
On Dec 9, 2007, at 11:02 AM, Paul Christensen wrote:
I believe the Argo V DOES use diode switching. The
pin diodes used are exotic 1N4007.
The Omni 6/6+ also uses a handful of 1N4007 diodes as part of the T/R
circuit, particularly for the 2nd Rx port. However, isolation
characteristics are not nearly as good as a true PIN diode
structure. Where
isolation is not a primary concern, the 1N4007 can be used as a low-
cost,
low/mid-power RF switch. On the Omni 6, I was able to increase port
isolation by better than 40 dB by replacing the 1N4007 diodes with
high
power UM2110 PIN types from Microsemi.
-...-
73 DE N7WIM / G8UDP
Kevin Purcell
kevinpurcell@pobox.com
|