EMRFD Message Archive 7170

Message Date From Subject
7170 2012-01-15 09:02:48 Vincenzo Calandra RTX switch trouble
Good morning to all.

I have recently ended to build a simple rtx qrp for the 40mt.

I find myself in difficulty with the trasmit-receive antenna switch
constituted from the classic two diode (1N4148), a coil (10uH) and a
capacitor (10 - 70pF trim-cap), places at the end of the filter low-pass
filter, like in EMRFD figure 6.125 at page 6.70.

Unfortunately, living in Europe, that configuration, in the nocturnal
hours, creates strong overload problems to the present of big
broadcasting on the 7Mhz.

In EMRFD a capacitor of shunt placed on the side of the inductor is
cited like possible remedy, but what it is the correct value?

Or... other suggestions?

Thanks in advance for the answers and... apologize me for my bad

My best 73 - Enzo IZ5GVP.
7171 2012-01-15 11:09:34 William Carver Re: RTX switch trouble
Enzo your English is molto bono, perfetto.
Much better than my Italian.
Guglielmo - W7AAZ
7172 2012-01-15 13:13:16 Jim Kortge Re: RTX switch trouble
7173 2012-01-16 01:19:07 Johan H. Bodin Re: RTX switch trouble

please take a look at the article "The Incidental Double Tuned Circuit -- A
Transceiver Anomaly" located on Wes' web page:



you may find the information useful if your front end has similar properties.

A figure seems to be missing on the first page, I don't know if it is really
missing in the file or if it is a bug in my version of Adobe reader.

Johan SM6LKM

7179 2012-01-16 11:34:07 Wes Re: RTX switch trouble
Hi Johan, Enzo, and gang,

Yes, it was the piece on the Incidental Multi-tuned Circuit that seemed to apply here rather than anything in EMRFD. But I didn't realize that there might be a missing figure. As it turns out, it was not a missing figure, but just a poor choice of words on my part. If you replace the phrase "a following figure" with "a following set of calculations," it all works. I'll fix the piece that is on line. I'll probably just convert it to an html so it is easier to read in a portable environment.

The use of a suitable capacitor (270 pF suggested by Jim) may well get rid of a resonance around 9.5 MHz, which could be death to a European listener. Alan suggested the use of multiple diodes, but this is just the information presented in EMRFD. A stronger mixer in the receiver is always another option.

Anyway, good luck with the experiments Enzo. Thanks Johan for the heads-up
7180 2012-01-16 12:07:42 Bill Noyce Re: RTX switch trouble
Steve Weber, KD1JV, claims that his method of T/R switching avoids the IMD
caused by the diodes. He uses a series MOSFET (2N7000) switched on to
enable receive, and a shunt MOSFET switched on to disable receive.
(Usually a third MOSFET is present as an inverter to drive one of these).
See, for example, http://kd1jv.qrpradio.com/ARRLHBC/MMR40.gif or most of
the other xcvrs on the site.
-- Bill, AB1AV

[Non-text portions of this message have been removed]
7181 2012-01-16 13:41:38 Pat Byers Re: RTX switch trouble
Hi Bill,

I'd like to add that Dan Tayloe N7VE used a similar scheme in the NorCal
NC2030 (Unfortunately now discontinued) and the FT-80 and FT-15
transceivers which are available from Hendricks QRP Kits. Dan used a
BSS123 as a switching transistor because of its low on resistance and
Vds of 100v which is higher than the 2N7000 (60v?). I'd be a bit wary of
using a 2N7000 with a 5W transmitter, particularly if the antenna has
high impedance.

The NC2030 receiver specs are outstanding so this T-R switching scheme
must work well.


Pat Byers VE3EUR

7183 2012-01-17 04:53:38 Fernando Krouwel Re: RTX switch trouble
Weird thing...

The day Johan's message was posted, I read the article in the Wes
website "Tech Notes" section (and found it very useful for my purposes).
Now, I can't find it anymore (missing link).
What could be happening? Is he making more tests, or more possibly, had
more ideas about this subject?

73' s,
Fernando - PY2ETT

7184 2012-01-17 11:57:17 Wes Re: RTX switch trouble
Hi Fernando,

It is still there, but now as a html rather than the old PDF. I did this when correcting the confusi
7185 2012-01-17 12:34:14 Fernando Krouwel Re: RTX switch trouble
Hi Wes, good afternoon:

Thank you very much for the info.

I have a small 40m transceiver with a DC receiver, which further I will
transform into an (also simple) superhet; in the receiver section I will
add a front end converter, probably with a NE602 (to keep things simple)
and VFO (followed by a 4.9152MHz xtal filter), so, it will be useful.
On the transmitting side, another converter to output the 40m freqs.

73' s
Fernando - PY2ETT