[600MRG] Receiver protection - FOLLOW UP

Fri Jul 13 15:31:47 EDT 2018

ED and all, 
It is good to see the interest in 630 meters increasing. 
I used a common-sense approach here with my primitive 600 meter experimental rig. 
I figured CW was the hardest work (most T/R switching) so I designed in three relays based on the principle that RF is always faster than a mechanical relay. 
I used two radio shack 12 volt DC DPDT relays and an old DC coil Dow-key with aux contacts. 
When the key was closed to transmit the first DPDT relay and the Dow-key were pulled in instantly. 
This then sent DC (delayed by the series-connected relay delays) through the contacts of the Dow-key and then to the third relay. The third relay keyed the RF. 
When the key was opened the circuit to the third relay was opened by the first relay and a capacitor across the coil of the Dow-key kept it in transmit for enough time for the transmit RF to end. It chugged along error-free for over 12000 hours and may be still in use by the someone who wanted it when family duties made the experiment end for me. 
I guess I must admit that narrow filters and an old man's hearing made my top CW speed on 600 meters with my S9 noise level about 13 WPM, so the milliseconds in the sequence were not a problem. 
73 all, 
Pat W5THT & ex WD2XSH/6 

----- Original Message -----

From: kl7uw at acsalaska.net 
To: paul at n1bug.com, "rsgb lf group" <rsgb_lf_group at blacksheep.org>, "rsgb lf group" <rsgb_lf_group at yahoogroups.co.uk>, lowfer at mailman.qth.net, 600MRG at mailman.qth.net 
Sent: Friday, July 13, 2018 7:49:46 AM 
Subject: Re: [600MRG] Receiver protection - FOLLOW UP 

preamp will not amplify if dc power is removed in Tx. That eliminates virtually any RF into Rx as preamp in OFF is an attenuator. If preamp is OK with +10 dBm into it then you are OK. But why not add relay in front so no RF reaches it? 

I run single Rx/Tx with open frame dc relay which is fine at 475-KHz as RF relay. I use an Elecraft K3 at 0.1mw to drive my 100w amp (modified NDB). I may someday build a shielded loop and use a preamp on Rx. The preamp will need protection as loop will pickup RF from my inverted-L used for Tx. I can measure that an decide what method to use. Perhaps removing dc will be sufficient as K3 has Rx protection. 

73, Ed - KL7UW (from St. Marys, MT - on the road by RV) 

On Fri, 13 Jul 2018 07:57:54 -0400, N1BUG wrote: 

I received an overwhelming number of helpful suggestions both on and 
off the lists. I also received a large number of requests to pass 
along any solutions found. Thanks to all who responded. Since there 
were so many requests for follow-up information I will do it through 
the lists rather than individually. 

I am *not* an expert by any means but will offer a few thoughts 
based on experience and what I learned after asking this question. I 
have extensive experience protecting remote (at the antenna) preamps 
on VHF and UHF. What I wanted to do at LF and MF is a bit different 
and I had little to no experience with it. 

The ultimate receiver protection is to disconnect the antenna from 
the receiver while you are transmitting. Very often a 50 ohm 
resistor or in some cases a short will be switched across the 
receiver input at the same time. You have to be sure the switching 
happens before the start of RF from the transmitter. 

There are various switching devices on the market under names such 
as front end saver, front end protector, T/R switch, etc. Sometimes 
such devices use 'RF VOX' where the presence of RF from the 
transmitter triggers the device to switch. Although this can work, 
it seems to me a bit like closing the barn door after the horse has 
escaped. 

Many of these devices instead require a logic input to initiate 
switching. That seems a lot safer to me. If you only use WSJT, I 
was reminded that you can use its RTS/DTR control signal and set the 
timing as required for your receiver protector. If you use modes 
that don't involve the JT software, you need another approach. 

If using a transceiver or exciter which provides external T/R logic 
(often somewhat inappropriately called a PTT signal), and if that 
logic is asserted some few milliseconds before the start of any RF 
output, and if you use a relay with sufficient switching speed 
(faster than the logic-to-RF delay) then implementation is simple. 
You connect the T/R logic to the relay or device which will switch 
in the receiver protection. The key here is to watch the timing! I 
have seen many devices come to my bench for repair because the user 
did not understand or did not think about the timing issues 
involved. Some older transceivers have no delay at all. W8JI 
discusses timing issues and how to measure delays here: 

http://www.w8ji.com/relay_timing.htm 

If you don't have a T/R logic signal from the exciter/transmitter, 
or if the delay is too short to be of use, then you need to create 
the switching logic and the appropriate delay yourself. I will leave 
it to you to figure out how to implement this with common LF/MF 
setups. On VHF and UHF where remote preamps are very common, we use 
devices called sequencers or T/R time delay generators to accomplish 
this. For example on my 144 MHz setup I have a four output 
sequencer. It takes a single low on transmit logic input from my 
transceiver and provides four outputs which go low in sequence with 
about 50 milliseconds delay between them (such that the last one 
goes low 200 ms after the input logic event. I have the first output 
switch my preamp to 'safe' mode (disconnect antenna and connect 50 
ohm resistor to preamp input), the second to my 1500 watt amplifier 
relays, the third to the 60 watt driver amp relays, and the fourth 
to a transverter enable line. Thus the transverter cannot produce 
any RF output until everything else is ready. The sequence is 
reversed when going back to receive. This has been standard practice 
on VHF and up for many years. 

That is all pretty simple when you have one receiver, one 
transmitter, etc. 

In my case I have two receivers (one LF, one MF) which are 
completely independent of the several different transmitters. This 
is a bit more complicated, as every receiver must be protected and 
this must be triggered by any exciter that might be used. Further 
complicating things, I have one very indispensable exciter which I 
don't entirely trust to get the timing correct every time. For these 
reasons I prefer some sort of RF limiting on the receiver input such 
that it can always be left connected to the receive antenna while 
transmitting without risk of damage. I didn't (and still don't) know 
how to design a limiter which would keep things to a safe level for 
all receivers. I was hoping someone else knew. :) 

Limiters can be as simple as the old school approach of two fast 
diodes connected back to back across the receiver input. With 1N4148 
diodes this should limit RF to about 10 dBm (10 milliwatts) but I 
use a 20 dB gain 2N5109 preamp which may be very capable of 
producing 30 dBm (one watt) output with 10 dBm input! Some receivers 
might not like that! I was reminded that such simple circuits often 
make use of a small incandescent lamp in series to provide some 
additional soft limiting. The idea, I think, is that the filament 
when cold (at normal receive signal levels) has low resistance, but 
as it heats up (strong RF from a nearby transmitter), resistance 
increases. 

I was cautioned that if f you have strong RF on any frequency 
reaching the diodes while receiving, they may create all sort of 
harmonics and other trash which will impair your receive ability! 
Also the harmonics could possibly be radiated by the antenna, 
perhaps causing interference to someone else. 

There are more complex (and better) techniques for the RF limiting 
approach. As noted in my original post, ICE once produced a receiver 
protector (ICE-196) which used a saturable core transformer along 
with didoes and other components to provide limiting. I have one 
permanently connected to the receive antenna input on my FT-2000. 
Nice, but the device has too much insertion loss at LF and MF. 
Reverse engineering it and designing one that works at our 
frequencies is beyond my level of expertise, but would be a worthy 
project for someone. DX Engineering currently markets a similar 
looking device but it is only rated down to 500 kHz and will pass a 
25 mW signal - well above the stated danger level for some SDRs. 

As it turns out, luck was on my side this time. ;-) The diodes which 
I already had should limit input to the 2N5109 preamp at around 10 
dBm. A 2N5109 will laugh at that. I was worried about the LF and MF 
receivers, but perhaps unnecessarily so. They are both modified 
Softrock Lite II SDR's, the input stage being a FST3253 mux/demux. I 
have been educated to understand these can take quite a beating 
without damage. This probably explains why neither receiver blew up 
last winter! I'm going to trust that will continue to be the case 
and leave my system alone. I am not worried about strong RF from 
other sources causing the diodes to conduct and make trash while I 
am receiving. I have no local LF/MF operators (yet), and I use 
bandpass filters before the preamps to knock down strong out of band 
signals. This might not work so well on LF in Europe where you have 
monsters like DCF39 and the LW BC stations to deal with! I am not 
too worried about radiating harmonics generated by the diodes since 
the didoes are inboard of the band pass filters which will attenuate 
the second harmonic more than 60 dB before it can reach the receive 
antenna. The receive antenna would be a most inefficient radiator. 

73, 
Paul N1BUG 

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