[600MRG] Receiver protection - FOLLOW UP

Sat Jul 14 13:08:40 EDT 2018

When you expect higher RF fields, consider placing two or more diodes in series on each side of the back-to-back diode assembly to raise the limiting threshold.  In other words two diodes in series, in parallel with two other diodes in series, reversed.

Steve WD8DAS

Sent from a portable electronic device.

> On Jul 13, 2018, at 6:57 AM, N1BUG <paul at n1bug.com> 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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