Simple Receiver Front End Protection Ciruit Idea

Inline with the antenna and receiver is a Daiwa CS-201 switch. One side of the switch is not being used. Before, I would simply disconnect the antenna at the receiver. Switching this switch to the un-used side would provide the same result, right? Doesn't it ground the antenna terminal? I wasn't sure about this - thinking that the center conductor would still be "floating". So, I made a short coax stub which terminates into a chassis mount S0-329 and the diodes.

If a relay is the answer, then I'm thinking that my switch solution would do the same thing. The only thing a relay would do is present the same condition to my receive antenna terminal each time I keyed the transmitter, right?

 

If I'm already switching the antenna out of the circuit, would this still be a problem if my end result is to protect the front end of the receiver? What you're saying is that, if I powered on the receiver and switched to this diode stub for a receive antenna that I would hear lots of intermodulation right? I can live with this since I won't be listening off of the stub - just want protection. Am I understanding this correctly?

 

If the receiver is connected to the common port of the antenna switch, just put a shorted connector on the switch port that is not connected to the antenna. That way, when the switch is thrown, the antenna input, to the receiver, will be shorted to ground potential.

Glen, K9STH

 

How long would it take to check this with a VOM?

 

Even though I can pass a test, I'm still new to building simple circuits & testing them. I think confidence will come with experience.

Anyway, I first tested the stub/chassis mount together with a multimeter: no reading between center conductor and shield - zero resistance center to center or shield to shield. Good so far.

Next, I switched the the meter to the diode symbol, and applied the test probes to center conductor and shield together, then alternating probes. I get the same reading of .56.

If I'm understanding this correctly, anything over this amount of voltage presented to these diodes will force the current to travel in the opposite direction, right? And, since I've alternated the diodes across the center pin and ground, both cycles of rf will be "trapped" at the diode (above this voltage) and will not travel down the coax unless the signal is high enough of voltage to overcome the diodes. Right?

I'm using the little glass diodes marked 148 for this simple stub.

K06WB recommends Shotkey diodes. N0SYA has recommended 1n4001s. Should I upgrade the diodes in this circuit I made? Am I understanding the operation of the circuit correctly?

 

Do you mean shorting the center/ground conductors of the un-used switch? One example would be a chassis mount so-239 with a jumper soldered between center and ground right? (this way I don't change anything in the switch internally). Am I understanding this correctly?

 

It wouldn't take me long - just inexperienced in interpreting the readings & using a VOM correctly.

 

This thread is killing me. It seems to me that two-way communication is compromised. To save my own life, I will try to summarize the main points as I understand them. I would appreciate any further clarification that others may wish to provide.

Said summary:

1) Two PN junction diodes across the antenna input of a receiver will not protect the first stages of a receiver from overload. The reason is this--overload is measured in microvolts; the signal required to switch the diodes to a conducting state is measured in tenths of a volt. Thus, the receiver will be in destructive overload long before the diodes can switch into a protective state.

Such a protective circuit could be built with P-Intrinsic-N (PIN) diodes, but those diodes would require DC bias to make such a circuit work.

2) The protective "antenna-to-ground" short circuit proposed by Mr. Zook can be easily implemented with a PL-259 UHF connector and a short piece of wire. That wire would connect the center pin of the PL-259 to the back of the body of the PL-259.

The "shorted" PL-259 would then be plugged-in to the un-used SO-239 of the coax switch. That port will now serve to protect the connected receiver from overload by shorting the receiver antenna to ground when that position on the coax switch is selected.

I hope this provides some clarity. I was never able to completely get my brain around "coax stub," "stub/chassis mount" and some of the other phrases used in the thread. I was never quite sure what was being measured with the multimeter or VOM, nor exactly what this meter is. I wanted it to be a Digital Multi Meter, but was never quite sure.

 

This is precisely the answer I was looking for! I shall make the described PL-259 tonight.

"Coax Stub" was simply a 3 foot coax jumper I had lying around. I used it to jumper the SO-239 of the switch to the SO-239 with the diodes I made. I now understand that, in order for this circuit to work, I would already have a massive overload state in my receiver.

You have provided clarity for me & this has been a learning experience, which for me is a big part of what Ham Radio is all about.

73

 

So when a 4001 fires in your tv set power supply (for sake of example), it's dc biased into operation?