Receiver Protection

[KM1H]

A single set of diodes can easily cause IMD in a multi transmitter contest enviroment, using a slaved spotting receiver (as I often do), lightning static, etc.
Just because the diode has an xxx conduction voltage doesnt mean it is doing nothing before that point, this is especially noticable with germanium.

The all purpose 1N5711 Schottky diode is pretty decent but I dont know of any suitable small PIN's that work well at 160 or 80.

[Steve - K4HX]

No one is talking about either a single set of diodes or a multi-transmitter corntest station. The original poster's question has been answered.

[KM1H]

All further discussion has been ordered to stop

[Steve - K4HX]

Wrong again Carl. Just stay on topic. If you aren't answering the original poster's question then there is no need to post. It's really pretty simple. If you're here to stir up trouble, hit the road.

[WA1GFZ]

Better to use a slow diode like a pin. Actually some power diodes like 1N645 make good pins. A slow Trr does not generate switching and mixing components when there are strong signals present.Search the net on pin diode limiters there are some application notes out there. Actually transorbs work quite well but it takes some extra hardware and plus/minus bias voltage to make them interface to the antenna due to their high junction C. 

[KM1H]

Do you have a couple of suggested PIN part # Frank? Ive not had much success with even biased diodes and not in a contest. I often have 2 radios going when Im on the hambands and a vintage GC one on the BCB or some SW station using a longwire. I dont trust much more than a single pair of diodes to protect a SMD or GaAs FET front end and at times IMD is pronounced.

[WA1GFZ]

Go to ebay and search on 4X4RB Baruch is selling some nice power pin diodes he removed from boards. You don't bias diodes on because they will load the signal. You back bias them both positive and negative so they clip when the sidnal increases above the bias plus junction voltage. Racal used a nice Microwave Associates diode in many receivers. I think the Trr was around 2us. Not sure if you can still buy them. 1N3379 I think was the HP part but they are low power.

[N1XBM]

http://www.k8nd.com/Radio/Planning/CQWW160/RG_2000M_SCHEMATIC_090201.pdf

I think this about covers what a few people mentioned. Found this nice schematic and thought I would share.

[Steve - K4HX]

Nice and simple and it obviously works considering the PJ2T operation is one of the tops in the corntesting world.

[w1vtp]

http://www.k8nd.com/Radio/Planning/CQWW160/RG_2000M_SCHEMATIC_090201.pdf

I think this about covers what a few people mentioned. Found this nice schematic and thought I would share.

I like this a lot

Al

[Pete, WA2CWA]

This protection using a #47 lamp, I think, was first introduced in QST's "Hints and Kinks" back in the late 40's or early 50's. Later versions just used a pair of back-to-back diodes. I've had the lamp protection in several of my tube receivers for many years.

[WA1GFZ]

#47 bulb in series with the antenna looks pretty useless
Cold series resistance will be low.

[Pete, WA2CWA]

#47 bulb in series with the antenna looks pretty useless
Cold series resistance will be low.

Why would you want the cold series resistance of the lamp to be high?

[K5UJ]

#47 bulb in series with the antenna looks pretty useless
Cold series resistance will be low.

Why would you want the cold series resistance of the lamp to be high?

you don't (in my opinion) but the problem is that the lamp probably won't go high until after the damage is done.

MFJ uses the lamp-as-fuse trick in their little noise nulling boxes.  It is supposedly there to protect the built-in preamp that sits on the line in from the noise pickup antenna jack which is supposed to be connected to a separate rx-only antenna.   I have never trusted them, immediately putting in a relay that opens on transmit with the coil in series with their T/R relay that switches to bypass the whole box on transmit.

[Steve - K4HX]

The diodes take care of that. The lamp is for big stuff the diodes for small stuff.

[W3RSW]

http://www.k8nd.com/Radio/Planning/CQWW160/RG_2000M_SCHEMATIC_090201.pdf

This is an exact copy of N8VB's circuit sent out to QS1R owners and others interested via Yahoo board, right down to the borders and signoff boxes on draft paper but with added logo's.

Now I Wonder who really drew it first.?

-And amazed that it wasn't referenced to previous posting in this very same thread.
oh well,
Our attention spans/memories are short these days.

[N1XBM]

There is a solution to the nay sayers. Build it and test it on the bench! Might make a very informative YouTube video!

[Steve - K4HX]

Nah, it's easier to nitpick and second guess w/o any data.    Don't ruin all the fun for the naysayers.

[WD5JKO]

Looking at that schematic, I see the antenna sees no DC return to ground, and two capacitors rated 25v, and 50v. Is this a problem? Many antennas like a simple dipole with coax feed don't have a DC return, and atmospheric events can put a pretty good charge on a floating antenna. That box might do more harm than good if the series cap to the RCV port suddenly shorts. Could shunt the ANT side with a 2.5 Mh pi wound choke, or maybe a 1K resistor, but leaving it open seems like an oversight to me.

Jim
WD5JKO

[ke7trp]

Jim, Thats exactly what I thought when is looked at the schematic a moment ago.  I would be worried about that kind of thing here in the Desert. I get 4 to 5 inch arcs off the back of the ant tuner to the ground lug. 

Then again, Maybe this is ONLY for Strong RF field and the was not meant to sustain ANY static build up? 

C

[WA1GFZ]

been a while since I checked the resistance of a bulb change with current but a couple hundred ms if I remember.

[N1XBM]

Jim, Thats exactly what I thought when is looked at the schematic a moment ago.  I would be worried about that kind of thing here in the Desert. I get 4 to 5 inch arcs off the back of the ant tuner to the ground lug. 

Then again, Maybe this is ONLY for Strong RF field and the was not meant to sustain ANY static build up? 

C

My original post was inquiring about protection when operating a receiver while I am transmitting on another HF band. So yes strong RF field.

[WD5JKO]

My original post was inquiring about protection when operating a receiver while I am transmitting on another HF band. So yes strong RF field.

   OK, good deal. So I guess you have a DC grounded antenna such that you don't need to worry about static build-up on the antenna when you are, or are not transmitting on any band? Others might use such a box and get very surprised when this thing could charge up inside, and the discharge could take out one or both capacitors, the diodes, and possibly even the receiver someone is trying to protect.  My suggestion of adding a bleed resistor, or an RF choke across the Antenna jack to ground might have some merit.

 
Jim
WD5JKO

[Pete, WA2CWA]

On many older receivers, the antenna input connection is an antenna coil with one side of the antenna coil to the antenna input terminal and the other side of the coil to ground so, in effect, the receiver antenna input is at ground potential. A simple pair of back-to-back germanium diodes across the antenna terminal to ground can aid in bleeding off static discharge. For additional receiver front-end protection, a lamp (or a fast acting fuse) and two diodes is really all you need. What's the purpose of the two caps in the circuit.

[WA1GFZ]

germanium diodes are useless. A good ESD hit will take them out.