The AM Forum

Maybe I haven't search right, but as a practice I always disconnect all antennas to any other equipment I am not using if I am on the air. My thought is to protect the front ends of my radios.

My question is if I did want to leave a receiver up and running to monitor another band or a shortwave broadcast, is there something I can build to protect my receiver from high levels of RF.

A lot depends on whether it's tube or solid-state receivers. Same band, different bands, how close the antennas, power levels, etc. Contest stations do it all the time.

So you want to have one radio and antenna receiving while you are transmitting on another radio on a different band?

Or are you talking about just a protection device for the front end for static, nearby lightning?

C

After the usual gap lightning protector or modern gas ionization model,  Phil, N8VB, recommends the following at the input of a receiver to prevent overload from nearby transmitters, etc. :

Ant input BNC (or whatever) to a 6.3v 150ma G-3/12 minature fuse style bulb (Mouser part #560-GF550)

Thence to a split of a 0.1uf/50 volt min. cap to receiver input terminal and
other side of split to a 1.0uf/50v. in series with back to back 1n4148 diodes (Mouser part 512-1n4148) to ground.  The diodes at time of pub. were in packs of 25 at $.02 each.

The bulb mounts in a Littlefuse holder, Mouser no. 576-03540101ZXGY

Thats a neat idea. 

I found another one that uses a simple Coax T.  You use SIX diodes.  Three one way and three the other way from Center of the unused T to the shield side of T.   Then you just plug it inline.

That device was intended to protect R390/A recievers from failed Coax relays and Static hits.

Ya I'm talking about a receiver on its own antenna listening to another band. A typical example for me is I am transmitting on 75m with 100 watts of carrier into my loop. I would like to be able to monitor 160 or a shortwave frequency on my R-392 or my softrock. Those receivers run on a separate dipole about 20 feet away from my loop.

Another example would be me transmitting on 75m with 100 watts of carrier and monitoring 6m with my G-50 with a halo about 30 feet away.

The R-392 needs less protection than the Softrock.

At least three people on here will remember a time we were in a QSO on 75 meters, when suddenly a station on 1885 "appeared" in the QSO.  None of us let on that we heard the interloper, except one guy who was first to mention it.  

We almost had him convinced he had an image, or that the interloper had a spur.

Turns out, while one of us was transmitting, a receiver on the other band mistakenly was fed through the transmit chain on the other band. Conditions were so good, it sounded indeed like the other station had broken in and was just randomly talking to himself. Well not randomly, he was replying with a full page of notes.

The erroneous receiver was an R390A.

Poor guy who commented may have stared at his bottle, then the receiver, then the bottle again.

During the days of the Friday and Saturday evening break-in sessions, ya never qiute knew what was gonna happen or who would pop in.  ;D  ;D  ;D

Lots of people use this or similar on their receiving antennas (Beverages, etc) for receiver protection. You can build it yourself.

http://www.qsl.net/n/n1eu//topband/rxprot.htm

Steve that type of clipper can produce lots of IMD. I suggest the diodes be changed to pin diodes. Trr should be nice and slow maybe greater than 500ns for 160m

IMD when receiving?

any strong local signal that can drive the diodes into conduction can produce IMD

Speaking of PIN diodes, where are sources for them??

                     _-_-bear

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It would have to be so strong as to cause damage to the receiver. I can't think of any real world situation like this unless you live next to a BC station.

Or you have an antenna changeover sequencing problem and accidentally feed something like a kw carrier right in to the receiver.

All bets are off then.  :)

Here's another one where the diode can be switched in or out.

http://pvrc.org/~n4zr/Articles/Simple%20Protection%20for%20the%20Fledgling%20SO2R%20Station.pdf

The Racal RA6830 can handle 20 watts input. The limiter is 2 banks of 1N4148 diodes biased with voltage regulators. It is a high level clipper. and generates some distortion but saves the front end.

The FT101 has survived since the 70s with a simple bulb like the one that was mentioned.  I use one on low power 160 meter AM (my friend is 10 miles away).  I found the bulb popped a few times now.  I left the 101 on the inverted L and fired up the 304TL rig on the inverted V. 

So maybe that is all he needs?  Lots of good ideas so far!

C

The FT-901 uses a really low milliamp fuse, and that thing won't blow, it glows. I've fired up a rig into one antenna while the 901 is hooked to another, and it lights up like a nightlight.

When I worked on military radios, the R-1051 receiver (to this day) used a 6BZ6 and 6AN5 in the front end. The rest of the receiver all transistor. Simply because hollow state is inherently robust. A spark or overload won't kill a 6BZ6, and it's simple enough to pop a new one in if you need to.

Bill

Good point Bill. I worry about coils.  We have an SX28A that was dead.  We found an RF coil blown open. That super small wire was a PAIN to fix.  Not sure if a diode device would have saved her or not.

C

Everything is a risk and a set of compromises if the standard of protection is set too high.
A direct lightning hit will fry diodes, hard switches, jump the gap of fuses and probably blow everything in the shack, maybe even the house, .... so pick your poison.

Try real hard not to get too fancy with multiple switched rigs on the same or multiple antennae too. Maybe a start menu similar to an aircraft pilot's would help.  ;D

...sooner or later. 

SWL 's have  1/4 the problem (inverse square).  8)

  I took a direct hit here a few years back. My neighbor saw the lightning bolt dancing to the tip of my antenna mast. I insulate the mast, and it is part of an antenna. I usually have a knife switch thrown outside, but this time not. I did have a coax switch in the shack thrown which grounded the antenna, and diverted my shack ham stuff to a 50 ohm dummy load. The next coax switch had my Icom R75 in line.

  The result? None of my ham equipment was damaged! But all around the house there was dead bisquets including: AC Programmable thermostat, AC ventilator control PCB, 1 garage door opener, and every HDMI port in the house. The HDMI ports were in multiple TV/GAME/Computer boxes. If a cable was plugged into the port, the port was fried.

   So one thing we can do, as a single layer of protection, would be to have a relay box in the shack that grounds the incoming antenna feed, and terminates the shack side into a dummy load whenever the shack power strip is powered OFF. So when that unexpected storm pops up suddenly, and your away and you forgot to disconnect things, at least there is 1 layer of protection in place.

Jim
WD5JKO

My shack is setup to ground everything when I turn the light off and leave the room.  The light switch controls a lamp socket pair.  Hooked to that is the power supply for my grounding antenna switch.  The feeders go to ground and the radios go to dummy loads all when I walk out of the room and turn off the light.   If I forget to turn out the light, then all bets are off..

C

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.

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.

All further discussion has been ordered to stop ;D

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.

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. 

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.

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.

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

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

I like this a lot

Al

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.

#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.

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

http://www.k8nd.com/Radio/Planning/CQWW160/RG_2000M_SCHEMATIC_090201.pdf (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.? ;D

-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.

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

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

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

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

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

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

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.

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

Gee, I've been using useless things for 40 years and never lost the front-end of a receiver.  I must be doing something wrong although I have seen one of #47 bulbs flicker during a winter snow storm. I better defer to the pin diode experts.

No airplane in the sky uses piss ant 1n34s or light bulbs for ESD or lightning protection because they are skroteless.
Trust me we test this kind of stuff all the time and have real protection methods rather than JSes

What do you use in a typical airplane box Frank?
Is it something like this that you mentioned earlier?

Biased transorbes isolated by a fast diodes works very well. The fast diode is reverse biased just below the transorb conduction point. The reverse biased diode adds very little C to the signal line. You need a diode and transorb for each polarity. The transorb could be replaced by a string of diodes if the clamp voltage needs to be low. The deal is you need a low impedance clamp to protect the electronics. A series bulb just adds a little series resistance. This really only reduces current a bit.
 

Suggested schematic would be nice?

A concern with bias is that this requires a power supply to be working properly, and/or the unit to be on for protection. Quite frankly I leave my ant connected often with the receiver off... eventually I may pay for this...

                   _-_-bear

I was unable to locate any auctions by 4X4RB on ebay. :(

The question was about protection in strong RF fields  - not ESD, lightning strikes, EMP, flight at 50k feet or any other such nonsense. Nitpickery is not productive. Whining about JS is not productive. Please take that crap elsewhere.

It is all the same voltage is voltage 1n34 are 60 year old nonsense
JS is JS
Science is science

For virtually all of the above mentioned protective circuits you will loose the desired signal when the protective circuit is shunting away the offending RF. Since you originally described transmitting on 75 while listening on a different band with a different antenna, then you have the classic multi-station contest interference issue. the best solution is to make up a coaxial trap for 75 and place it in the listening receiver's antenna input. An open circuit quarter wavelength (electrical) line T'ed to your input (stub) will get you something like 20+dB of attenuation for the undesired RF. An antenna tuner on the receiver will also help by rejecting some of the 75 meter RF. Many tuner configurations act like low pass filters and will do a good job at rejecting 75 when tuned to 160.

But, stay away from diode or other clipping type of circuit on the receiver as it will generate, and the receiving antenna radiate harmonics of the 75 M RF. It is just like the old rain gutter or wire fence with bad electrical (corroded) joints acting like diodes.

WOW  If my 4-811a homebrew amp can generate as much strap as lighting bolts to a near by receiving antenna, then I must of botched something on the build. I never mentioned 1n34s in my original post.

I've already made my best attempts at lighting protection, but thank you for your input/concern. The original post is about strong RF fields at my home station.

I have a feeling your one of "those guys" who will keep replying for the last word. I'll do it my way and you can do it your way, if it doesn't work you can say " I told you so."

Will that make you happy?

Good info here, something to think about and watch out for.

I've always just used a relay to short the antenna input to the receiver during transmit.

Can still monitor my TX , and never lost a front-end yet.

Why over-complicate things?


Even is the RX is on another band... do you really need to monitor it
while you are transmitting?


If so, it seems like proper filtering on the transmitter, and reciever should eliminate any problems.

Exactly as I tried to refer to it a while back on the thread and got crapped on and was not trying to stir up trouble. Since I often use 2 stations running in NON contest times I thought it was relevant to mention the primary use by many hams and was going to next get into the stubs next which I also use and are currently an active topic elsewhere. I can often be on one radio chasing DX on 160 CW and the other monitoring an AM QSO or looking for a DXpedition on another band/mode.

However they are not applicable in many antenna feeds without a lot of switching so a decision of the various methods suggested has to be made since the OP has several radios and antennas involved.

There are also several articles on bandpass and band reject passive filters for receivers or barefoot transmitters/transceivers. OTOH I doubt if anything will get into or out of the G-50 that would affect HF.

Carl

OK show us some science. I'm sure the crew at PJ2T would love to be educated. Put up or shut up.

Best solution yet.  No PIN diodes or "science" required.  ;)