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[Articles Home]  [Add Article]  

Killing Radios

Pedro M.J. Wyns (ON7WP) on March 6, 2013
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How rich amateurs are in danger of blowing up their entire transceiver collection...


A true story... by ON7WP-AA9HX


I am one of those guys that was 16 and as a student out of money in the eighties. I wanted radios and couldn't afford them. Now 30 years later I only spend my money on radios, buying them all in order to catch up. Last time I counted I had 53. That was early 2000. And I only counted the HF rigs. So far the bragging.


Recently I redecorated my shack. I decided only to display radios connected to antennas. I knew about crosstalk but always believed that it would be not such a fuzz with barefoot radios. Until today...


The day I started measuring....


In a moment of weakness I decided to hook up my network analyser to both ZS6BKW multiband wire antenna and my Moseley PRO67B 7 element 6 band beam, in transmission analysis mode. Below you can see the crosstalk between both antennas:


On 40 meters there is 16 dB crosstalk, so in numbers, if I transmit with my IC775 200W barefoot on the beam I get 5W on my Kachina 505DSP. Must be hell of a strong receiver frontend. Not to mention what happens if I decide to kick in my Alpha 91... 50W pumped into that poor radio... And yes it is still alive. Who said something about bad Amercican designs (me...).


I did some more tests between my two beams. My tower carries a PRO67B at 26 meter and a Cushcraft A4S at 12 meter (fixed to the US) Below the results:

Minus 23 dB at 20 meter, so 200 Watts results in 1 watt on the other antenna. Not too bad compared to the earlier situation.


And now what ???


I still want to have multiple radios connected to multiple antennas. What can I do ? Maybe use triplex filters, connecting like three radios to a single antenna, with ISOLATION between them this way.


Or I could use an antenna matrix, that isolates all transceivers from the antennas when one of them goes in TX. Not a readily available product but something nice to design maybe.


Anything better than ruining radios. You bet we will ruin some if we continue like this..



Pedro M.J. WYNS - aa9hx@arrl.net

Member Comments:
This article has expired. No more comments may be added.
 
Killing Radios  
by IZ4UFQ on March 6, 2013 Mail this to a friend!
Having only one HF rig but two V/U rigs (with three antennas) and an active APRS digi at my site, I have considered this issue, and I have installed the V/U antennas in a way that reduces crosstalk (putting my vertical GPs at different heights, so that the lower is under the radials of the higher). I have no amplifiers, so I use 100 W max on every band. I have measured crosstalk in a far less professional way than you did: I connected a wattmeter to the "receiving" antenna (Antenna on "TX" input, dummy load on "ANTENNA" output) and set it to its lower scale (20 W) and then transmitted at 100W on the "transmitting" antenna.

I could see the needle move on the wattmeter, somewhere below 100 mW. This should be something near 30 dB of separation, which is not bad, but I really don't know what is the maximum energy in terms of mW that the receiving radio can stand without being damaged.

Having a limited number of radios and antennas, I can easily set them up do avoid damages by antenna crosstalk, and anyway I disconnect all of the antennas I do not use to avoid static discharges or induced currents from nearby lightning. I have set up a simple patch panel with N connectors (low loss at V/U) for this purpose.


Fabio Muzzi, IZ4UFQ



 
Killing Radios  
by K1KP on March 6, 2013 Mail this to a friend!
You can protect a receiver front end with a 1/16th amp (.063A) fuse in series with the antenna. This limits the power to less than 200mW. I have used this in my contest station for decades in a 2-position maximum legal power setup. I have blown many fuses, but zero receivers. Fuse is DigiKey F2380-ND or equivalent.
 
Killing Radios  
by N6JSX on March 6, 2013 Mail this to a friend!
The issue boils down to when you TX you fear your RX radios will get front-end damaged due to proximity.

The easy solution is to use your TX radio "Amplifier control-signal-out" to switch voltage controlled coax switches (one common [RX] to two outputs [Ant & GND]) on all your RX radios.
When you hit the TX PTT all the RX radios antennas will get switched to OPEN and the RX switched to the other COAX input that will be connected to ground. I'd put this coax switch near the rear of the RX's to minimize coax RF pick-up on the jumper. (cost the most - but safest)
You may need to make a TX switching relay circuit to accommodate the coax switches for proper voltage/current.

Another method but not as safe as my first, is to put from the antenna side an inline 1KV capacitor then dual germanium diodes to ground (flipped) to the RX, This should first block the DC component and then clip max signal (to ~0.3V) into the RX radios. (the cheapest - but not as safe)

An interesting issue - that I'm sure multi-radio FD'ers have had to overcome - but there difference is most FD radios are setup for mono-banding OPs/ant so a stub notch filter could work for that application. I'm sure you are running multiple bands so the stub method would not work.

I often have my TS-790 scanning on a J while operating my IC-746P on HF antennas never finding cross-talk issues.

I may run into this in the future as I plan on putting my Pegasus as a dedicated PSK31 only station on a dedicated multi-band inverted V, while playing SSB HF (with ~600W Amp) on other antennas. An issue I've not though about until this article. But in my case I need to protect both directions as both will TX just not at the same time.


 
RE: Killing Radios  
by AA4PB on March 6, 2013 Mail this to a friend!
Using coax switches you should consider the switching delay time. It only takes a matter of milliseconds (or less) to blow a FET in a receiver front end. You want to make sure that the relay has time to close and ground the receiver input before the transmitter puts out any power. On SSB that's probably not an issue, but it could be on RTTY or other constant carrier modes.

One common protection method was a series "grain of wheat" bulb in series with the receive antenna and a pair of back to back diodes on the preamp side of the bulb. Under normal conditions the signal is not large enough to make the diodes conduct so the bulb is cold and pretty low resistance. If the signal voltage gets large enough for the diodes to conduct then bulb resistance limits the current to keep from blowing the diodes. If the voltage and resulting current gets high enough then the bulb will get hot, increasing its resistance, and limiting the current even more. If its a very large surge such as nearby lightning then the bulb may burn out like a fuse but the diodes and front end devices are still protected.
 
Keeping it very simple.  
by AI2IA on March 6, 2013 Mail this to a friend!
There is really no reason to constantly keep several radios connected to antennas, except in unusual situations.

It is a good and simple practice to ground connect all antennas when not in use. After all, how long would it really take you to disconnect an antenna, or reconnect one? How much exercise is that? You don't need components, coaxial switches, fancy arrangements or additional gadgets. Just disconnect the coax and connect it to a good earth ground and you will have peace of mind and safety too, especially in summer months with lightning.

Keep everything as simple as possible and as neat and clean as possible, and you will be rewarded with a nice looking shack, a safe shack, and minimum failures and repairs.
 
RE: Keeping it very simple.  
by WB5JWI on March 6, 2013 Mail this to a friend!
Actually, that is why I have coax switches on all my antennas. After a received a surprisingly large shock as I disconnected a beam from my rig as a thunderstorm approached (still 15 miles away or so). At that point I decided to ground the antennas when not in active use. No use paying repair costs with money that would be better spent buying more radios and keys!
 
RE: Keeping it very simple.  
by AF6AU on March 6, 2013 Mail this to a friend!
Very interesting article. Makes me think of ARRL Field Day. Your shack sounds like what many clubs have during field day.

2 years ago My club ran a delta loop on 40 meters and a cube quad on 20 meters, rigs manned with ops running at the same time. I was on a Kenwood 570 for 20 meters SSB, and could hear the 40M crosstalk in the rig, and I wonder how much actual power crosses over. The quad swings around over the delta. But if you consider that at some point of the compass the loops of the antennas are not quite aligned, but still parallel, I wonder if that was partially the reason for the killing of a processor driven band actuation circuit in the 40 meter rig, an Icom 746 Pro, that morning. A well designed receive front end may handle the stray RF power, but maybe other components like sensitive MOS chips and tiny switching diodes elsewhere cannot. Perhaps a coincidence? Maybe. The 746 was replaced with a beater Icom 735 that took the RF abuse with stride.

How many Hams out there have had rigs die during ARRL Field Day events with close proximity antennas?

Hmmmm...
73's
AF6AU
 
RE: Killing Radios  
by K6AER on March 6, 2013 Mail this to a friend!
Most HF antennas depending on resonance, location and distance will have between 15-45 dB of isolation. This is almost never a problem between HF and VHF/UHF antennas.

At my location I run a 4 element StepperIR at 100 feet and have a G5RV connected to the TS-2000. It only becomes a problem if the radio is tuned to the same band as the other HF operation. At that point the radios bandpass filters are passing the same in-band energy. I just have to remember to switch the antenna switch to the grounded position for the second radio.

I notice on my wire line dipole that is connected to a Dentron 3000, I see energy on the reflected meter all the time. Most radios will handle up to +30 dBm of energy before the front end is popped. As for the fuse front end I would not hold much credence in that approach. In the radio world generally transistors protect fuses.

When you blow the front end or first mixer your radio will be numb about 20-25 dB.

In multi band contest site it is generally a good rule to have external bandpass filters (ICE) for each band. This cuts the IMD by a considerable amount.
 
RE: Killing Radios  
by N6AJR on March 6, 2013 Mail this to a friend!
If you use the switch with a ground position, or a remote rf switch that grounds the non used antennas, or even put each radio with a 2 position switch and when in use put it on position 1 ( antenna) and when not in use use position 2 (either no attachment, or a dummy load or such) so if not in use, switch out antenna.
 
Killing Radios  
by AC4WY on March 6, 2013 Mail this to a friend!
I'd say that if you need 53 transceivers you've got worse problems than crosstalk. LOL!
 
RE: Keeping it very simple.  
by K1CJS on March 7, 2013 Mail this to a friend!
"There is really no reason to constantly keep several radios connected to antennas, except in unusual situations...."

That is the best advice. So you would have to connect and disconnect your rigs, but they would be safer all around, and so would your shack.

On the lighter side, maybe if you stopped using fertilizer on your antenna farm, the number of antennas you have would be manageable, and the chance of between antenna crosstalk would be kept to a minimum! ;-) 73!
 
RE: Keeping it very simple.  
by AA4PB on March 7, 2013 Mail this to a friend!
"In the radio world generally transistors protect fuses"

Only when the circuit is not properly designed.
 
Killing Radios  
by NY7Q on March 7, 2013 Mail this to a friend!
"cross talk"?? So our radio/electronics verbage has finally been lowered down to "computer talk"
So sad that computers are killing radios and electronics.
 
RE: Killing Radios  
by AA4PB on March 7, 2013 Mail this to a friend!
"crosstalk" actually comes from the analog telephone circuits - well before computers were popular.
 
Safety first, last, and always.  
by AI2IA on March 7, 2013 Mail this to a friend!
I have seven rigs on my radio bench. All are ready and able to transmit and receive, except only one is connected to an antenna at any given time. I have a coaxial switch with a ground position connected to a good earth ground.

I also have a junction box attached to an earth ground with coaxial connectors on the box able to take six cables from rigs.

I never have a static voltage problem or a lightning problem, or an RF receiving overload problem.

My shack is equipped with a hefty CO2 fire extinguisher and on the ceiling above is a smoke detector. I never leave wall warts plugged in when not in use. I never charge batteries unless I am nearby.

I am a happy ham and I intend to stay that way by putting safety first, last, and always.

Be safe and be happy!
 
RE: Killing Radios  
by W4VR on March 7, 2013 Mail this to a friend!
How far apart are your antennas? When I transmit on one antenna, all the others are grounded, except for the beverage. The beverage is at least 300 feet away from any transmit antenna. I've never had any problems blowing front ends.
 
RE: Safety first, last, and always.  
by K6AER on March 7, 2013 Mail this to a friend!
"In the radio world generally transistors protect fuses"

"Only when the circuit is not properly designed."

I don't care who designs the solid state amplifier the fuses are always intact after the transistors short.
 
Killing Radios  
by K5SBR on March 7, 2013 Mail this to a friend!
This is a REAL problem, not just for the rich/famous and multi-multi's, but anyone with more than one receiver at a transmitter site. Not long ago, I replaced the rx front-end transformer in a receiver that had been connected to a 25 foot wire within 100 feet of a KW station. The litz wire in the secondary worked well as a fuse, since the radio was not powered at the time. No devices were damaged, just the input coil. Be aware and beware.
 
Killing Radios  
by K1WCC on March 8, 2013 Mail this to a friend!
I use a Delta-4 coax switch-a high quality switch.It has good seperation specs.
I leave both rigs connected to the Delta when operating (one radio at a time), and switch it to the COM position when done operating. The Delta is grounded to a good earth ground with #8 wire.
Whenever I'm away for awhile, or if the forecast is for stormy weather, I unplug the antennas from the switch. The Delta is set up with a surge protector which would probably work for one close lightning strike, but I'm not taking any chances.
BTW, the ARRL rig insurance policy is a bargain.
 
RE: Safety first, last, and always.  
by WB6DGN on March 8, 2013 Mail this to a friend!
"My shack is equipped with a hefty CO2 fire extinguisher and on the ceiling above is a smoke detector."

I use a Halon fire extinguisher instead of CO2 because I understand that it is more effective in fighting a fire. I realize that you have to "fire and run" as the halon will suffocate you in a confined area (as it does the fire) but it leaves no residue to have to clean up afterward just like a CO2.
Since my work area has a door to the outside, I consider this a safe alternative. I would not consider using one deeper inside the house, though.
Tom
 
RE: Safety first, last, and always.  
by WB6DGN on March 8, 2013 Mail this to a friend!
"My shack is equipped with a hefty CO2 fire extinguisher and on the ceiling above is a smoke detector."

I use a Halon fire extinguisher instead of CO2 because I understand that it is more effective in fighting a fire. I realize that you have to "fire and run" as the halon will suffocate you in a confined area (as it does the fire) but it leaves no residue to have to clean up afterward just like a CO2.
Since my work area has a door to the outside, I consider this a safe alternative. I would not consider using one deeper inside the house, though.
Tom
 
RE: Safety first, last, and always.  
by WB6DGN on March 8, 2013 Mail this to a friend!
"My shack is equipped with a hefty CO2 fire extinguisher and on the ceiling above is a smoke detector."

I use a Halon fire extinguisher instead of CO2 because I understand that it is more effective in fighting a fire. I realize that you have to "fire and run" as the halon will suffocate you in a confined area (as it does the fire) but it leaves no residue to have to clean up afterward just like a CO2.
Since my work area has a door to the outside, I consider this a safe alternative. I would not consider using one deeper inside the house, though.
Tom
 
Killing Radios  
by K1DA on March 8, 2013 Mail this to a friend!
I don't worry much about my Collins and Drake tube stuff. Four receivers on a 4 way splitter, never had a problem. Solid state, another matter, although a front end with bandpass filtering and a passive mixer is pretty tough.
 
Killing Radios  
by N4LRA on March 8, 2013 Mail this to a friend!
No Smoke, No Worry right?
 
RE: Safety first, last, and always.  
by K1CJS on March 14, 2013 Mail this to a friend!
"In the radio world generally transistors protect fuses."

"Only when the circuit is not properly designed."

Doesn't matter. Even if the circuit is designed properly, most transistors will short or open faster than a fuse can act. It isn't the fault of anything or anybody, it's just the way it is.
 
Killing Radios  
by W8LV on March 15, 2013 Mail this to a friend!
"In the radio world generally transistors protect fuses"

I have found this to be an "always true" sttement, no matter the design. The only way I have found to ward off front end receiver damage is to burn some incense, and place a few Radio Shack FETs nearby in a little tray far from the receiver as an offering to Electra, the RF Goddess, in hopes that she will take your tribute in lieu of your front end. (No doubt the cost of purchasing multiple "Electra Offering" FET's for all of those worldwide listening installations is a large part of the NSA's annual black budget... ) I have found nothing, however, will deter Thor the Lightning God, but that Puckish little God-Imp side kick of his, Staticus, can be somewhat appeased with an EMP rated lightning discharge unit.

But seriously, what do they do in Commercial/military installations to control this? What about all of the monitor receivers and "guard" receivers out there running 24/7? What if you live across the street from a Class A 50 kilowatt AM station?
 
Killing Radios  
by KA8DKT on March 26, 2013 Mail this to a friend!
Most modern transceivers have diodes paralleled front to back across the inputs to the input filters, specifically to protect the front end of the receiver.
Thus the input voltage never exceeds 0.6V or so.
Often when a receiver goes "dead" it is because one or both of the diodes have shorted.
-gary
 
Killing Radios  
by VA3OC on April 1, 2013 Mail this to a friend!
There is and old trick for multi transmitter contest stations and Field Day setups.The use of coax stub filters to attenuate the unwanted transmitted signals from the other stations stopping receiver de-sense or complete receiver overload. See the following link to get an idea of how it works:
http://www.k1ttt.net/technote/k2trstub.html
Building and testing them is fun but if you don't have the tools or capabilities here is one supplier:
http://www.qth.com/topten/Stubs.htm
There are other approaches to the filters using L C component filters. They are smaller but more limited to power handling. If you try the coax stub filters the lower the loss of the coax higher Q the more attenuation the filter will give. Use old helaix or similar if you can get it. Be sure of the velocity factor of coax in the calculation and be careful that switching and connection lengths to the transceiver have to be included.The filters could be applied at your antenna switch facing out to monoband antennas or you could choose to filter specific problems where two antennas are are in close proximity. The combinations are endless. Do some research on coaxial stub filters. If you find any really good sites please post them.
73 DE VA3OC Brad
 
RE: Killing Radios  
by VR2BG on April 2, 2013 Mail this to a friend!
Before I QRTed, at my last station I was able to transmit at full legal limit on one band of my tribander whilst at the same time listening with that tribander on another band (the drivers of the tribander were ~40 centimeters apart at the center [perhaps 10 cm at the ends] & fed with separate feeders).

Or instead with a 30/40/80m vertical that tips of the tribander cleared by only centimeters as it rotated.

And vice versa.

In extreme near-field interaction situations like that, even shorting the feeder inside the shack can be sensed by the change in SWR on the OTHER antenna.

Of course, doing that makes it impossible to listen on the other antenna whilst transmitting on the other, so isn't a solution. At least not for radiosport!

73, ex-VR2BG/p.
 
Killing Radios  
by KC6ZUT on April 6, 2013 Mail this to a friend!
I've killed the front end of one of my HTs just that way. Transmits just fine but it is now deaf as a post. I am more careful now. I am thinking about building notch filters to protect the front ends.
 
Killing Radios  
by KL7HIM on April 6, 2013 Mail this to a friend!
Make yourself a patch bay. Route all the radios and antennas to their own connector on a panel and then connect any one of the radios to any one of the antennas with a jumper. It's like a manual matrix switch. If you want multiple receivers on at one time, add an rf distribution amp to the panel the same way as the radios and antennas.
 
RE: Killing Radios  
by AE6RO on April 12, 2013 Mail this to a friend!
Modern solid state gear is oh-so-complicated and clever but oh-so-delicate. In the real world of lightning, and uh, other strong electromagnetic forces. Long live the vacuum tube! John
 
RE: Killing Radios  
by AE6RO on April 12, 2013 Mail this to a friend!
Electra seems to be a Greek Goddess of storm clouds and a source of lightning. Not to be trifled with, dudes. And of silver linings. John
 
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