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Author Topic: Multiple Radio/Antenna Safety  (Read 1069 times)

Offline i_hear_you

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Multiple Radio/Antenna Safety
« on: June 28, 2019, 1901 UTC »
I'm looking into my first mobile install with a CB and a VHF radio.  I've also just acquired a second HF radio and am interested in using one for "ears" and the other for mostly TX.  I'm looking for some advice, feedback, and/or life experience regarding ensuring systems don't harm each other.

Questions I have:

-For radios and antenna systems in the same band (e.g. 20m) what sort of distances between TX and RX antennas are generally safe, and where on the wattage spectrum does danger of the TX hurting the RX radio approximately begin?  I've been using the pl880 with the antenna retracted and only TXing at most with 15w to test my sound, but I've been paranoid I'm hurting the pl880.  I intend to be TXing with 100w.

-For radios and antenna systems in different bands (e.g. 11m and 2m) the same question as above, and this will be on a vehicle so spacing will be tighter.  I don't suppose I have to worry much about the 4w from CB hurting the VHF, but what about 75w going the other way? Also, are transistors less vulnerable to damage from something like this if they aren't in use?  Would turning the CB off while using the VHF help protect it?

-Recommendations for a TX/RX radio switch are appreciate, or inline attenuators for the RX radio.  I believe these would allow one to blast away on TX and ensure a damaging voltage doesn't make it to the RX radio, but I have no experience here.

Offline ThaDood

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Re: Multiple Radio/Antenna Safety
« Reply #1 on: June 29, 2019, 1918 UTC »
Hmmmmmmmmmmmmmmmmm... In my truck, I have a 2M 1/4-wave MAG mount about 2ft away from my Lil' Wil CB Antenna. The CB is a stock Uniden Pro520XL and the 2M rig is an older Kenwood TM-241A, with MAX OUT at 32W. Neither rig interferes with each other, but the CB does go over the trucks Chinese made stock in-dash stereo. (Hmmmmmm, rarely a problem with AC Delco stereos I've had in the past.) 2 years ago, I've tried a B-Tech UV-24X4 multi-band VHF / UHF rig, and the CB played total havoc with it, BAD. So, I took that B-Tech rig out, and put the Kenwood back in. Might as well use what works. Your problem might be a big one with the Tecson PL880. With little, to no protection on the front-end, you might fry some MOSFET's. What can you try? Add a coupler with parallel opposing ultra fast switching diodes on the ANT IN of the PL880. The diodes will shunt RF to GND when they reach .7V. Where did I learn this trick? From those that have protected the ANT IN's on Sony ICW-SW2010 portables. I did this also to my Sangean ATS-803A portable. Try a search on the Sony 2010 Yahoo Group's page for this, and apply that technique to other RX only radios. The other transceivers? Well, modern ones have PIN diodes already installed for super quick TX / RX switching. Now, whether they can protect the RX of a transceiver from high ANT RF IN is debatable. (Could someone else ring-in on that?)
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline i_hear_you

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Re: Multiple Radio/Antenna Safety
« Reply #2 on: June 29, 2019, 2250 UTC »
Thanks for the input. The tecsun wont be the ears, but I suspect the advice suits the radio I intend to use.


Offline Josh

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Re: Multiple Radio/Antenna Safety
« Reply #4 on: July 01, 2019, 1852 UTC »
On diodes worthy of rx protection circuits, the 1n4001 has a PIN structure, wich is great for such use, far less likely to create imd when saturated than a PN diode, and most 4001s are designed to withstand much higher voltages than the common 1n4148 and similar PN switching diodes. In areas where lightning strikes are common, don't expect small signal diodes to last very long.

Gas discharge devices should also be considered, you can get them with leads to hard solder into a circuit, they're very hardy and can withstand huge amounts of voltage and current for brief periods self resetting unless they're blown, when they blow they short permanently. Some guys just pop in a ne2 neon bulb, and enjoy the light flashes that leak out the case of the rx protector when tstorms are around. Prob with ne2s devices is they arc pretty much after the voltage has risen high enough to damage semiconductors, hence the employment of diodes. Purpose made gas discharge devices are biased at the factory to have a clamp action almost fast enough to act on hanb/nemp.

Using diodes to clamp at a lower voltage and gas dischargers to take the brunt of a large spike is a good idea.

As I'm lazy, I use these factory made things;
https://www.polyphaser.com/solutions/hemp-tested
https://www.mfjenterprises.com/Product.php?productid=MFJ-270
We do not encourage any radio operations contrary to regulations.