XPost: rec.radio.amateur.moderated

Amateur/Ham Radio

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Spent More Than $188 On A $23 Radio And I'm Happy About It

Posted: 17 Jun 2022 04:48 PM PDT https://www.reddit.com/r/amateurradio/comments/vetfha/spent_more_than_188_on_a_23_radio_and_im_happy/

submitted by /u/mwfoutch1
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[OC] How to isolate and by how much?

Posted: 17 Jun 2022 06:00 PM PDT https://www.reddit.com/r/amateurradio/comments/veur4f/oc_how_to_isolate_and_by_how_much/


If you connect the antenna ports of two radios together and transmit from
one into the other, that would be bad, right? Just how bad would it be and
what could you do differently?
Before I dig in, you might ask yourself why on Earth this question even
arises.
Consider having two radios and one antenna. You couldnt use a T-piece to connect two radios to the antenna unless both were receivers. So, after connecting and disconnecting coax for a decade, you might decide to use a
two position coaxial switch instead. Set the switch to one port and the
first radio is connected to the antenna, flick it to the other port and
youve just avoided swapping coax between radios.
Ill point out that in most cases a coaxial switch can be used to connect multiple antennas to one radio, or in reverse, connect multiple radios to
one antenna.
When you do start looking for a switch it would be good to test that at no point it connected any two switching ports together, potentially causing
the magic smoke to escape from your radio.
A less obvious issue is that a coaxial switch has a property called
isolation. Its a measure of what part of a signal leaks between ports and
youll see the isolation or cross-talk of a switch described in decibels or
dB.
If you recall, a dB is a relative measure. It means that its something in comparison with something else, in our case, the amount of signal going
into one port compared with the amount of signal leaking through to a disconnected port.
Youd think that in a perfect switch none of the signal would leak through,
but it turns out that under different frequencies a switch responds differently, even one specifically designed for switching radio
frequencies. It might be that a 1 kHz signal is completely isolated, but a
1 GHz signal is not, which is why when you look at the specifications of a
coax switch, youll see something like "greater than 70 dB isolation at 200 MHz". Its worth noting that the lower the frequency, the higher the
isolation, indicating that in the worst case, at 200 MHz, theres 70 dB isolation, but at lower frequencies it has higher isolation, sometimes much higher.
If you were to transmit into this switch with 5 Watts at 200 MHz, the
amount of signal that can leak through would be 70 dB less than 5 Watts.
You might recall that you can convert Watts to dBm to allow you to do some interesting calculations. As with other dB scales, its in comparison to something else, in this case a dBm is in reference to 1 milliwatt and 5
Watts is the equivalent of 37 dBm. This means that if you had a switch with
70 dB isolation, youd start with a 37 dBm transmission, take 70 dB
isolation and end up with a -33 dBm signal leaking through. Thats the same
as 0.0005 milliwatts. In other words your 5 Watt transmission leaks through your coax switch to the tune of 0.0005 milliwatts.
Is that enough to damage your radio?
Well, that depends on the radio, but lets put some numbers against it.
S9 on VHF and UHF was defined in 1981 as -93 dBm assuming a 50 Ohm
impedance of your radio.
So, our leaking signal, -33 dBm, is 60 dB higher than S9. Youd report it as
a 60 over 9 contact, a tad excessive, but not unheard of. So by that
metric, you should be fine.
Many, but not all, radios specify the maximum radio frequency or RF power
that they can handle. For example, according to the documentation, both the NanoVNA and a Icom IC-706 can each handle a 20 dBm or 200 milliwatt signal without doing damage. That means that your -33 dBm signal shouldt do any
damage to those two devices.
Im off to see what the isolation is for cheap 12V relays to see if I can construct a cost effective, modular, remote control antenna switch with lightning detection.
What are you building next?
Im Onno VK6FLAB
TL;DR This is the transcript of the weekly Foundations of Amateur Radio
podcast - for other episodes, see http://vk6flab.com/ submitted by
/u/vk6flab
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