Remote antenna switch (4 antennas, no additional control cable)

[A.Z.]

hello all, I was considering the idea of putting together a remote antenna switch and while seeking for ideas I stumbled upon this circuit

http://www.hayseed.net/~jpk5lad/K5LAD%20Memories/Vols%2031-35/Memories--Vol33.htm

which, I think, is a really smart one; as is it allows to select one amongst 4 different antennas and the whole thing is controlled over coax, so there's no need for an additional control cable



plus the circuit could be modified by adding an additional relay between the 12V line and the rotary switch and connecting the "normally closed" contact of such a relay to the power line, at that point, powering the relay through PTT would result in the antenna #0 being always used as the "TX antenna" while in RX the rotary switch would allow to select any of the 4 antennas (by the way antennas #1...#3 in such a case would be "RX only"), additionally it's possible to add a bit of circuitry at the remote unit to pick up the power controlling the relays and use it to feed whatever remote preamp (only for #1...#3 by the way), so I think that it's definitely a slick idea

now... my doubt; when selecting the antenna #3 the coax line will carry 12VAC, and I wonder if the AC hum may then be picked up by the antenna when receiving; so introducing undesired noise, do you think it's a real risk or am I worrying about nothing ?

[Ian.M]

The antenna and the transceiver are coupled to the feed by 10nF coupling capacitors.  Their impedance at 50Hz is nearly 320K, so the 50Hz is attenuated by several orders of magnitude due the impedance mismatch.    The receiver needs to be able to operate normally with a significant 50Hz signal on its input anyway, due to the prevalence of line frequency ambient E and H fields.   

[A.Z.]

Thank you ! And then, if I'd ever have "AC hum" issues, I may always add an high-pass filter at the shack side, the filter would be only inserted in RX (PTT relay) and could have a cutoff around 1KHz (just to stay on the safe side), but looking at your note, I don't believe it will be needed; thank you again

[A.Z.]

Here's a quick initial sketch of the modification I'm planning to do to the original circuit (didn't add components values, refer to the original schematic)



the modification allows to use the unit in default mode (with both switches open) or to allow for different modes of operation; by closing SW1 (and connecting the PTT circuit), the TX antenna will always be the #0 since when the PTT powers the relay, the latter will cut-off power from the remote unit so selecting antenna port #0, while in RX one will be able to use any of the four antennas.

By closing SW2, the separate TX/RX port feature will be enabled so that it will be possible to use a separate receiver, the "RX mute" contacts could then be used to "mute" the receiver when transmitting.

I believe that with such a simple mods the switch could become even more flexible; another mod, if desired, could be picking up the relay power at the remote unit and use it (with a little bit of circuitry) to power a remote active antenna or RF preamplifier, such an option could only be used with SW1 closed and PTT connected, in such a case antennas #1, #2 and #3 will be RX only ones, while the #0 will be the one used for TX (and, if selected, RX)

[fourfathom]

Some nitpicks: 

That "47pfd" capacitor across the green LED must surely be 47 uF (it's even polarized on the schematic).  And I would move that 100 Ohm resistor to go between that 47uF cap and the LED.

Depending on your frequencies of operation, you might want to add a smaller choke or ferrite core in series with those 25 mH chokes.  At least check the self-resonant frequency of the chokes you use.

I see no need for "600V / 1KV" rectifier diodes.  They could be 50V diodes and you would still have plenty of margin.

I also see no need for the 500V rating on those 0.1 uF caps used on the 12V side of the RF chokes.  The capacitors in series with the RF (at the input and output) probably don't need a high voltage rating either, but they do need to have a high current rating.  You can use multiple caps in parallel to improve the current-handling.

Be sure of your PTT timing.  You probably don't want to briefly start transmitting on your receive antenna before the relays activate.

Those LED series resistors look like they are sized for old low-intensity LEDS.  If you use more modern high-intensity LEDs you will probably want to increase the R values.

[A.Z.]

Some nitpicks: 

That "47pfd" capacitor across the green LED must surely be 47 uF (it's even polarized on the schematic).  And I would move that 100 Ohm resistor to go between that 47uF cap and the LED.

Depending on your frequencies of operation, you might want to add a smaller choke or ferrite core in series with those 25 mH chokes.  At least check the self-resonant frequency of the chokes you use.

I see no need for "600V / 1KV" rectifier diodes.  They could be 50V diodes and you would still have plenty of margin.

I also see no need for the 500V rating on those 0.1 uF caps used on the 12V side of the RF chokes.  The capacitors in series with the RF (at the input and output) probably don't need a high voltage rating either, but they do need to have a high current rating.  You can use multiple caps in parallel to improve the current-handling.

Be sure of your PTT timing.  You probably don't want to briefly start transmitting on your receive antenna before the relays activate.

Those LED series resistors look like they are sized for old low-intensity LEDS.  If you use more modern high-intensity LEDs you will probably want to increase the R values.

Thank you very much for the notes; it's still "all in my head", that is I didn't start building anything, but I very appreciate the informations; as for the PTT, I was considering to use a solid state relay for the one controlling the switch, given that it doesn't carry RF, it shouldn't be a problem, and then, by the way I'll need to check if there's a delay at the "accessory" port when the PTT is depressed, I don't think so, and if I recall it correctly, there's a delay when you release the PTT, not when you depress it, so it should be ok

[A.Z.]

here's a simplified version of the same circuit

http://n1al.net/ham/ant_switch.htm