A FOOLPROOF ANTENNA SWITCH

[glovisol]

SIMPLE AND RELIABLE RSP RECEIVER PROTECTION

Please see next post, this done for amendment purposes.

[glovisol]

SIMPLE AND RELIABLE RSP RECEIVER PROTECTION

The RSP play specification for RF input is:

0 dBm - 1 mW - permanent maximum.
+10 dBm - 10 mW - short periods.

In a 50 Ohm system,
0 dBm correspond to an RMS voltage of 0.223 V and to a peak-to-peak voltage of 0.632 V.
+10 dBm correspond to an RMS voltage of 0.707 V and to a peak-to-peak voltage of 2V.

In my opinion a shorting relay is still the best possible antenna switch to be used to protect the RSP receiver input from local transmitted power. With a transmitter power of 1 kW (+60 dBm) and assuming a worst case transmit - to - receive antenna decoupling of 30 dB, we could end up with 1 W (+ 30 dBm) at the receive antenna terminals, which must be reduced by the antenna switch to less than 0 dBm, e.g. -10 dBm.

Therefore the switch must withstand a power of 1W and provide an attenuation of 40 dB or more. The proposed antenna switch provides a minimum tested attenuation of 70 db between 900 KHz and 29 MHz: this means that, with a power of 1 kW nearby, our RSP receiver input will be equal to or less than -40 dBm.

The additional advantage of using a a relay in place of switching diodes is total absence from intermodulation and spurious products that diodes in the antenna lead are alwayst at risk of generating in presence of strong signals, even far removed from the receiver's frequency.

Additional details in the next post.

[sdjack]

Waitung for the next post.

[glovisol]

SIMPLE AND RELIABLE RSP RECEIVER PROTECTION

The schematic diagram is shown below.

The switch is built around a miniature relay with contacts rated 10 A @250 VAC. +12V supply voltage is connected to terminals B and C and is monitored by LED D2, to warn about the absence of supply voltage which would render the switch inoperative. In this condition K1 is OFF and the antenna is connected to the receiver. As soon as a positive PTT voltage is applied to terminal A, transistor Q1 turns on, actuates K1 and connects the antenna to resistor R1, 1 Ohm/3W. Any power coming from the antenna terminal dissipates into this resistor. LED D1 monitors the PTT command voltage.

With the worst case value of 1 W coming from the antenna terminals, switching current must be limited to less tha 10 A with the limiting resistor R1: 1W produces a voltage of 7V RMS across a 50 Ohm resistor and a maximum (still safe) worst case current of 7A will flow through the relay contacts.

With a jumper between A and C the relay will switch by applying PTT only, provided the PTT voltage is above + 9V.

This antenna switch can be made totally foolproof by inverting the contact connections and keeping K1 always ON when the antenna is connected to the receiver(command phase inversion). With this scheme a failure in the power supply will not cause danger to the RSP.

[vk3alb]

Thank you to Glovisol for his excellent and elegant circuit. As he says in his text there is a way to protect the receiver in case of failure of the 12 volt supply to the circuit. I think my version of his circuit was his intention.

RX Isolator.png (45.28 KiB) Viewed 98591 times

Consider the modifications of his circuit as shown above.

First state: The power is off so the antenna is switched to R1 via the normally closed (nc) contact. RX safe.

Second state: The power is on. As in the original circuit, Q1 is held off by R3. Q2 on the other hand is held on by R4 so the antenna is now switched to the receiver via the normally open (no) contact of the relay. RX in operation.

Third state: With a positive voltage (PTT) applied to "A", Q1 is now turned on by the current through R2. This in turn pulls the base of Q2 very close to ground turning it off and in turn turning off the relay. Once again, the antenna is switched to R1 via the normally closed (nc) contact. RX safe.

[Roger]

The PTT output (sometimes marked SEND) on most ham transceivers goes to ground during transmit.

Roger

[vk3alb]

Hi Roger,

Yes I see that now. In that case, as long as the PTT (SEND) line can sink at least 10mA it can connect directly to the base of Q2. Then, remove D1, R2, R3 and Q1.

Thanks for the feedback.

[glovisol]

Hi Lou, excellent diagram which fills the bill for a failsafe antenna switch. I would only add 0.01 to 0.1 uF (not critical) bypass capacitors across the 12 V power supply input and between base and emitter of Q1 and Q2 to avoid malfunction due to excess RF rectification.

I would also add another 0.1 uF capacitor across the relay windings, in parallel with a reverse diode, to protect Q2 from inductive kicks. With negative going PTT, LED D1 should be placed in series with the base of Q2 with cathode connected to a limiting resistor. Finally, just to be safe, Q2 should be a TIP31 with a series diode in the emitter lead (1N4001) to offset the forward voltage of D1. Considering your much "cleaner" and clearer schematic, you should incorporate these refinements in your drawing and post it.

And yes, Roger, you are right: with ground going PTT, Q1 and related components are no longer necessary and can be eliminated, adding to reliability.

[vk3alb]

Hi All,

Thanks for the comments and suggestions. It shows that even in a simple circuit there may be room for improvement. Here then is the revised circuit with bypass caps and back EMF diode across the relay coil. I was lazy this morning and left them out. Not surprised that I was caught.

RX isolator 2.png (48.44 KiB) Viewed 98476 times

A much simpler circuit than before, but essentially the same behavior.

With no power the RX is safe.
With power applied RX is now in operation and the LED warns us that the relay is active.
With the PTT line pulled low, LED and relay are off and the RX is protected.

[n7esu]

If running a RSP2 would you have to build 3 of these ant switch's or incorporate 3 relays in parallel with appropriate circuit?