Antenna sharing with transceiver, what I am doing

Useful information regarding antennas for SDR products.
TOROSERVICE
Posts: 7
Joined: Tue Jan 09, 2018 3:12 pm

Antenna sharing with transceiver, what I am doing

Postby TOROSERVICE » Wed Jan 10, 2018 11:24 pm

Consider this still a bit "experimental" but I think I'm on the road to success, and thought I'd share

In another thread, I explained how I'm wanting to use the RSP2pro as a diagnostic tool for signal strength evaluation. That hasn't changed.

But I also want to use it as a panadapter for my HF rig.

Here's what I've got going right now. The rig is an ICOM IC-7200 and AL-811 amp. I've been playing with a receiver, an ICOM IC-75, sharing the same antenna as the IC-7200. Results have been ok using the MFJ-1708 antenna switch. More on the MFJ 1708 later.

Timing is everything, and the IC-R75 has a "mute" function with kills the front end of the radio. Basically, it shuts off the +5vDC power going to the front end. That, along with the MFJ-1708 has given great results, but .... I've been thinking about the RSP2pro and its lack of a mute function.

So here's my simple work around, and remember - timing is everything.

Instead of letting the IC-7200 key the MFJ-1708, I have completed a little relay box, to pre-trigger the MFJ-1708.

What I did was build a small box with two relays in it. There are corresponding ICOM mic jacks on the box (about $6.00 each). There are input and output RCA jacks on the box (10 for $7.95 on Amazon). I built up a short "jumper" to go from the box to the IC-7200, which used a bit of HeilWire and two ICOM mic plugs (maybe $10 total). There is also a 12v barrel jack ($1.50) on the box. Inside the relay box are two 12vDC DPDT relays.

Here's how it works. I plug the ICOM mic into the "box" and tap into its PTT line. That line is routed inside the box the coil of one of the relays. The NO contacts of the relay are wired to trigger the MFJ-1708. The remaining set of contacts on that relay energize the coil on the second relay. That relay's NO contacts are used to key up the IC-7200. In that manner... the second relay is acting as a time delay, while the first relay triggers the MFJ-1708. Its a bit of insurance, and cheap, and easy to do. My mic does not use the +8vDC supplied by the IC-7200, so I've got a capacitor to block the DC and allow the AC of the mic through. I used a Sprague Orange Drop, .01uF @ 600v, but that's because I had one handy! Any old ceramic or silver mica or polyfilm capacitor of similar value should do. Err on the large capacitance if need be.

In operation, the mic triggers relay #1, which does two things. First it triggers the MFJ antenna switch, and also triggers relay #2. Relay #2 gets the radio into transmit mode. I've got enough time delay built in, to give it "insurance" upon pressing PTT

But wait... how about when you release PTT?

No problem! The MFJ-1708 provides a 0-2 second time delay to switch back to "recieve mode". So, just set the delay to something reasonable to assure the PTT is off, the radio has stopped transmitting, and your amp is off, before allowing the transfer of the antenna back to receive mode.

So we've got a bit of extra time delay on xmit, and a bit on recv too.

This all has worked exceptionally well on the IC-R75. I'll know in a day or two about the RSP2pro.

A little insight to the MFJ-1708. There are two models, the 1708 and the 1708SDR

The "SDR" model has a jumper that shorts the incoming from the transceiver to the antenna. This is to allow the transceiver to "receive" even as the SDR is acting as a panadapter. There is a 3db loss in so doing, not a huge deal for receive. If you get the regular "1708" you can install the jumper yourself. Again, no big deal. The "SDR" model also nixes the regularly unused switching port that the normal model 1708 has. They both list for $79.95 and more my money, I got the normal model, as I don't need a jumper. But if you do, just add it and keep the switching port.

As extra-extra insurance, I put two 6amp diodes between the receive antenna port's center and ground. Should something unthinkable happen, those diodes will give a little bit of extra protection to the receiver.

Well that's about it.

This is long, but hope it can help someone.

Charlie B, K1VCT
Last edited by TOROSERVICE on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Roger
Posts: 82
Joined: Fri Dec 29, 2017 7:59 pm

Re: Antenna sharing with transceiver, what I am doing

Postby Roger » Fri Jan 12, 2018 7:34 pm

A couple of comments....

1. SDRUno and SDR-Console both have a mute function in the software that will operate if you have a CAT connection with the transceiver. When you push the PTT button on the transceiver it will send a Tx command via CAT to Omni-Rig which forwards it to the SDR software. The SDR software mutes the radio and puts in 30 to 60 dB of attenuation so that the AGC can recover quickly when PTT is released.

2. Here is a link to the TX timing on the 7200 http://www.w0qe.com/amplifier_timing.html. You can see that there is the possibility of RF being output within 3.6 milliseconds of Send going low. Typical switching time for relays is 5 to 15 ms. On SSB speech this would not be as much of a concern because of the natural operator delay in speaking after PTT is pressed. However for AM, FM, RTT or CW RF could be output very quickly. There is a delay after Send goes high so this does not pose a problem when it comes to receiver protection.

3. The MFJ-1708SDR is not well designed as an SDR switch for several reasons.

- The schematic diagram shows that the transceiver is ALWAYS connected to the antenna http://www.mfjenterprises.com/support.php?productid=MFJ-1708SDR. This is different than the 1708 model which disabled receive during transmit. The 1708SDR uses a jumper between the Ant and Transceiver sockets. During receive the Transceiver and SDR receiver are DC coupled together and to the antenna. As noted above the switching time is not specified for the MFJ relay but delays of 5-20 ms. are typical. So for a few milliseconds the SDR receiver could be getting RF from the transmitter. This was not the case for the 1708 model

- Since the transceiver and SDR are DC coupled the user has to be careful to never accidentally turn on the Bias T, on an antenna port with this feature, in the software. If Bias T is activated 5 Volts will go into the front end of the transceiver. If the transceiver receive section has AC coupling this will not be a problem but if DC coupled or equipped with back to back protection diodes on the input this could cause a component failure. MFJ should have used AC coupling to mitigate this and used a .1 microfarad capacitor of suitable voltage rating instead of a wire jumper.

- Connecting both receivers to the antenna in a T arrangement is not good for several reasons. Receivers have local oscillators, microprocessor circuits and switching power supplies in them and can radiate undesired noise and signals out their Rx antenna port. Connecting one receiver directly to another means they will each receive this undesired spurious and noise from the other. Secondly, there is a misconception that there will only be a loss of signal strength of 3 dB. This is only true if both receivers have an input impedance of 50 ohms. This will only be the case with well designed receivers and if BOTH receivers on and are tuned to the same frequency band. If the impedance of one is less than 50 ohms the other will have much lower received signal strength as shown in this graph. https://drive.google.com/file/d/1r61IoHewQKbJlszmbOdFpXVIoXqitovw/view. The solution to both of these problems is to use an RF splitter to feed both receivers. This will give RF isolation (typically 30 dB) and impedance isolation between the two receivers. Commercial units are available or they can be built by homebrewers using a toroid.

4. The use of back to back protection diodes needs to be done carefully. They need to be high switching versions so that they can react to high frequency signals (especially if VHF radios are used in the shack). There needs to be resistors or a fuse element in the circuit in order to prevent burnout. Also the user needs to be aware that diodes are inherently a non-linear device and will generate intermodulation products which are highly undesirable even at signal levels much below their saturation voltage. Given that SDR receivers have a wide bandwidth front end and signals can sum together this can become an issue in areas with high power stations nearby. This is why high performance receivers do not use them.

Roger Need

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TOROSERVICE
Posts: 7
Joined: Tue Jan 09, 2018 3:12 pm

Re: Antenna sharing with transceiver, what I am doing

Postby TOROSERVICE » Sat Jan 13, 2018 12:09 am

1. SDRUno and SDR-Console both have a mute function.....

I reasonably certain that the SDRuno mute function does not shut off the front end of the RSP1/2 as the ICOM circuit does with the IC-R75. Also, to the best of my (current) knowledge, the 7200 doesn't have CAT capability, but... I could be full o' prunes. Either way, I believe the timing issue exists.

2. Here is a link to the TX timing on the 7200 http://www.w0qe.com/amplifier_timing.html. You can see that there is the possibility of RF being output within 3.6 milliseconds of Send going low. Typical switching time for relays is 5 to 15 ms. On SSB speech this would not be as much of a concern because of the natural operator delay in speaking after PTT is pressed. However for AM, FM, RTT or CW RF could be output very quickly. There is a delay after Send goes high so this does not pose a problem when it comes to receiver protection.

You are correct. The 3.6 or 5.5 millisecond delay (off the ACC connector) is mighty fast. I don't trust any of it, and frankly I'm rather concerned about hotswitching the ampat 3.6 milliseconds. The release delay isn't a while lot better.

That's why I decided to activate the antenna switch from the mic PTT, and also cascade the activation of the transceiver. There's that 3.6ms, plus the relay say at 10ms, and whatever the time is between PTT activation and the RF envelope, which isn't stated but bound to be a few more milliseconds. Add it all up, maybe I've got 16-20milliseconds between activation of the antenna switch and RF envelope. Timing stays put for the amp. On release of the mic PTT, the relay activating the switch will trasfer first, then the latency time between release of PTT and no RF envelope. That time is not stated, except, in terms of the SEND relay, which I'm not interested in, because its activating the amp. Because of that, a small delay, hardly noticeable, on the MFJ-1708 should insure that the PTT-Envelope delay has occurred before switching back.


3. The MFJ-1708SDR is not well designed as an SDR switch for several reasons.
- The schematic diagram shows that the transceiver is ALWAYS connected to the antenna http://www.mfjenterprises.com/support.p ... FJ-1708SDR. This is different than the 1708 model which disabled receive during transmit.


I think thats what I said. And also a good reason to key the 1708 series off the mic PTT

- Since the transceiver and SDR are DC coupled the user has to be careful to never accidentally turn on the Bias T


Very good point! Best to use Port A only from the RSP1/2

4. The use of back to back protection diodes needs to be done carefully. Also the user needs to be aware that diodes are inherently a non-linear device and will generate intermodulation products which are highly undesirable even at signal levels much below their saturation voltage.

Yes, choose diodes carefully.
Last edited by TOROSERVICE on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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