Latest Full-Duplex Node Build

Thought I’d post some pics of my latest full-duplex node build. See the following for details:
How To - Build a High-Quality Full-Duplex AllStar Node for Under $150


This is a neat setup, great job!

Jim, K6JWN

Nice, neat, compact example.

I might add for those that desire better full duplex performance,
If you stack the radios vertically as opposed to horizontally, you will limit de-sense of the receiver and thereby extend the usable ‘full-duplex’ range. Adding some more distance between them is also a plus.

The area above and below the antenna’s have the least concentration of signal.
Having a more profound effect where your freq pair are closer together like a 2m 600khz split.

Mike, in theory it is true that orienting the antennas axially would minimize interaction, however by having one radio and antenna higher than the other, cabling (at Ground potential) would have to pass across the length of the lower antenna, likely resulting in a significant performance impact on the RX antenna or a null in its receive pattern. Also if you stacked the radios vertically the node would be quite tall, and thus not very portable or easy to move around. BTW the distance between the antennas will not make much difference when < 1/2 - 1 λ (1-2m). Thus if the antennas are ~8" apart as here, or even 2-3x that, performance will likely not be much different. Most radios eg. cross-band full-duplex HTs use just a single antenna in fact ie. zero spacing.
In all these cases I can be miles from my house and the node picks up 500mW very well on 70cm even though it’s transmitting 1.5W on 2m.
This is a simple low-cost node intended for home/mobile use within a range of a couple miles and works great for that. If you wanted more like 10+ miles of range that would be a different story and in that case I would either use a cross-band full-duplex mobile radio, or run the HTs into a good quality duplexer and outdoor antenna.

I was not suggesting any cabling.
Just place them in line vertically is simplest no cost solution to the same.

To have 2 radios in a line vertically would require a cable or cables of some sort to go past one of them. Both radios have power and ground wires and an audio cable that has to go to the node electronics. Even if you put the node computer and radio interface between the radios, there would still then be a power and ethernet cable going past the lower radio. Anyhow just a minor detail. It would be an interesting experiment to build a full-duplex node with the radios stacked vertically and see how much performance difference there is. I suspect the net result would not be much different though, and have noticed zero desense issues in the current configuration. The 3rd harmonic of the 2m Tx frequency is several MHz away from the 70cm receive frequency and these are FCC-certified HTs whose harmonics should be well filtered.

I have done this many times to produce emergency portable repeaters for field use in the 90’s.
I live in a multi valley terrain where the PS radios do not function. I used them in search and rescue.

Yes in that sort of application a larger antenna configuration would definitely be good. Though I wonder in that case what the performance difference would be to instead use a duplexer with 1 nice high-gain antenna at the highest practical height with no obstructions…

Finished testing the node today and am now packing it up for shipment to the buyer (a ham in AL). I did need to add one additional ferrite filter on the Rx audio cable. Round-trip (parrot test) Signal-to-Noise Ratio on this node is 48dB. This is quite impressive considering that very few radios are capable of that even without being connected to a node computer, USB interface, and power supplies. (In fact you almost never see published SNR specs on radios and on other nodes because they’re usually pretty bad.)