No, not possible. There is really no substitute for a big expensive six cavity pass/reject duplexer for a VHF 600KHz split.
If you can find some affordable band pass cavities you can run 600KHz split with the transmit and receive antennas separated vertically and I've pulled that off with good success in the past. I've seen individual VHF cavities go for between $25 and $50 at ham swapmeets.
There will be a minimum amount of isolation you will need depending on your transmit power and receiver quality. I would say the minimum goal would be around 75dB with more being better and you can look up the specs of various cavities to see how they perform at 600KHz. You might get 10dB isolation with a single 5" cavity set for 1dB loss or about 15dB with two cavities. One cavity on the transmitter and two on the receiver would be about 25dB isolation.
20ft vertical separation with two antennas having good pattern rejection up and down might get you 45dB and the cavities mentioned above would be another 25 for a total of 70dB. With a GE Master II or III repeater operating at low power that might work. With a low quality repeater it may not.
You can add more cavities to increase isolation but at the cost of insertion loss. You also need specific tuned lengths of coax between cavities and if they didn't come with interconnect cables its a hassle to make and test them until you get the lengths just right for maximum performance.
In the end you will have spent some money and a lot of labor and you will wish you had just bought the big duplexer.
prcguy
I have a VHF repeater that I am trying to get on the air. My issue is that my budget is low, and I cannot find a surplus or used VHF duplexer around that can do .600khz spacing. I wanted to use a Chinese notch duplexer (I already know about them) but the min spacing is 3.8 mhz. Cannot get a frequency pair coordinated with that.
So, my question is this.. can I use two of the Chinese duplexers, one for each frequency, and somehow combine the two antenna ports out to the antenna? To achieve the .600 mhz spacing? With the Chinese ones going for around $100 each, it is something I could swing to get the project up and running.
Thanks!
