WA Radio Scanner Users' Group

Hi Guys,

Does anyone know what type of duplexers are being used on the UHF CB repeaters around town? I went to speak to RFS about them but the minimum channel separation they support is 2.5MHz for an "off the shelf" product and the pricing for a custom designed set of can type cavities is pretty high. The 650KHz separation on 477MHz is a real PITA!

Is there a relatively easy way to do this by using dual antennas and a lesser specified filter?

VK6ZMB wrote:Is there a relatively easy way to do this by using dual antennas and a lesser specified filter?

Hiya.

Just like the 600Khz spacing on 2 metres. Argh! Too close!

I know that the Perth channel 8 CB repeater used to use seperate TX/RX antennas for years and still had cavity filtering. If you don't actually need to duplex into one antenna you can get further isolation by vertically seperating the TX and RX antennas. I think the ARRL handbook covers it with all the maths in the repeater section.

This is an ancient photo of the channel 8 repeater antennas... the photo distorts it but they were a few metres distant on the tower. (I think the upside down antennas was the RX)



73
Gavin.

Found a picture of the PER08 duplexer too. I think it cost them in the order of $3k around 6 or 7 years ago.

Thanks Gavin,

Those cavity combiners sure look familiar. The irony of all this is that for 3 years I was the National Sales Manager for Radio Frequency Systems and could have had as many "spare" combiners as I wanted at practically no cost. I wasn't really into radio then, sigh.

In the meantime, I have been doing some research and I think perhaps I am going to homebrew a set of notch filters and bandpass filters. My requirements are for low power (so high insertion is OK) narrow bandwidth and the noise floor is as low as it gets so I can probably get away with a lot less isolation that would typically be required for a split this close.

The antenna diversity isn't going to be an option since portability will be key here so I will just have to live with a 6 cavity combination but hopefully I can get away with 75mm cavities instead of the 150mm ones shown here.

And no, this unit isn't for operation in Australia and won't require ACMA licencing!

Marcus

VK6ZMB

Oh, and do you happen to recall what additional isolation the diversity on the tower gave to PER08?