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Sunday
Oct092011

New TX/RX duplexer arrangement at VK5REX

About a month and a half ago I took the plunge and bought a "cheap" Chinese notch-style mobile duplexer via eBay.  I had heard how other D-Star repeater owners had used them to augment their existing cavity duplexer to improve the TX-RX isolation. The price on eBay was low enough that if this "experiment" didn't work... I wasn't out of pocket too much.  My eBay experience was good, the vendor looked after me and also tuned the duplexer to my nominated frequencies before shipping.

Prior to installing this notch duplexer I had split the exiting cavity duplexer and was running with two identical folded dipole antennas vertically separated... I had no luck getting the cavity duplexer to work satisfactorily on a single antenna, even with the addition of the 1MHz wide DCI bandpass filter. If you look back through my YouTube channel you can find a video showing how I tested for receive desensing.  The main remaining issue was due to low-level broadband noise emanating from the D-Star transmitter affecting the receiver. Without notching the receive frequency in the transmit path... I was still experiencing ongoing trouble with weak signals.

The combination/hybrid notch/cavity duplexer gives me the best of both worlds. Each side of the notch duplexer has a very nice narrow but deep notch tuned to the "opposite" side frequency, but it does not really stop out of band transmissions from reaching the repeater receiver socket.  By itself, the cavity filter duplexer didn't give enough isolation between the TX and RX frequencies for the Icom equipment to be "happy". The cavities do keep other on-site RF crud away from the receiver input and also help me be a good RF neighbour.  Combining the the notch duplexer and the cavity filters works very well.  The 1 MHz wide DCI filter in the receiver leg is added insurance and the AAR preamp helps claw back some of the losses from all this filtering.

I still have to do proper "scientific" measurements and tests but initial real-world testing seems to indicate improved performance, even better than the two antenna arrangement.  When I was using two antennas, I had the folded dipole at the top of the tower receiving and the one on top of the building transmitting.   Now I am using the single antenna at the top of the tower (~45m) as the TX/TX antenna.

Since the addition of the notch duplexer, I find that I can access VK5REX B in more places that I ever could before. I use my IC-91AD for everyday D-Star operation and was unable to reliably access VK5REX B from inside the building at work (~6km away) even on high power... now I can wander around inside with the radio on low power and talk to my hearts content. 

I have been able to access VK5REX B with the same handheld radio on low power connected to an 8 element commercial UHF Yagi antenna from over 35km away with no line of sight between the two locations.

One word to describe the increase in performance?? "Outstanding!"

 

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Reader Comments (1)

Analogue repeater controllers using Raspberry Pi, low power Linux box.

Do you have experience, or intention?

VK2ZIW

beardal (at) unixservice (dot) com (dot) au

February 21, 2014 | Unregistered CommenterAlan Vk2ZIW

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