Updated 11Sept 2018

6M band helical diplexer   VK5RDX then VK5RSB



By Adrian VK5ZBR 2004

Back in 2004 I want to build a 6m repeater to play with on FM.


I was in the South Coast Amateur Club at this stage and they let me put the repeater on their Chandlers Hill site.    


One part of the repeater system is the antennas.  Do I use one or two antennas.


The main disadvantage of two antennas is the cost of two of everything.


One problem with a single antenna or two was I still needed a filters, the TX and RX frequencies are only 1MHz apart.


How Hard could it be.

Well it took me 6months of playing around to get  what I needed in isolation.  Lots of used coax and scrap copper in the bin trying all sorts of designs.


Some of my designs I melted the acrylic formers I made for the helical's.  Other designs I could not get to tune or were very hard to tune.




First two photos show my second design using the larger helical cavity to get more Q.  I made them from copper for easy prototyping.




I ended up using aluminium for weight and cost.


The cavity box's I bend up was with a new bender I made for the job. 



 I then anodised the box sections at home to protect the surface.

Anodising aluminium is very easy and finishes the cavities really nice. These photos are taken of the filters after 8 years of service, they still look new.



The helical coils are  made from 6mm copper tube.  Easy to bend and light weight.  I never tried aluminium tubing but may work, the losses may be higher.  The copper tube was polished before the were rolled into shape.  After I soldered the bolts to the coils I dipped them in clear lacquer.


The original version of this design used copper bar, this would work harden as I bend the bar around a mandrel.  This made it to hard to bend.





The mounting nut's and bolt's are made from hexagonal aluminium bar.  These I turned up on the lathe then use a tap a die to form the thread.   I was worried about dissimilar metals effect on the cavities you see.

Left you can see the helical units I made up as a diplexer unit. One antenna for the transmitter and receiver. this configuration  has one transmit filter and 3 receiver filters. 

This gave me the isolation I needed for 25Watts on transmit with no de-sence on the receiver.



I found out a short while into testing the were a little microphonic, I reduce this down to minimal by place Styrofoam spacers between the cavities box walls and coils.  This stopped all movement and fixed the problem.


The reason I didn't use a former for the coils was losses. I did measure quite a few dB of losses in my trials and this cause the Q to drop.


At some stage I will go through my notes and draw up some plans to show how to make them, its been years so good luck to my self.


The Summertown repeater VK5RSB has been for at least 15 years been using a new diplexer built by AREG members Peter VK5TZX and Graham VK5GH, these use helical loaded cavities.  These are 1.5m high.  these have a higher power rating and the losses are down to 1.2dB compared to my 2.3dB.  Since we wanted more power and less loss we switch over to these.


This whole project started because someone said it can’t be done.  Don’t every say that to me :)   it’s like calling Marty Mc Fly chicken.


My secret weapon was a good group of smart ham friends and the Handbook of Filter Synthesis, chapter nine has a helical filter design section.  Some one lent me the book and I copied the whole lot, so it must of been too expensive for me at the time. 


My test equipment was quite crude at the time.  I had a analogue signal generator then upgraded to a Marconi analogue service monitor  and a Electronic Australia spectrum analyser kit using my CRO. The home made stepped attenuator was the key in calibrating the spectrum analyser this was a project in it own right.   I would manually plot my results on paper.  


I did a lot of work on a 10m repeater diplexer with a 100Khz split,  But I gave up after many hours.  I got as good as 250kHz split.  If get really bored one day I may give it another try, the maths said it’s possible with enough Q.



73's Adrian VK5ZBR

Prototype helical filter with anti resonance notch can for sharp skirt.