Post mortem review of a 144MHz combiner / splitter

This article is a post mortem review of a 144MHz splitter combiner that was made using RG6 coax. It is post mortem (ie post death) because the combiner was stored outdoors without checking that the connectors were protected from weather.


The combiner was used successfully for over 10 years on a 144MHz four over four antenna system (above) without any maintenance problems.


Above is a close up of the Tee point of the network. The coax cables are protected by HDPE sleeving to reduce the chance of damage at the hands of Sulphur Crested Cockatoos, in the event there was no damage.

Note that a combiner using quarter wave of 75Ω coax transforms two 50Ω antennas to 56Ω, or two 56Ω antennas to 50Ω… so best to match the Yagis closer to 56Ω than 50Ω when used with this match. You can match two 50Ω to 50Ω, you need quarter waves of (50*100)^0.5=70.7Ω.

As mentioned earlier, the combiner became spare on the move to Bowral and was stored indoors at first… but was moved outdoors without checking thoroughly (these BNC connectors are waterproof… only when mated). The rotator loop section was fitted to the box properly, and the other end of that cable was properly capped. The two BNC connectors to the antennas were not capped (they should have been joined in a F-F bullet).

comb02Above is a picture of the salvaged Tee box. There are signs of surface corrosion on the die cast box, but the connector is in excellent condition and resistance from connector body to box is very low, no sign of corrosion, it was assembled with a thin smear of marine grease to exclude water and oxygen.

The cover came off cleanly, screws and threads in excellent condition (again protected by marine grease).


Above, a view of the interior. No sign of corrosion, the weep hole strategy has worked in preventing vapour pumping filling the interior with condensate.

comb04Above is one of the BNC connectors with cable cut and stripped back. The cable is dry and corrosion free, the braid and foil are bright a free of any signs of corrosion.

comb05Above a view of the mating end of the BNC connector reveals white corrosion products on the outer surface that makes contact with the inside of the outer conductor of the female part. That surface is protected by the gasket when they are mated. Though measured resistance from cable shield to this metal is low, the surface corrosion results in a high resistance and unreliable connection to a female part. The combiner’s non-working status is entirely due to this corrosion problem.

The combiner can be repaired my making two new RG6 sections with new connectors (the existing connectors cannot simply be changed as they are tuned lengths of line).

In review, the design worked very well. It was easy to tune in the first place, used low cost components, was weather resistant in service, and would still be good if it had not been left out in the weather with two uncapped connectors.

A footnote: I capitulated and replaced the two coax tails and BNC connectors… a total cost of less than $3 and half an hour’s work and it is available for use if / when I need it.