Antennas

Passive Repeaters

This article was inspired by a talk recently given to Coventry ARS by member Iain G7III. It reminded me of some conversations I had with Phil G4CFG and a QRP club member who's name and c/s I can't remember. ( It was over twenty years ago!) He asked me how he could get a VHF signal out of the Yorkshire dale he lived in, so I suggested a passive repeater.

What is a passive repeater? Basically it is two antennas connected back-to-back. Usually each antenna would be a high gain type eg a Yagi or dish antenna. The technique is best suited to VHF, UHF & SHF.

What can they be used for? They can be used to get around or over an obstructed path.

Advantages: They do not require a power source. They don't require a licence or NoV. They can be used simultaneosly by multiple users ( using different channels/frequencies ). They can be used with ANY mode of modulation and will re-radiate mixed modes simultaneously. They do not suffer cross-modulation, unless faulty or poorly constructed*. They can also be used to change the polarization of the incoming and outgoing signals. They do not suffer from de-sensing,so can easily used in full duplex mode ( a pair of repeaters say one on 2m & one on 70 cm is required.

*If an accidental diode ( “rusty bolt”) is included in the antenna then there is a possibility of cross-modulation.

And disadvantages: The re-radiated signal is nowhere as strong as the re-transmitted signal from a conventional repeater. In fact the signal re-radiated/received will be directly related to the original transmit power of the user. You need access & permission to a site near the obstructed path which can be “seen” by both Trx A and Trx B

How to make one. Simply erect a pair of antennas at a location which can be seen by both Trx A and Trx B. Join the driven elements with a short piece of co-ax. Point one antenna at Trx A and the other at Trx B. This can be used to get your signal over a hill on a straight path or around an obstruction on a dog-leg path.

An alternative construction is to use a bidirectional Yagi ie a Yagi with no reflector, but two sets of directors in front and behind the driven element. No co-ax necessary, thus reducing feeder losses. This is best suited to getting over a hill/building in a straight line but a dog-leg could be accomodated by using an assymetrical bidirectional antenna ie one side with many elements and the other side having only a few, thus giving a wide beamwidth on one side enabling a dog-leg path to be worked.

Of course dish antennas need to be connected by co-ax or waveguide.

The improvement in performance will be a reduction in path loss equal to the combined gains of each antenna. For example two Yagis of 10 dB gain will reduce the effective path loss tby 20 dB giving at least three and a half S points increase.

Does it work? YES! Iain, G7III did some experiments using HB9CV antennas for 2m and 70 cm. He placed his passive repeater on Riddian Bridge which enable signals to be worked from The Dilke PH to Lime Pits, a short but obstructed path. These locations are on or near the Daw End branch of the Birmingham Canal Network. See here ( Passive_Repeater.pdf) for details of experiment.

I've also done some tests of my own. I have been accidentaklly using a passive repeater for years. By pointing my 2m beam antenna at a repeater 5 miles away in Leamington, I can use my low power hand held to access the repeater from anywhere in the house with any polarization. My beam is a 9 X 9 crossed Tonna which is phased for circular polarization. Using the repeater as a beacon, as I turn the beam I can watch the S-meter rise and fall as the signal from the repeater goes through peaks and nulls on my beam antenna. I've also tried hiding behind a metal central heating radio and turning the power down on the hand held so the repeater doesn't “see” the direct signal from the handheld.

Diagrams to be inserted. ( I need to draw them first!) Dec 2018