VHF receiving intermittent fault - non-boaty

[chrisclin]

Sorry to clog the forum with non-boaty questions, but I feel this is definitely related.
We've got a problem with the radio microphones used in our church. Both of them drop out for a fraction of a second from time to time and no-one seems to know how to sort them.
I am convinced it is an aerial fault - there are 2 separate aerials feeding into a distributor. While it could be the distributor that is the problem, I would like to discount the possibility of it being an aerial fault.
As the aerials are only used for receiving, can they be tested with a VSWR Meter? Can they be tested with a multimeter with the aerial still in place?

 

[William_H]

I think the usual aerail for this kind of system is just a 1.4 wavelength whip. ie a rod of conductor so aerial itself is not likely to be a probblem. The cabling to the aerial is a likely candidate. The distributor is designed to give the best signal over a larger area of roaming. I woud not imagine that a likely suspect. It may be possible to by pass the distributor or even substitute a piece of wire in the receiver aerail socket to confimr the source of problem. Length of wire should be similat to the aerails.
My own experience with these devices is that it is battery conatacts in the microphone that are the most likely culprits. not much help but good luck olewill

 

[fisherman]

Sorry to clog the forum with non-boaty questions, but I feel this is definitely related.
We've got a problem with the radio microphones used in our church. Both of them drop out for a fraction of a second from time to time and no-one seems to know how to sort them.
I am convinced it is an aerial fault - there are 2 separate aerials feeding into a distributor. While it could be the distributor that is the problem, I would like to discount the possibility of it being an aerial fault.
As the aerials are only used for receiving, can they be tested with a VSWR Meter? Can they be tested with a multimeter with the aerial still in place?

I recommend you PM mikefleetwood, he makes this stuff.

 

[wooslehunter]

To use a vswr meter you need a transmitter on the right frequency. One of these may work if you're using VHF license free mics: http://www.mfjenterprises.com/Product.php?productid=MFJ-259B

Licensed VHF goes above the frequencies they will do. MFJ make a higher frequency version but it's much mo expensive. A local radio ham may have one.

The first question is, has it always been like this or just started recently. If the former, the you may just have a dead spot & need to play around with antenna positions or if you can, change the mic frequencies. If the latter then you have a fault. If you a spare antenna & cable, try swapping one out. If that makes no difference, then it could be the combiner. Only do one thing at a time.

A multimeter is generally little good with antennas. All you do really is continuity & finding shorts between conductors.

 

[chrisclin]

Thanks for the helpful replies.
I optimistically tried shaking the transmitters today hoping the problem would be as simple as poor contact with the batteries. Unfortunately nothing showed up.
I don't think these outages are because of dead spots. We recently had another spur aerial installed actually in the pulpit but it has made no difference to the drop-outs.
Could it be as simple as interference from other transmissions? It has got worse recently but I don't know of any new transmitters close by.
I understand that because the government are selling off radio bands we are going to have to change our kit. Unfortunately they don't seem to have made up their minds what are going to be the licence-free frequencies so we can't change the equipment over. I do wonder how long it will be before we have to give a credit-card number before calling Mayday on Ch16.

 

[winsbury]

A single antenna vhf receiver will experience null signals due to reflected signalls cancelling one another out depending on the relative location of the transmitting mic and various rf reflecting surfaces. A Diversity receiver uses two or more antennas to reduce this effect but unless its a very sophisticated one with at least three antennas you will still experience dropouts. Simply adding more antennas in parallel on a non diversity system is likely to make things very unpredictable, most likely worse. Vhf is not usually used in pro radio mic systems nowdays , uhf being preferred as the shorter wavelength makes for much smaller dropout zones and higher bandwidth gives higher fidelity audio into the bargain. If your system is digital, you might also be suffering poor signal to noise caused by poor wiring or interference which in turn causes the system to mute briefly when it detects an error.