- Patwah81 wrote:
- Thank you guys, the help is much appreciated. If you have the time for a more technical reason to the wives tale that would be great, but if not that's ok too.
First of all you have to look at an antenna
as being a complete system. You have the bit that sticks in the air which you want the signal to go out of, you have the bit it mounts to and the bit that sends a signal to the antenna
. Everything in the system affects the tuning of the antenna
(and therefore SWR), some of it which is intentional, some of it which happens because of an issue with the installation.
The physics behind it is a bit complicated but basically it's all about the flow of electrons and Kirchoffs Law (which I'll let you read up about), in particular his first law.
For this next bit I'll quote from W8JI who explained it simply much better than I can.
- Quote :
When we force charges up into an end fed antenna (making current flow), we have to move an equal number of charges out of some ground system or counterpoise at the feed point. We always must have the same current coming into the feed point from some sort of counterpoise as the current that moves up into the antenna at that point! There is no way around that rule.
Another way to view this is feed lines or feed points of our end fed antennas must have something to push against to force current into the antenna. It is very much like pushing a car. If we have very poor footing, our feet will move and slide as we push. We not only waste energy that could be used to move the car, we have movement or motion where we don't want movement. The same is true for a ground system, as feed line power "forces" current up into the antenna the other terminal of the feed line has to be held steady. We waste energy that COULD be in the antenna, and we have RF movement where we don't want it....on the outside of the coax shield and on anything connected to that shield.
So reading the above you need a RF ground and normally to do this correctly we use radials which could be made from wire, ally tube, whatever.
When we don't put in an adequate RF ground then the antenna
system will try to compensate for it. It'll use anything it can source electrons/current from to compensate. This inevitably means it'll use the mast it's mounted on if there'a an electrical connection and any wire connected to it which means your coax which we don't want.
When it's using the mast and, more importantly, the coax we have RF spewing out everywhere we don't want it to and not that much going out of the antenna
. That's caused by what is known as common mode RFI which involves RF travelling on the outside of the braid of the coax and it means you pick up more noise, you're more likely to cause interference to things like TVs, burglar alarms etc, and if it's bad enough it'll distort your transmit audio and if you're running power it can lead to RF burns. Some people have reported a tingling sensation from the mike when transmitting, that's actually RF burning caused by common mode RFI and the RF ground system being so poor for the power being transmitted that the antenna
system is using the coax AND the radio and the power cables and the mike cable to try to compensate.
So now how the old wives tale came into being. A lot of CBers noticed that when they cut their coax to certain lengths that the SWR dropped and that it was lowest at multiples of 9ft or thereabouts. Now remember how I said at the start that the antenna works as a system.
What they didn't realise is the reason why is that they had a poor or non-existent RF ground so the antenna
system was using the coax as a RF ground. By altering the length of that coax, because it was forming part of the antenna
(which we don't want) then it was altering the tuning of the antenna
. Lo and behold, the old wives tale is born. What they didn't know was that the reason it did was because of an actual problem, that if the SWR alters because you change the length of the coax that the antenna
system is telling you you don't have an adequate RF ground at the antenna
So how do we avoid the coax becoming part of the antenna
? A decent RF ground and using a RF choke at the antenna
end of the coax. You can either build or buy one using a ferrite core or you can make it from the coax, winding 5 turns around a 4.25" diameter former (110mm plastic ground waste pipe you can buy from MKM is ideal). That'll present a high impedance to RF at 27MHz, preventing or seriously reducing the ability for RF to travel down the outside of the coax and preventing common mode RFI.