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Call Sign : 26-CT-3382 Posts : 3 Times Thanked : 0 Join date : 2020-03-13 QTH or Location : Macclesfield Equipment Used : Amstrad 901 Fidelity 2000
Subject: Short circuit in 1/2 wave Sigma Venom Mon Aug 09, 2021 6:28 pm
I’ve been setting up a Sigma Venom 1/2 wave silver rod bought new from eBay a few years ago. Checking with a multimeter shows continuity between the mounting bracket/outer thread of the connector and the radiating element. I’m not overly knowledgable about the subject, but this surprised me. I’d be grateful for your comments as to whether this is normal for an aerial of this type, or whether it’s a possible fault. Many thanks in advance for your comments. Regards from Dave
Call Sign : 26-CT-3228 Ham M7VIC Posts : 2483 Times Thanked : 169 Join date : 2019-11-10 QTH or Location : Bedford Equipment Used : Various
Subject: Re: Short circuit in 1/2 wave Sigma Venom Mon Aug 09, 2021 8:20 pm
As already said it's not unusual and nothing to worry about.
There's a matching coil or actually an inductor in your antenna which measures as a short on a multimeter at DC or Direct Current but at RF or radio frequencies it appears as a 'reactance', so along with capacitance, (either added or part of the antennas capacitance which also has a 'reactance'), it gives you a RF 'impedance' which should be about 50 ohms to match your coax and radio.
I know, all sounds complicated.
You should see the maths involved!!
Again, as mentioned by our helpful members, you should only really worry about getting a good SWR.
I wouldn't worry either, catches everyone out the first time and as they say 'every day is a school day', as in we always learn something new.
It's when you delve into antenna design or a better understanding of antennas and feed lines that it all gets interesting....you can have a good SWR but a terrible antenna! (For example a 'dummy load' has a great SWR reading but won't get your transmissions out very far. )
There's always someone happy to provide help here at Charlie Tango so you're in the right place for any questions you have.
Call Sign : 43-CT-016 Posts : 238 Times Thanked : 15 Join date : 2019-11-17 QTH or Location : Perth Equipment Used : iCom IC-7610/IC-9700
Subject: Re: Short circuit in 1/2 wave Sigma Venom Tue Aug 10, 2021 9:49 am
If you have a look at most antennas of that type (I've popped a picture up of a very common one used over here below, as it's easy to visualise what we're saying on this model) you'll see the centre conductor goes to a tap on the coil, and the outside of the SO239 and one end of the coil connect to the chassis, with the other end of the coil going to the radiating element. So in effect, you have a DC short through part of the coil from the inside and outside of the SO239, as you have noticed.
However, at RF frequencies, as others have said, what you really have is a simple matching network as that coil appears as an inductive reactance, and helps to tune the antenna for a 50ohm impedance match with the other variables involved.
Victor, Alan Pilot, SangueG and Chickenfoot@1987 like this post