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Short circuit in 1/2 wave Sigma Venom Icon_minitimeToday at 3:14 pm by Wireless

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 Short circuit in 1/2 wave Sigma Venom

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Alan Pilot
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PostSubject: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 6:28 pm

Hi guys,

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 Smile

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Alan Pilot
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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 6:34 pm

You will find that with most aerials.
You need to check with an SWR meter in between the radio and aerial coax.

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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 6:42 pm

I THINK THAT HOW IT SOULD BE, the  multimeter does register a short(dc ground)check SWR and I'm sure if I am wrong someone will give better advice
take care 
mark

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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 8:20 pm

Hi Dave,

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. Wink

You should see the maths involved!!  Shocked


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. Short circuit in 1/2 wave Sigma Venom 1f44d 


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. Very Happy)


There's always someone happy to provide help here at Charlie Tango so you're in the right place for any questions you have.


All the best,
Victor
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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 9:12 pm

Yes it does catch us all out victor ,I had a right royal wobble. But its all part of the learning curve.PS just been on 20m and heard OSCOR OSCOR OSCOR,did a double take. Short circuit in 1/2 wave Sigma Venom 1f602  Short circuit in 1/2 wave Sigma Venom 1f602  Short circuit in 1/2 wave Sigma Venom 1f602 
take care 
mark

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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeMon Aug 09, 2021 9:28 pm

Ha! Ha! 
Did he shout out "ONE KEELO-WATT!" at the end too? Very Happy


I've heard all sorts on 20m Mark including CB club call signs. Rolling Eyes


As for the antenna question and pretty much every discovery they aren't always heralded with the word "Eureka!" but usually by the words "Woah! That's weird!?!" Wink

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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeTue 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. 

Short circuit in 1/2 wave Sigma Venom IMG_2031_1024x1024@2x

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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeTue Aug 10, 2021 10:03 am

Nice one Jeff. Short circuit in 1/2 wave Sigma Venom 1f44d 

As they say a picture is worth a thousands words.
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PostSubject: Re: Short circuit in 1/2 wave Sigma Venom   Short circuit in 1/2 wave Sigma Venom Icon_minitimeWed Aug 11, 2021 9:22 pm

Many a perfectly working antenna has been thrown in the bin by people testing them with a DC multimeter and seeing continuity between inner and outer.
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