>Darrel Morisse wrote:
>
>> I have a standard 3 hole comm antenna, Dorne and Margolin Model DMC63-1/A.
>> A friend was fooling around with it using an ohm meter and said the antenna
>> is no good because he wasn't getting any resistance between the ground and
>> hot receptacle. He claimed it should be at least 30 ohms continuity.
>> Is what he's saying true?
>> Is this an effective way of proving an antenna is shot?
>> He also said that if I used it I would burn up the transmitter on my radio.
>>
>> Any help out there?
>> Darrel
>
>forgot to mention that the antenna imperdance (in ohm) can be inductive or
>capasitive, ie. if you are using an ohm-meter on an inductive antenna you can
>measure zero ohm!
>If you are measuring a capacitive antenna you are measuring in an open
circuit
>with infinity resistance. This is a little bit of a short and practical
>explanation as the antenna theory is very complex, but from a practical
way of
>view its ok.
>To really check your antenna and feeder installation (coaxial cable) we
use to
>have a SWR (standing wave ratio) check, this check confirm that your
antenna is
>workin properly, cause in this way we can see that if all the power delivered
>from the transmitter is radiated out through the antenna.
>If you are using your transmitter without any antenna, your transmitter may
>quit, also if you are transmitting with a grounded/shorted antenna same might
>happend.
>As you see, using a ohm meter does not prove the antenna is ok or bad.
The last statement is true but not quite for the reasons
mentioned . . . you can have both "inductive" and "capacitive"
antennas that measure either infinite ohms or zero ohms or
anywhere between with an ohmmeter . . . a device for measuring
DC resistance.
Antennas are characterized with an AC impedance value that does
require equipment designed for radio frequency AC measurements.
Betware of low cost SWR meters sold by Radio Shack and CB radio
outlets. While "rated" for a wide range of frequencies, I've found
a number of devices that showed an SWR reading of 1:1 (ideal)
when in fact it was much different!
The best device is an impedance meter like the one we rent from
our website catalog. See . . .
http://www.aeroelectric.com/Catalog/catalog.html
Go down to "Tools and Test Equipment" and click on "Rental
Antenna Analyzer." While you're checking out the device,
you can click on "download operating instructions" to see
how the thing is used.
This tool costs about $250. You only need it for a short
time when installing home made antennas so we decided to
make one available for rent.
Getting back to the original post . . . If you have a manufactured
antenna that has not been badly beat up . . . it is
99.99% probability of being okay. Bolt that feller on and
give it a try. If the system doesn't work after installation,
and you suspect the feedline or antenna, buy, rent or borrow
some test equipment equal to the antenna analyzer I just
showed you and it will help you deduce the fault.
Bob . . .
////
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