[AMRadio] 50 ohms simple question


Jim Wilhite w5jo at brightok.net
Thu Apr 5 16:49:06 EDT 2007


Most of the tuners in the solid state rigs are only good 
from about 35 ohms to 150 ohms reactive, which takes care of 
about 90% of the installations a ham can put up.  Alan if 
you put up a straight dipole at the proper height, 1/2 wave 
length or so over proper ground, it will be approximately 
70-72 ohms in free space.   If you lower the ends to about 
12 ft. of the ground the impedance will drop to near 50 
ohms.

But what have you accomplished in most cases is to put the 
voltage nodes near the grounds and the worms love it because 
it keeps them warm.  The higher the ends the better even if 
it results in a minor mismatch to your tuner or radio.  More 
room for the RF to leave the wire.

Therefore the difference between 50-52 and 70-72 ohms will 
yield a 1.3: swr provided there is no other reactance in the 
system.  If you put up a straight dipole and have the right 
conditions, I would use 70 ohm coax and any tuner will 
handle it so the loss of power in the tuner circuits will be 
minimal and the resultant heat dissipated should be in the 
tuner not the radio final.

Bob is right about knowing what your antenna looks like from 
the feed end of the feedline.  SWR is a catch all that only 
tells you there is a mismatch.  You will almost be chasing 
your tail to try to achieve a match using an SWR bridge. 
In other words, you will always wonder why the best match 
you might be able to achieve is 1.2 to 1.

Do some reading of the handbooks to learn a bit more about 
impedance and the necessity of proper installation, end 
effect, ground, etc.  Look at coefficient of coupling for 
example and so forth.

Jim
W5JO
> Geoff, Alan, Et al,
>
> Both of you are RIGHT - BUT - almost EVERY radio/amplifier 
> will tune from 50 ohms to about 75 ohms.
>
> So futzing with the antenna MAY not be NEEDED!
>
> An inexpensive way to check is to borrow an MFJ antenna 
> analyzer, I.e., MFJ-204B currently going for $89.95.
>
> OR simply put a GOOD VSWR bridge between the Final and the 
> antenna.  See what it tells you!  My bird with peak 
> reading tells me that my HT-32, Heath TX1, DX-60, and 
> TS-930SAT, TS-820S -> and FINALLY the T-599 will ALL tune 
> to the simple standard dipole.
>
> From OLD BAs to newer <groan> Sand State, I have found 
> that ALL will tune in this range OR more.
>
> Bob - N0DGN
>
> Geoff/W5OMR wrote:
>> Alan Beck wrote:
>>> Basic question
>>>
>>> Why is it that my radios are 50 ohms and the impedance 
>>> at the feed point at
>>> the antenna (pure dipole) is 70 ohms.
>>>
>>> Am I mistooken
>>
>> No, yer not mistaken.... but, if you want to lower the 
>> input impedance to your dipole, bring the legs down to 
>> around 40 degrees, and you'll drop the input impedance to 
>> the antenna down around 20 ohms.
>>
>> -- 
>> 73





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