Need some very old time information and a 6m radio
metavox at earthlink.net
Mon Aug 27 14:40:23 CDT 2007
Mr. Kahn was right, at least about the resistive value. But then on
short antennas there is a reactive component which is capacitive. For
each meter of length a whip antenna has around 10 to 12 pf. of
capacitive reactance. That virtual capacitor appears in *series* with
the resistance. The result is a high impedance, especially at low
For Example, 10 pf at 10 kHz has a capacitive reactance of 1.5 megohms.
At 530 kHz it is down to 30k ohms, still a lot compared to the 50 ohms
we have learned to love and use. This is true if you are trying to drive
the antenna as a transmitting antenna or using it as a receiving
antenna. It works both ways. If we go to a 2 meter whip that will be
halved but is still high.
If you were to just plug a short whip into a 50 ohm receiver input at
LF, the matching loss is huge and we need to correct the mismatch. Put
some coax between the antenna and the receiver and it just gets even
worse. This is why we use a FET on the input of an active antenna and
place it just below the whip. Typically, the active antenna amplifier
input at 10 kHz is 10 to 30 pf which is much better than 50 ohms.
Feel free to quote my active antenna article in the September, 2001 QST.
Of course, I got that from another who got it from another...... Some
say 10 pf per meter and others say 12 pf per meter. I measured it with
an impedance bridge and the value is pretty close to either one
depending on surrounding objects.
I typically use a 10 pf capacitor in series with a signal generator to
test the active antenna electronics. Radio Shack sells a nice pack of
small value ceramics and you can sort through to try different values
and their effect on the electronics.
Go get 'em now that you have the facts!
Michael Chisena wrote:
> I got some tips on this Saturday, but I'm hoping there are some more.
> Back Story.
> Some years ago I worked for Mr Kahn of ISB AM Stereo fame.
> He taught me that car radios are set up for 10ohm antennas.
> Has to do with impulse noise suppression.
> We even had a little adapter that worked with our 50 ohm signal
> generators for setting up car radio receivers.
> Fast forward to last month or so.
> I got nailed on a round table about this.
> I wasn't believed and was asked to 'name my source'.
> I sputtered, didn't have a good answer.
> When 'they' are right, they are right.
> Just what was my source?
> I'm looking for an old spec that is not being found on the Internet.
> Experimental Data
> As an alternative, I tried my MFJ antenna analyzer on the trucks antenna.
> It peaked at 87MHz with a 10 ohm impedance.
> It also showed a good match at 27MHz, I don't know why.
> Didn't get much of a reading at about 1.8MHz, that's the lower limit for
> the instrument.
> Current plans.
> The guy that works in the next cube here is your classic 'go to' guy for
> a lot of things.
> I'm going to ask Dr. D this question and thumb through some of his text
> Second topic
> About two weeks ago I was asked about a 6m radio.
> What Ive got is a 50 watt under dash type.
> I'm willing to loan it 'on spec'.
> That means, take it, use it for a few weeks, and decide if you want to
> buy the thing from me. I spent a ripping 40 bucks for it, so it's not
> exactly an expensive item.
> It's set up with a power plug and power pole connectors.
> If you want an external speaker, that's a snap.
> I don't have a proper mounting bracket.
> If you want some additional frequencies shot into the thing, I will need
> that info as well.
> Right now it's got about 50 of 80 channels occupied.
> So who ever wanted it, kindly call me at
> 703 863 4574 after 16:30.
> I will hook up with you and hand the item off.
> Thanks in advance
> "You are, what you do, when it counts"
> The Masso
> "Gravity, the quickest way down"
> Mayor John Almafi
> "You ever drop an egg, and on the floor you see it break?
> You go and get a mop so you can clean up your mistake.
> But did you ever stop to ponder why we know it's true?
> If you drop a broken egg you will not get an egg that's new?"
> MC Hawking
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