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Old November 28th 15, 06:13 PM posted to sci.electronics.repair
Chuck[_20_] Chuck[_20_] is offline
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First recorded activity by DIYBanter: May 2012
Posts: 70
Default Right length for table radio antenna

On Sat, 28 Nov 2015 00:47:50 -0500, Micky
wrote:

On Thu, 19 Nov 2015 04:25:32 -0500, Micky
wrote:

On Tue, 17 Nov 2015 12:59:43 -0500, Fred McKenzie
wrote:

In article ,
Micky wrote:

I have a table radio that used to have one stranded wire coming out
the back, about a foot long, which I think was the AM or FM antenna.

I want to replace it. Is there an optimum length for such a wire? Is
that measured from the variable capacitor? (I haven't opened this
radio yet, but that's where I usually see antenna wires connected.)

Micky-

As others have stated, it is not critical. For nearby stations, it may
work without the wire!


Yes, there's a local station, WYPR 88.1 Baltimore, that just about
every radio gets.


The wire you want to replace would ideally be one quarter wavelength
long. But that length would be "ideal" at only one frequency. So
whatever you choose is a compromise.

Suppose the FM band covers the range of 88 to 108 MHz. The middle of
the band would be 98 MHz. A wavelength would be 300/98 = 3.06 Meters.
A quarter wavelength would be 76.5 CM or 30.1 inches.


30 inches is a lot longer than the wire was. I'm glad I asked. Even
15 inches, if 1/8 wavelengths are useful, is longer than the wire, i'm
pretty sure. (Someday I may find the wire on the floor in the corner
and I'll measure it. )


BTW, a friend just gave me another radio, second hand, that indeed
does have a wire about 34 inches coming out the back, so some makers
care about this. I think that's what it shows, even if it's not 30.

He bought it at a Goodwill store to dock his iphone or ipad but he
says it doesnt' work. I don't have one to test with, but it also
doesn't get many radio stations. It's also missing its AM antenna and
the only source I have for a plug is maybe my auto burglar alarm from
the previous car, which I'm saving for the next car. For the record
it's an Imode model IP200A but I don't really plan to fix it

I posted mostly to say that its antenna was 34 inches, not just 15.




Besides 88.1t there are two other stations, I only get easily on the
car radio (no matter what car I have) and on a 250 dollar table radio
(KLH maybe), but not on some fancy digital tuner/preamp. Some other
table radios get one or both, at least if tuned perfectly, and it
doesn't seem to matter how expensive they are, so I've taken to buying
used radios for $3 to see if they will get those stations.

(But this radio I bought new for my mother because it had digital
tuning and I thought it would make it easier to change stations. )

Anyhow, the stations I want are in DC, maybe 35 miles away, on 88.5
(WAMU) and 90.1 (C-Span) so I think I'll use 88.5 in your formula
above and see how that goes. Maybe I'll trim a little off until I
reach the 90.1 length.

Several projects ahead of this, no space on the work bench, so I won't
be able to post back for a couple months or more.

Thanks and thanks everyone.


Fred

One reason those radios have short antennas is the front ends overload
near powerful transmitters.

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