[sci.electronics] TV Antennas, the knob

hbg6@citek.mcdphx.mot.com (12/12/90)

In article <12765@milton.u.washington.edu> amigo@milton.u.washington.edu (The Friend) writes:
>
>     I'm wondering if some gizmo like that of the rabbit ears's "dial"
>     that helps pull the signal in exists for outdoor antennas as well..
>     Seems like this could be the key to locking the station in or not.
>

Now that you mention it, I've always wondered what that knob on the antenna
actually did. I guess it might rearange the phasing between the 'ears',
anyone out there know for sure?

John

billn@hpcvaac.cv.hp.com (bill nelson) (01/01/91)

depolo@eniac.seas.upenn.edu (Jeff DePolo) /  9:04 pm  Dec 16, 1990 / writes:

>>	b) Terminated it poorly or are using the wrong impedance cable
>>	    (you'd better be using 75 ohm coax (not 50 ohm!) or 300 ohm twin
>>	    lead).

>You'll realize little difference between 75 ohm and 50 ohm coax.  When
>you consider what a wide range of frequencies you are attempting to
>receive, even with a well designed log periodic, the feedpoint 
>impedence is going to vary over quite a wide range.  Choose the cable

Absolutely correct - I believe the primary cause of the impedence variation
is due to mounting height variation.  I know the effect decreases as the
height (in wavelengths) increases.  I don't think the 25-40 ft above ground
level is enough to minimize this factor.

Bill

tad@ssc.UUCP (Tad Cook) (01/03/91)

In article <6690003@hpcvaac.cv.hp.com>, billn@hpcvaac.cv.hp.com (bill nelson) writes:
> 
> Absolutely correct - I believe the primary cause of the impedence variation
> is due to mounting height variation.  I know the effect decreases as the
> height (in wavelengths) increases.  I don't think the 25-40 ft above ground
> level is enough to minimize this factor.

Maybe at HF, but at VHF 25-40 feet above ground without nearby obstructions
should be just fine for minimizing impedance variations.  I have seen
charts in the ARRL Antenna Book which shows impedance variation based
upon height above ideal ground, and they are talking about at a wavelength
or less.  One wavelength at lowest frequency (TV ch2) is something around
17 feet.



Tad Cook
Seattle, WA
Packet: KT7H @ N7HFZ.WA.USA.NA
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