transciever TM751

[Jeff Wooding]

Wow this thread went to a whole other realm then what I live in. I'm going
to put a pin in it and save it for later because it is beyond me, but I'm
sure I will need it in the future. :)

Thank you smart guys of the XTension group.

Thanks,
Jeff

On Mon, Sep 29, 2014 at 8:11 AM, Ed <edsha at ktsllc.net> wrote:

>  However, there is a problem with this strategy: The input impedance will
> be poorly matched to the coax feedline if you use a half-wave vertical
> element. On the other hand, if the element is 1/4 wavelength, the input
> impedance is close to 40-50 ohms which is not too bad a match to 75 ohm
> coax and a pretty good match to 50 ohm coax.
>
> The exact formula for the wavelength of a radio wave in free space is
> 300/F where F is the frequency in MHz. (Well, actually, the numerator is
> <really> 299.792... but Maxwell will forgive you if you use 300.) The
> result using this equation is in meters. When you plug in the frequency 315
> MHz, you get that the wavelength is 37.495 inches. A quarter wavelength is
> then 9.37 inches. Again, this applies to a radio wave propagating in free
> space.
>
> For an physical antenna, the length where the antenna is resonant is
> affected by its surroundings and by the size of the conductor as Tom has
> said. An infinitely thin ground plane antenna would be resonant at very
> close to 9.37 inches. Engineers often use "rules of thumb" and one that
> applies here is to multiply the free space calculation by 0.95 to 0.97 to
> get the length to use for an antenna. That would make the length of the
> element for a ground plane antenna about 9 inches.
>
> There is one problem with using ground plane antennas for this
> application. Their maximum radiation (or response) is in a direction
> perpendicular to the vertical element. A true ground plane antenna would
> have NO response either below its ground plane or directly overhead.
> Therefore, a GP antenna is a poor choice for this application if you are
> planning to place it in your attic and your sensors are on the floors below.
>
> On the other hand, the turnstile antenna described earlier in this thread
>
> http://www.shed.com/tutor/mr26ant.html
>
> is pretty much omnidirectional and is an excellent choice for this
> purpose. I built one of these about 15 years ago and mounted it
> horizontally, centrally located, between the joists in my basement
> connected to one of the original W800s. Once I set this up, I never had any
> problem receiving X10 motion sensors (of which I have a bunch!). It works
> for sensors outside the house also, as long as they are not too far away.
> BTW, the folded dipoles (half wave elements) in this turnstile are 18" long
> (just measured them).
>
> One of the first things my antenna professor told us was that designing
> antennas was a mixture of analytical tools and black magic with an emphasis
> on the latter.
>
> Regards,
> Ed
>
>
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>
>  On 9/29/14 10:27 AM, Thomas Arman wrote:
>
> Oh, and one more thing.  You should be able to get 2-3 db gain over the
> 1/4 wave by cutting  the vertical element (ONLY, the radials stay 1/4 wave)
> to 19”.
>
>  Tom
>
>  On Sep 29, 2014, at 10:18 AM, Thomas Arman <tarman at me.com> wrote:
>
>  Hmmm, this is what I thought was the  formula for 1/4 wave resonant
> antennas
>
> 2992/freq in MHZ EXAMPLE 2992 divided by 155.0 = 19.3 (inches)
>
> You may think this isn't exactly correct, but the 2992 is compensated for
> "end effect" - which means - the traveling 'wave' tends to 'stretch' itself
> a bit beyond the end of the antenna (like a ghost image). The THINNER the
> wire, the more pronounced the effect, AND the narrower the bandwidth. This
> is why most TV antenna elements are THICK. It's not just the support
> thickness gives, it the bandwidth of the reception frequency and the
> reduced end effect.
>
>  So 2992/315 = ~ 9.5"
>
>  Tom
>  On Sep 29, 2014, at 9:59 AM, Thomas Henry <tjhjr at mac.com> wrote:
>
>
>
>
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