transciever TM751

[Ed]

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 
ithorizontally, 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













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 
> <mailto: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 
>> <mailto:tjhjr at mac.com>> wrote:
>>
>
>
>
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