[TowerTalk] putting a reflector below a 75 meter inverted

[K4SAV]

I think the 2.53(0.05 dB) number is a typo.  He doesn't even say which 
antenna he is talking about in that paragraph.  Assuming it is the 
Figure 4 graph, the gain difference, looking at his graph versus the 
calculated gain of a dipole at 0.07 wavelengths, should be about 4.2 dB.

His article is missing some information.  He didn't describe what the 
counterpoise was, and it makes a big difference.  The counterpoise could 
be small, large, dense or not dense, and the results will be different 
for all.

You could also just use a single wire for the reflector instead of a 
counterpoise.  In that case, taking his example of Figure 4 and 
translating the numbers to 3.6 MHz, the dipole is at 19.2 ft, and the 
reflector is at 5.5 ft. then the gain without the reflector is 2.34 dBi 
and with the right length reflector it is 6 dBi, assuming average ground 
and no resistive losses.   The length of the reflector is somewhat 
critical, just like designing a Yagi.  Get it the wrong length and no 
gain, or gain in the reverse direction, and the gain varies with 
frequency.  A large counterpoise could be made to look less frequency 
dependent but that is a big effort.

In general for low antennas these reflectors (or counterpoise) just 
decrease ground loss, so the gain goes up for all elevation angles.  It 
doesn't change the take-off angle, the whole radiation lobe just becomes 
a little larger.

Jerry, K4SAV

Bill Turner wrote:

>ORIGINAL MESSAGE:
>
>On Sun, 9 Sep 2007 09:10:46 -0500, "Robert Chudek" <k0rc at pclink.com>
>wrote:
>
>  
>
>>You guys discussion prompted me to look into the "counterpoise" for horizontal wire antennas. I found a brief article here, including some graphs.
>>
>>http://www.arising.com.au/people/Holland/Ralph/counter.htm
>>
>>73 de Bob - KØRC in MN
>>    
>>
>
>------------ REPLY FOLLOWS ------------
>
>One paragraph in that article states:
>
>"The horizontal dipole system has a minimum gain of 0 dBi at 51
>degrees elevation, while the counterpoise system has a minimum gain of
>0 dBi at about 42 degrees of elevation. Alternatively, antenna gain
>comparisons at all elevation angles show that the counterpoise gain is
>greater than the dipole system by 2.53 ( 0.05 dB. This demonstrates
>that the counterpoise is a more effective ground system; the
>counterpoise reflects more energy into the half-space so less is
>wasted in the intrinsic ground resistance and more is radiated."
>
>
>I don't understand the "by 2.53 ( 0.05 dB." part. Is that a typo?
>Where is the missing close parentheses?
>
>Bill W6WRT
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