[TowerTalk] SP7GXP antennas?

[Larry Banks]

Are they mixing up dBi and dBd?

73 -- Larry -- W1DYJ


-----Original Message----- 
From: David Gilbert
Sent: Thursday, October 21, 2021 14:48
To: towertalk at contesting.com
Subject: Re: [TowerTalk] SP7GXP antennas?


I own an Optibeam OB16-3 tribander as well as an Optibeam OB2-40 shorty
40.  I modeled both antennas in EZNEC+ using the published dimensions,
as well calculating the impedance of the short square tube transmission
line that connects all three driven elements on the OB16-3 and
calculating the inductance of the "hairpin" matching coil used on the
OB2-40.  My modeled patterns matched very closely the patterns that
DF2BO had sent me.

With the help on K9RX, who at the time lived about 30 miles line of
sight distant from me (we both lived on elevated ground), I then
measured the horizontal pattern by rotating my antenna while he took
signal strength readings.  With him transmitting, I again rotated my
antennas and took direct voltage readings using the microvolt mode (no
AGC or other compression) on my K3.  In all cases the measured patterns
closely matched the Optibeam specified patterns.

It is a fact that you can't get additional gain without changing the
pattern, and 2 db would require a pretty significant narrowing of the
pattern.  It is also a fact that the vertical pattern is predominantly
determined by height above ground for similar antennas.  My conclusion
is therefore that either DF2BO has understated the gain ratings for his
antennas, or that SP7GXP has overstated the gain ratings for his antennas.

73,
Dave   AB7E



On 10/21/2021 7:38 AM, john at kk9a.com wrote:
> I have modeled some OptiBeam antennas and the published specs seem 
> accurate.
> Of course getting 2db more gain from the same boom length would circumvent
> the laws of physics. I recently installed a 4el 40m OptiBeam on my tower,
> the SP6GXP version is 37% lighter.
>
> John KK9A
>
>
> David Gilbert AB7E wrote:
>
> His 16 element 20/15/10 tribander looks like an almost complete copy of
> my Optibeam OB16-3, except that SP6GXP claims about 2 db more gain on
> each band and uses lighter weight materials.
>
> Dave   AB7E
>
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