On Sat, 15 Feb 2014 01:44:31 -0800 (PST), wrote:

Myth:
A 5/8 wave antenna has 3dB
more gain than a ground plane.
False

This can be true in many cases.
But it can also be false in many cases.


*
The losses in the required matching coil at the base
of the 5/8 wave antenna reduce the gain difference
to a max of about 2dB (with a perfect ground plane)
to zero difference in some installations. **
--------------------------------------------------

This is fairly absurd.. The loss of the loading coil
is quite negligible. I bet not even enough for most
people to accurately measure.

The coil has nothing to do with why some types of
5/8 verticals show little or no gain vs a 1/4 GP,
or isotropic.

Just a minor note on this issue. If the coil actually did contribute
a 1dB power loss, the lost power would need to be either reflected or
dissipated by the coil if it's not being radiated as RF. I haven't
seen any high VSWR 5/8 wave antennas, so it's not VSWR. I haven't
seen the matching coil on my Larsen 5/8 wave antenna, with a 45 watt
VHF mobile, become warm. If it was dissipating 4.5 watts, I should be
able to see it get quite warm[1]. I pointed an IR thermometer at the
coil and transmitted for about 5 minutes. No change. However, I did
see a very slight temperature rise from a mercury thermometer taped to
the stainless steel antenna rod (which could be attributable to my
sloppiness). The test is easily reproduced. Have fun.

However, he is right about the minimum VSWR point shifting, but
neglects to quantify the effect. A typical 5/8 wave antenna is fairly
narrow band and will not cover the entire 2m band. If you miss the
minimum VSWR point, there may be additional losses.
http://vk2zoi.com/articles/single-five-eighth-flower-pot/
His graph shows 1.5:1 at the band edges which is a mismatch loss of
0.18dB. Not even close to the claimed 1dB loss.

I consider the article quite good in that it's a survey of things to
consider when building or using antennas. It covers quite a range of
topics and is therefore understandably lacking in detail.


[1] For entertainment value, take a 33 ohm resistor and put it across
a 12v battery. Find something that has approximately the same mass as
the Larson loading coil. Give it a few minutes to get warm. Feel the
heat? That's what a 1dB (10%) power loss with a 45 watt transmitter
should feel like.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558