[NLRS] 2M Big Wheel antenna info

[Dr. Gerald N. Johnson]

On Mon, 2007-01-08 at 13:52 -0600, Doug Reed wrote:
> 
> The original performance test article from Oct 1961 compared the various 
> Big Wheel versions against a mobile turnstile antenna at its normal 
> height 20" above the rear deck of a station wagon. The single Wheel was 
> at 15' and the stacked bays at 24'. This obviously gave a serious 
> advantage to the Wheel antennas. But it was meant to be a comparison 
> against a known performer, the turnstile, rather than a dB gain race.
> 
> The numbers reported in the article were 5.7dB gain for the single 
> Wheel, 6.2dB gain for the double stack, and 8.1dB for the quad stack. 
> The quad stack was what the article compared to a small yagi for gain.
> 
> The numbers that Donn suggests are probably closer to the actual gain 
> compared to a dipole. The QST figures merely show they performed well 
> against the competition and give an indication of the gain advantage of 
> the stacked Wheels.
> 
> To me, the most interesting aspect of the QST gain numbers is that the 
> difference between single and double stack is so minor, only .5dB. This 
> is where I'd normally expect to see a 2dB gain from stacking, but it 
> doesn't appear. Proper stacking gain does appear when going from double 
> to quad stacked versions. One clue might be that the first article says 
> to offset the two antennas by about 60 degrees so they will fill in the 
> dips in the radiation pattern and achieve a more circular pattern. That 
> probably would be enough to make the difference.
> 
> It is also interesting to read the praises of the double stack as a 
> miracle for digging out the weak ones that couldn't be heard on the 
> single Wheel or the turnstile. That sort of advantage doesn't seem to be 
> shown in the reported gain difference. It does make me want to try a 
> double stack at home, and seems to indicate that you'd want your mobile 
> antenna as high as practical, allowing for bridges.
> 
> When I was searching, several references to CSVHFS 2006 came up and 
> seemed to show one Big Wheel tested .8dBd and another at 1.8dBd. But I 
> didn't read the web page to confirm that.

On the WB0TEM antenna range, measuring a dipole or a simple array of
dipoles or the big wheel is filled with errors. His range is intended
for antennas with relatively narrow patterns, preferably under 30
degrees beam width to avoid ground and building reflections. Hence his
source antenna is as directional. When you go there with a low gain
antenna its far more sensitive to right angle bounced signals from the
tent, watchers, light poles, utility wires, and buildings. And since
those are a longer path the antenna position can be super sensitive with
a quarter wave of motion changing the strays from subtracting to adding.
I've seen my 2 half waves in front of a 1 wave length square screen, a
NBS style 7.7 dBd gain reference, come up with over 10 dB gain because
it was too heavy to get up out of the ground reflection.

What it takes for true gain measurements of low gain antennas like
discone, FM verticals, halos, and big wheels with no inherent azimuthal
directivity is a measuring place with the source antenna at a decent
distance (though it can be closer than for a long yagi) but with several
times as much clear space around the antenna under test as the distance
from the source IN ALL DIRECTIONS. Which means it may have to be an
elevated range with the instruments under the deck. And you have to
compare the reference antenna at a separate time from the antenna under
test to avoid any reflections that the near omni antenna will detect
just as fine as the direct signals.

Omni antennas are the hardest antennas to measure for gain, and the most
lied about in advertisements.

And its possible that the limited length of the WB0TEM range
discriminates against the long yagi but not being long enough. But many
home brew long yagis discriminate against themselves by not working as
planned.
> 
> I've been interested in the Big Wheel since I first saw the articles 
> back in the late 60's. They don't seem particularly difficult to build 
> if you have a good supply of tubing and a fixture or pattern to aid in 
> bending. Worst case, you can draw the pattern on a large piece of 
> cardboard using a string and magic marker, then use a small hand tubing 
> bender for the rest.
> 
> With all my talk of single stack, double and quad stack, I feel like I 
> should order fries to go with it....
> 
> 73, Doug Reed, N0NAS.
> 
73, Jerry, K0CQ