[TowerTalk] measuring F/B

[Tom Rauch]

> An easy way to measure F/B is as follows:
> 1. Have a local direct-path station transmit at a constant power level. 

Remember that is groundwave F/B, which is useful for low angle 
signals but does not necessarily reflect the "working F/B" you see 
on the air

The major flaw with groundwave measurements is the earth greatly 
attenuates horizontally polarized signals, while vertically polarized 
signals have much less attenuation. Because of that, any feedline 
radiation or scattering has an exaggerated and unpredictable 
influence on results.

The most accurate F/B and gain measurements would occur with 
distances just outside the Fresnel zone, and hopefully at the 
desired wave angle. Ground level measurements would be best 
with the absolute minimum amount of wavelength separation, 
before the earth has a chance to affect the results. The longer the 
path along earth and the more cluttered the area around the 
antennas, behind the antennas, and in the path between the 
antennas the less likely results would be accurate. 

.
> Point your antenna away from the transmitting station.

But remember if any conductors (power lines, buildings, hills, etc) 
are in that direction, F/B will be changed. This is especially true if 
the path is longer than necessary to get out of the Fresnel zone of 
the antenna. 

 3. Note the s-meter
> reading. 4. Point your antenna at the transmitting station. 5. Insert
> attenuation in the receive line until the s-meter reaches
>    the first reading.
> 6. Read the F/B ratio off the attenuation setting.

 
> If a good attenuator is not available, I suppose it could be done
> with a wattmeter, by transmitting, and then reducing power until the
> distant station's s-meter matches, and then, noting the two power
> levels, use dB = 10 log P1/P2.

In this case remember the accuracy of a properly calibrated Bird 
meter is only rated at plus or minus 5% of full scale, anywhere on 
the scale. A 1000 watt slug would be + or - 50 watts anywhere on 
the scale you read it, so it would be best to switch to a lower 
power slug when calculating F/B ratio. In other words, set power at 
1000 watts off the backside, and if head-on power is 9 watts for the 
same reading use the low power slug to make the reading.

It's always important to understand the limits of the test method.


73, Tom W8JI
w8ji@contesting.com

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