Topband: Variable Flag Termination?

[n4is]

Dan


Here what I noticed working on my Waller Flag. The WF is 2 flags in phase so
this explanation applies to Flag, EWE, K9AY or Pennants, they are all the
same kind of antenna and works based on2 small vertical n phase.

1- The value of the resistor gives you best F/B when it is close to the
impedance of the Flag. 

2- When it is close to the impedance of the Flag, small variations only
changes the angle of the null at the back of the antenna.

3- A local signal or a ground wave signal arrives at zero degree elevation;
tuning the resistor for max F/B on the flag will move the deep to zero
degree elevation.

4- When modeling the RX antenna you must watch the plot using a 3D diagram.
The resistor changes the F/B at different elevations angles and the RDF.
Using 3D it gives you the average gain, and decreasing it from the frontal
gain you get the RDF value.

5- The trade off depends in what you want to do, best rejection from local
noise or from propagation noise. Local noise is always zero degree elevation
but propagation noise can come from any elevation angle. It changes by the
hour. In my case I tune the resistor for best RDF; it is a good average
point.

6- The resistor can also change the phase when you have 2 Flags in phase
like a WF. Making the back flag resistor 13 - 20 ohms bigger then the front
Flag increase the F/B, however decreases the F/S deep. I like to pick up the
two values that balance F/B = F/S for max RDF.

7- The impedance can change from 160m to 80 m and it depends on the high and
the distance between the two vertical sides. Some shapes can give you almost
the same impedance on 2 bands. Each band could have a different optimized
resistor value. 

8- Big Flags gives you more gain, small Flags less gain. You can go very
small if you know what you are doing and if you have common mode noise low. 
It is common to get great performance from small rotatable Flags, 10x14 ft,
(3mx4.5m) Using appropriate feed line choke to reduce common mode noise.
Small flags can keeps the phase better, the horizontal part pick up less
noise and you can check the pattern using a PC. I use PolarPlot program.

9- Other structures or antennas can change the impedance and the best
resistor could be different for different directions.

I prefer to keep it simple, close to the average value for best RDF and flat
SWR, not minimum, I should be flat, around 1:1.5 from 160m to 80m and a
little raise to 1:1.8 or 1:2 on 40m.

Regards
Carlos
N4IS