Bandwidth impact of a gamma match / NEC2 calculations
 

Hi,

I have been running many automated NEC2 runs to optimize a 5 band
quad. My criteria included bandwidth and all my runs to date have
been done assuming direct feed with 50 Ohm coax.

I'd like change the criteria and allow the feedpoint impedance to drop
below 50 Ohms now, with the expectation that I would use a gamma match
at the feedpoint. Using a gamma match at the feedpoint, for me, would
be more advantageous because this means the driven elements of the
inactive bands will truly be closed loops (eliminating the need to do
other things to achieve this, relays, specific lengths of coax, etc.).
I would place a choke at the gamma match feedpoint as well.

Since (2.5:1 SWR) bandwidth is still something that is important to
me, I need a way to calculate it. Would it be a mistake to take the
real impedance at the center design frequency, make this the line Zo
and then take the reactance at the center frequency as well and offset
the reactance at all frequencies this amount?

For instance, if the frequency was 25 - j10, If I make the line Zo =
25 Ohms and then add +10 to all the reactance values, can I use the
resulting SWR value to judge bandwidth. I know this assumes that the
gamma match is a perfectly linear circuit, which I am sure its not. Is
there a better way to estimate the bandwidth and how off could I end
up off? Is there any better way to do this?

-Scott, WU2X

Bandwidth impact of a gamma match / NEC2 calculations
 

"Scott M" wrote in message
...
Hi,

I have been running many automated NEC2 runs to optimize a 5 band
quad. My criteria included bandwidth and all my runs to date have
been done assuming direct feed with 50 Ohm coax.

I'd like change the criteria and allow the feedpoint impedance to drop
below 50 Ohms now, with the expectation that I would use a gamma match
at the feedpoint. Using a gamma match at the feedpoint, for me, would
be more advantageous because this means the driven elements of the
inactive bands will truly be closed loops (eliminating the need to do
other things to achieve this, relays, specific lengths of coax, etc.).
I would place a choke at the gamma match feedpoint as well.

Since (2.5:1 SWR) bandwidth is still something that is important to
me, I need a way to calculate it. Would it be a mistake to take the
real impedance at the center design frequency, make this the line Zo
and then take the reactance at the center frequency as well and offset
the reactance at all frequencies this amount?

For instance, if the frequency was 25 - j10, If I make the line Zo =
25 Ohms and then add +10 to all the reactance values, can I use the
resulting SWR value to judge bandwidth. I know this assumes that the
gamma match is a perfectly linear circuit, which I am sure its not. Is
there a better way to estimate the bandwidth and how off could I end
up off? Is there any better way to do this?

-Scott, WU2X

Hi Scott

Have you thought of recording the antenna's input impedance at several
frequencies in the band of interest, then plotting them on a Smith Chart?
The plot will be a smooth circular trace going clockwise with increase
frequency so it will be easy to know how many frequencies are needed to
plot. maybe 5 is sufficient.
When the antenna's impedance is displayed on the chart, it becomes easy to
estimate the effect of something like that + j10. Reactance added in
series will move the impedance along the lines of constant R. When added
in shunt, the impedance moves along the lines of constant 1/R (admittance).


Jerry KD6JDJ

Bandwidth impact of a gamma match / NEC2 calculations
 

Hi,

I need to be able to add this to my program that runs automatically .
Is there a publicly available formula available or is this some
proprietary formula unique to winSmith?

-Scott, WU2X