Of course spacing also has an effect on impedance, gain and F/B. I think
2el beams are typically designed for gain rather then F/B. If I recall
some 2el 40m beams with reflector lengths close to the DE would flip
direction high into the phone band so be careful. I would trust your
model, you can verify the frequency of each element to see of your loading
model is accurate and design the beam for the criteria that you want.
John KK9A
John Keating (no callsign) wrote:
I'm getting ready to make another, hopefully the last, adjustment on the
reflector length of my 2 element 40M coil loaded short boom yagi. Looking
for recommendations or calculation methodology to optimize F/B and pattern.
Here's what I know:
- I operate mostly around 7.150 - 7.160. Driven element is tuned there, and
its length is 408.25" tip to tip.
- Reflector length was originally 462.25" which resulted in maximum F/B at
6.700, with a basic dipole radiation pattern at 7.150.
- Most recent adjustment shortened the reflector to 420.25", resulting in
maximum F/B and good gain at 7.060; modest gain and less pronounced pattern
at 7.150. At this point, the reflector is tuned about 1.5% lower than the
driven element, and its physical length is almost 3% longer.
- coils are nominally 13.5uH with Q ~ 100 at 7.150, resulting in about
200kHz bandwidth. Coils are approximately in the center of each half
element. Feedpoint impedance is close to 50 ohms with no match. Boom length
is 15.6 feet. Antenna height is 50 feet.
N6BT's Array of Light states that shortened yagis require tighter coupling,
and recommends that the reflector elements be tuned about 1% lower than the
driven element. In my case that would mean shooting for around 7.090. If
behavior followed the linear relationship implied by the first instance of
shortening (8.22 kHz/inch), reflector length should be 413 inches, or
shorter by 7.2 inches. But I don't know if the length vs resonance frequency
relationship is linear for a coil loaded element.
Alternatively, my EZNEC model indicates best F/B, gain and pattern will be
obtained with the reflector only one inch longer than the driven element
(yes, one inch!) but the SWR will deteriorate substantially due to change in
feedpoint impedance.
Any further shortening of the reflector elements will require cutting
aluminum while in the air, so I want to be careful what I do next.
Looking for any advice or suggestions other than "remove a half inch at a
time and measure."
73, John
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