Marc,
I've been doing some 402-CD modeling/tweaking for my station.
Since my AO (MININEC) model was available, I ran it for your case:
MININEC input impedances assume perfect ground. Radition patterns
are based on dielectric constant of 13, conductivity = 0.005.
The SWR trend is similar to what you describe. You also lose
a few dB of F/B going from 24 to 28m....
24m height (Gain & F/B at 24 deg el.)
Freq. Real Imag. SWR Gain F/B
----- ---- ------ --- ---- ----
6.950 25.9 -j32.5 3.1 9.6 8.0
6.975 31.5 -j21.1 2.1 10.0 13.4
7.000 37.4 -j12.3 1.6 10.0 14.7
7.025 42.0 -j5.3 1.3 9.9 11.8
7.050 45.0 +j1.3 1.1 9.7 9.2
7.075 46.6 +j8.2 1.2 9.5 7.5
7.100 47.6 +j15.8 1.4 9.3 6.3
7.125 48.2 +j24.1 1.6 9.1 5.4
7.150 48.7 +j32.9 1.9 9.0 4.8
28m height (Gain & F/B at 21 deg el.)
Freq. Real Imag. SWR Gain F/B
----- ---- ------ --- ---- ----
6.950 23.7 -j31.9 3.1 9.6 6.4
6.975 27.9 -j20.5 2.2 10.0 9.6
7.000 32.2 -j10.9 1.7 10.0 10.3
7.025 35.7 -j2.2 1.4 9.8 9.1
7.050 38.4 +j6.3 1.4 9.6 7.8
7.075 40.5 +j14.9 1.5 9.4 6.7
7.100 42.1 +j23.7 1.7 9.2 5.8
7.125 43.5 +j32.9 2.0 9.0 5.2
7.150 44.8 +j42.4 2.4 8.9 4.6
hope this helps,
--
Steve Miller N8SM millersg@dma.org http://www.dma.org/~millersg
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