Hi Lee,
Since I have two excellent receiving antennas to compare with (a 900 foot
Beverage and a full size 4-square), I'm satisfied that I'm getting the expected
performance from my BSEF receiving array. Its excellent directivity extends
well down into to AM broadcast band.
As you noticed after writing your 0013Z email, I'm using three 75 ohm Magic-T
combiners as suggested by W1MK. The four RG-6 coaxial transmission lines
connected to the verticals are cut to precisely identical lengths, 1/2
wavelength at 1840 kHz. The only potential source of VSWR induced phase shift
would be the RG-6 coaxial cable that provides part of the phase shift between
the front and rear pairs of verticals (along with a W8JI phase inversion
transformer), but I believe that line is also operating at (or close to) 1:1
VSWR.
I will perform a few more VSWR measurements just to be certain that all of the
RG-6 transmission lines are operating at (or close to) 1:1 VSWR as planned.
Thanks for your helpful comments.
73
Frank
W3LPL
---- Original message ----
>Date: Sun, 3 Feb 2013 00:13:48 -0800
>From: "Lee K7TJR" <k7tjr@msn.com>
>Subject: Topband: New 160M high performance receiving antenna at W3LPL
>To: <topband@contesting.com>
>
> First let Me say this passive 8 circle antenna is a great antenna. I am
> sure
> it is outstanding compared to most receiving systems.
> However, It seems to me a moot point to be discussing the merits of
> minutia
> in terms of the pattern from this 8 circle antenna. If you model this
> antenna with
> all the transmission lines and transformers etc., you will find that the
> pattern
>changes dramatically form 1.8 to 1.9 MHZ. Yes, it is perfect at the design
>center
> frequency. And this is modeling without the effects of the ground radials
> often
> used in the system. They will only make the following problem worse. I do not
> have the NEC 4 engine so I cannot evaluate the radial influence.
> So someone correct me if I am wrong, but here is why this happens. It is
> incorrect to expect the delay in a transmission line to be as calculated
> unless
> one or both ends of it are terminated in its characteristic impedance. Also
> unless it is multiples of 1/4 wave which we do not have here.The delay line in
> this design is 37.5 ohms and the two elements feeding it are 37.5 ohms, a
> match only at the design frequency. If you look at the output side of the
> delay
> line it is driving the output 18.75 ohms output transformer and it is also
> seeing
> the other two elements paralleled (37.5 ohms) so the load on the line is 12.5
> ohms. The delay line is not terminated on the load end for sure. And not on
> the
> source end off the design frequency as the frequency of the array changes. The
> output impedance of the elements change naturally with frequency, so unless
> you are at the design frequency, source impedance will be off 37.5 ohms and
> the delay will be incorrect in the delay line. The delay varies with frequency
> bottom line.
> A combiner with a little more complicated impedance matching would help
> this problem.
>Lee K7TJR
>_________________
>Topband Reflector
_________________
Topband Reflector
|