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TopBand: Pennant f/b ratio

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Subject: TopBand: Pennant f/b ratio
From: (P&V Nesbit)
Date: Fri, 02 Oct 1998 10:10:51 +1000
[MODERATOR NOTE:  You can requst the ZIP file from the author.  It has
                  been deleted from this message.]

Intrigued by the observations of Earl K6SE on pennant antennas, regarding
f/b ratios ranging from one to seven "S" units on local stations, I decided
to plot the elevation pattern of the recommended pennant (1, 2) over a
range of terminating resistances using NEC2 (NEC-WIN Basic). The ground
chosen was "urban and industrial", with a conductivity of .001 s/m and
dielectric constant of 5. The results show clearly what is happening.

Firstly, these antennas "want to work". They have useful directivity and
are not overly reactive, over a range of terminating resistance of at least
750 to 1000 ohms, making them relatively foolproof.

Observations over this range of terminating resistance include:

1. almost constant gain of about -37 dBi, with a broad front lobe centred
40 degrees, and the lower -3dB point 12 degrees;

2. a rear lobe, whose amplitude and vertical angle varies significantly
with terminating resistance;

3. a deep rear null, whose vertical angle varies widely with terminating

4. limited rejection of high angle signals arriving from the rear, as
expected from the short array length and semi-cardioid pattern.

The results are as follows (rear lobe is dB with respect to the main lobe,
taken visually from the graph):

Rterm      Zo         Rear Lobe    Rear Null
 750   712 + j106        Nil          0 deg
 800   746 + j74    -30dB @ 12 deg   20 deg
 850   779 + j41    -28dB @ 14 deg   29 deg
 900   809 + j8     -24dB @ 15 deg   33 deg
 950   837 - j27    -22dB @ 16 deg   37 deg
1000   864 - j61    -20dB @ 18 deg   40 deg

One should avoid using local signals for f/b measurements, unless the
skywave is definitely absent i.e. the critical frequency must be well below
1.8 MHz. In particular, the resistor should NOT be adjusted to null out
high angle signals arriving from the rear, because the rejection of noise
and signals arriving at low angles will be significantly degraded.

The plots show that Earl's choice of 900 ohms provides a good compromise
between high and low angle rejection, and should be close to ideal in terms
of maximising the overall SNR for wanted signals.

I would expect similar overall results for the flag, and will repeat the
process for that antenna over the next few days, when time allows.

For those who are interested, I have attached a zip file containing the
various elevation plots for the pennant.

Earl should be commended for bringing this most useful antenna to public
notice, not forgetting of course the originator of the concept EA3VY.

Peter VK3APN

(1) Earl W Cunningham, "High Angle vs. Low Angle" (posting to Top Band
Reflector), Sun 02 Aug 1998 22:50:32 EDT

(2) Earl W Cunningham, "Pennants and Flags", Tue 04 Aug 1998 15:25:36 EDT

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