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[TowerTalk] New HF Beam Comparison

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Subject: [TowerTalk] New HF Beam Comparison
From: (Bruce Horn)
Date: Sun, 18 May 1997 11:48:05 +0100
I thought it might be useful to compare the claimed specs for the Bencher
Skyhawk tribander with the modeled results for a similar antenna from
Force-12, the C3XL. Both antennas are trapless designs.

Force-12's C3XL is an extended boom version of its C3 that covers only 20,
15 and 10-meters, no WARC band. It has 10 elements on a 33-foot boom, with
3-el on 20, 3-el on 15 and 4-el on 10.  Therefore, the elements per band
are identical to the Bencher Skyhawk. Unlike the C3, the C3XL has separate
feeds for each band. No details were given of the feed arrangement for the

Based on NEC-2 modeling using the W6QHS taper algorithm, here's the
comparison for free space performance (the claimed performance data for the
Skyhawk appears to be free space performance). Remember, this is a
comparison of modeled results for the C3XL with claimed results for the


         Effective     Gain      Gain
           Boom         @         @
Antenna   Length       14.0      14.35     Front-Back
-------  ---------     ----      -----     ----------
Skyhawk    22'0"      7.2 dBi   7.4 dBi    20 - 21 dB
C3XL       16'2"      7.5 dBi   7.6 dBi    22 - 29 dB


         Effective     Gain      Gain
           Boom         @         @
Antenna   Length       21.0      21.45     Front-Back
-------  ---------     ----      -----     ----------
Skyhawk    15'6"      7.0 dBi   7.6 dBi    16 - 25 dB
C3XL       16'5"      7.8 dBi   8.0 dBi    21 - 28 dB


         Effective     Gain      Gain
           Boom         @         @
Antenna   Length       28.0      29.0      Front-Back
-------  ---------     ----      -----     ----------
Skyhawk    16'9"      7.7 dBi   9.3 dBi    14 - 15 dB
C3XL       10'4"      7.3 dBi   6.8 dBi    13 - 18 dB


The Force-12 C3XL, like other multi-band Force-12 antennas, uses a forward
stagger design that places the reflector for the next higher frequency band
between the lower frequency band's driven element and and director. For
example, the 15-meter reflector of the C3XL is placed between the 20-meter
DE and 20-meter director; the 10-meter reflector is placed between the
15-meter DE and 15-meter director. On the other hand, the Bencher Skyhawk
appears to be placing its elements differently:  almost the entire boom
length is used on 20-meters, whereas the C3XL only uses about half of the
boom length on 20.

Interestingly, the C3XL has better performance on 20 while using a shorter
effective boom length. On 15, where the effective boom lengths are similar,
the C3XL appears to out perform the Skyhawk by almost a dB at the CW end.
On 10, where the Skyhawk's effective boom length is significantly longer
than the C3XL, it claims higher gain, as would seem reasonable.

What makes this comparison interesting is how close the claimed performance
of the Skyhawk comes to the C3XL (except 15 CW) on an overall boom length
that is 10 feet shorter.  Because of the reputation of the individual
responsible for the design of the Skyhawk, I would personally be more
interested in seeing what performance could be derived from an overall boom
length of 32 to 36 feet. Could the design out perform a KLM KT-34XA if a
similar boom length were used?

I look forward to hearing more details about this antenna and hope that the
manufacture is willing to share dimensional data so that the antenna can be
modeled by others.

73 de Bruce, WA7BNM   (

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