[TowerTalk] 20 meter elements interacting with 17?

n4kg@juno.com n4kg@juno.com
Wed, 5 Jul 2000 07:34:16 -0600


When monoband antennas are interlaced, the lower frequency 
directors act as inefficient REFLECTORS when placed in front
of higher frequency antennas.

This is why you see the "Forward Stagger" used by Force 12.

I tried a 3L15 inside my Telrex 3L20.  The best I could achieve
was about 2 dB below what the 3L15 would do when in the clear.

Reflectors line up just fine since the highest band reflector is closest
and the lower frequency reflectors are 'out of the picture'.  This is
the secret of the popular 2L Force 12 C3 and it's derivitives.

One way to overcome the undesirable effect of low frequency directors
is to use TRAPS in the director(s).  The new CC X7 and X9 are
good examples of this technique.  These antennas use Log Cells
for the driven elements with separate reflectors for 15 and 20M
(the log cell provides adequate F/B on 10M rendering an additional
10M reflector unneeded).  The X7 uses a single trapped director
for 10 / 15 / 20 and the X9 uses a pair of trapped directors with
a single 10M director between the two trapped directors.  To my
mind, the X9 is a trap version of the popular KT34XA.

Bottom line:  interlacing higher frequency 3L monobanders 
inside lower frequency monobanders results in reduced 
gain on the higher bands.   Another technique is to use
'open sleeve' decoupling for the higher bands on each side 
of the lower frequency directors.  This is the technique used
on the longer boom C31 tribanders.

Placing a higher frequency director on each side of lower
frequency directors also helps to (partially) overcome the
destructive interaction.  This is what HG did on the TH6 / TH7
where there are 10M directors on each side of the 15M director.
(I have no idea how the 10 / 20 director affects 15M performance
of the TH6 / TH7 but my guess is that it is not totally invisible).

There is No Free Lunch in multi-band antennas.  Everything is
a compromise.   Stacking monobanders is also a compromise.
The BEST solution is separate towers for each band, given
enough real estate and resources.  Stacked monobanders for
each band on separate towers is even better of course.

de  Tom  N4KG

On Tue, 4 Jul 2000 06:38:07 -0700 "Lane Zeitler" <Ku7i@worldnet.att.net>
> Put up a homebrew  yagi yesterday on an 18 foot boom. 3 elements on 
> 17 and 2
> on 20. The 20 is using a reflector. I was afraid that the director 
> for 20
> mignt interfere with the 17 meter band so I left it off hoping that 
> 2
> elements on 20 would be enough. The ant is a flame thrower on 17. On 
> 20 it
> works "okay" but not nearly as well as on 17. I will add the 20 
> meter
> director tomorrow and hope that it will not interact with the 17 
> meter band
> and improve the performance of the 20 meter band. The ant was 35 
> feet in the
> air during the testing.
> Is there any reason why the two bands can not be interlaced on the 
> same
> boom. They are about 23% different in frequency (14 mhz vs. 18 mhz).
> Lane
> Ku7i

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