Dear Joe (and the list)
>I am looking to replace my Hygain DB10/15 Dualband beam with a
>Tennadyne T7 18-33 MHZ log. It would be located under my Hygain
>204BA (20 Meter Monobander).
>On the Tennadyne website it give the gain figure as
>"6.2 dBd"? I am not sure what dBd is? Should I subtract 3 from
>the figure to get the real DB gain?
>Anyone have this antenna and have a comment about it?
>Thanks
>Joe K0BX
I have a setup similar to the one that you propose Joe and I would make the
following points:
(1) The directivity (and hence gain) will be slightly lower than your
DB10/15 on 28 and 21 mhz (at their design frequencies), but to all intents
and purposes, you will not notice the difference.
(2) The front to back on 10mx and 15mx will be slighly worse on the log,
when compared with the design centre frequencies of the DB10/15, but at band
edges, the log may well be better.
(3) In general, the front to back on the log will be worst at 18mhz (the
lowest design frequency) UNLESS the log has a SEPARATE close spaced
reflector added to the rear.
(4) Take-off angle maximum(for the same height) will be the same on either
antennas.
(5) The SWR will vary upwards to about 1.8:1 at some frequencies on the log,
so you will need to provide some sort of matching, so as to preclude the
possibility of power reduction from the finals on the rig (assuming you are
running solid state). If you are running a valve linear (or exciter) then
the tuning controls will cover that aspect, with minimum losses.
(6) You may have some frequencies where there are "RF hot" spots on the
feeder cable, depending on line length. A good line isolator will help with
this, including making a coaxial choke coil up near the antenna.
(7) There has been some discussion lately about the best place to put the
balun on a Tennadyne, but from a performance point of view, the effects are
marginal.
(8) BY FAR THE MOST IMPORTANT THING TO DO IS AS FOLLOWS:
On 18mhz, there is definitely interaction between the log and the
directors of the 204BA, causing all sorts of pattern interference,
especially from a front to back point of view.
I WOULD STRONGLY RECOMMEND THAT YOU ROTATE THE LOG AT 90 DEGREES TO THE
204BA AND MAKE THE SEPARATION AS LARGE AS YOU POSSIBLY CAN ON THE MAST.
(8) While you are at it, give serious thought to adding two gamma matches
to your 204BA boom and feeding it for 30mx and 40mx via a separate feed
line. In that way you can have rotatable antennas from 7 to 28 mhz on the
one tower.
With over 2000 band/countries on those frequencies, I can CERTAINLY
RECOMMEND SUCH A SETUP.
Cheers
Peter VK3QI
With over 2300 BAND/COUNTRIES
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