On 16 oct, 03:00, Ed G wrote:
Our ARES station has had a NVIS antenna on the roof of the building
we are in. It is a large, one story structure with a rubber coated on
steele roof.

The dual dipole type antenna, center at 14' and ends at about 3'
high, was fed with a long run of coax and has historically exhibited a
large SWR when operating below about 3825 KHz.

We recently replaced the entire antenna assy with a 122' half wave
dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the
base of the center mast. Heights are the same as previous.

The SGC 237 coupler tunes the antenna well above 3825, and all other
bands, too. However, we still exhibit a very high SWR when going below
about 3825 and the tuner fails to tune... in fact the Radio's power drops
way down, possibly preventing the tuner from working properly. A
different radio shows the same problem.

QUESTION: Can anyone offer reasons we may be having this tuning
problem below 3825?

Thanks.

Ed K7AAT

Hi Ed,

I assume that the metal roof extends over the full length of the
dipole.

Your antenne is very close to ground, that results (as mentioned by
other posters) in a very low radiation resistance (about 5..10 Ohms).
When using this construction at 40m, the impedance will be very High
(certainly above 6 kOhms, when losses are low).

I would recommend you to raise the antenna. This increases the
radiation resistance at 80m significantly. This lowers the impedance
at 40m also (making it easier for the tuner, resulting in higher
overall radiation efficiency).

When raising the antenna is not possible:
Construct the quarter wave sections of about 3 wires in parallel,
about 1m separated. Connect the wires at the feed point. This
"emulates" a thick strip dipole.

The result is that the radiation resistance at 80m does not change,
but the input impedance at 40m reduces significantly (a factor 6 is
possible). This enables the use of a transformer (for example 2(feed
line) : 1(dipole side) ). Now the 5..10 Ohms becomes 20..40 Ohms and
the 6 kOhms will remain, or drops a little. As the impedance ratio
has been reduced now, matching is easier. Please note that your
transformer experiences high voltage (so high core flux) when operated
on 40m.

Other option: use the transformer on 80m only, and remove it (relay)
when operating on 40m.

I hope this helps a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl