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

"Ed G" wrote in message
92.196...


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

I ran EZnec on it, and the impedance at resonance is about 5 Ohms (at about
3.8 MHz). Most of the energy goes straight up, which might not be bad for
local contacts. If you have your heart set on it, try a 9:1 balun at the
feedpont.

Tam/WB2TT

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

Ed G wrote in
92.196:



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


Ed,

The way in which you dealt with the transition from the open wire
feedline to the SGC237 unbalanced tuner is relevant. As is whether or not
you bonded the tuner 'ground' terminal to the roof.

Your description is short on relevant detail.

Owen

Ed,

The way in which you dealt with the transition from the open wire
feedline to the SGC237 unbalanced tuner is relevant. As is whether or
not you bonded the tuner 'ground' terminal to the roof.

Your description is short on relevant detail.

Owen



SG237 tuner is not "bonded" to the roof ground, SGC does not suggest
doing such a thing, and in this case, it would not be possible anyway
since the "ground" side of the tuner is also one side of the balanced
antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open
ladder line, spaced about 1 foot off the aluminum mast, up to the
feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is
sitting in a plastic box, about 1 foot off the roof.

Ed

"Ed G" wrote in message
.89...

Ed,

The way in which you dealt with the transition from the open wire
feedline to the SGC237 unbalanced tuner is relevant. As is whether or
not you bonded the tuner 'ground' terminal to the roof.

Your description is short on relevant detail.

Owen



SG237 tuner is not "bonded" to the roof ground, SGC does not suggest
doing such a thing, and in this case, it would not be possible anyway
since the "ground" side of the tuner is also one side of the balanced
antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open
ladder line, spaced about 1 foot off the aluminum mast, up to the
feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is
sitting in a plastic box, about 1 foot off the roof.

Ed

If I had a huge metal roof like that, I would be tempted to try one of the
shortened verticals. The kind that requires radials - which would be your
roof.

You are running coax from the tuner to the shack, right; with just the 14
feet of ladder line?

Tam/WB2TT

Tam/WB2TT wrote:

If I had a huge metal roof like that, I would be tempted to try one of the
shortened verticals. The kind that requires radials - which would be your
roof.

Unfortunately, a vertical makes a poor NVIS antenna.

. . .

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Tam/WB2TT wrote:

If I had a huge metal roof like that, I would be tempted to try one of
the shortened verticals. The kind that requires radials - which would be
your roof.

Unfortunately, a vertical makes a poor NVIS antenna.

. . .

Roy Lewallen, W7EL

Still, a waste of a ground plane.

Tam/WB2TT

If I had a huge metal roof like that, I would be tempted to try one of
the shortened verticals. The kind that requires radials - which would
be your roof.


That is something we had not considered. Will keep that in mind.
Tnx.


You are running coax from the tuner to the shack, right; with just the
14 feet of ladder line?

Yes, 14' ladderline from tuner to antenna center. About 100 ' of
RG-213 on roof and another 30' or so going down to station inside
building. We have a coax choke balun located in line on the roof about
8' before the coax goes down in the building.


Ed

Ed G wrote in
.89:


Ed,

The way in which you dealt with the transition from the open wire
feedline to the SGC237 unbalanced tuner is relevant. As is whether or
not you bonded the tuner 'ground' terminal to the roof.

Your description is short on relevant detail.

Owen



SG237 tuner is not "bonded" to the roof ground, SGC does not
suggest
doing such a thing, and in this case, it would not be possible anyway
since the "ground" side of the tuner is also one side of the balanced
antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open
ladder line, spaced about 1 foot off the aluminum mast, up to the
feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is
sitting in a plastic box, about 1 foot off the roof.

Ed

The feedline is doing more than transporting energy to the dipole centre,
it almost certainly carries a significant common mode current and in that
case is just as much a part of the radiating system as the dipole itself.

So, isn't the coax and control line to the SG237 carrying current mode RF
current, ie contibuting to radiation.

Inch by inch, the detail unfolds.

Owen