My thanks to those who replied directly. It has been suggested I first
adjust the phasing lines for low band. It has also been suggested I leave
well alone and accumulate experience in behaviour of the array before
making further adjustment, given perceived F/B and F/S are so dependent on
signal direction and arrival angle. Sadly I don't feel I have the luxury
of the latter as rains might come anytime. When they do they tend to come
in the form of a deluge which can make the antenna site an unworkable
quagmire for months.
In view of the likely deterioration in the weather I have pressed on. I
provide details of my findings for those who may be interested and in the
hope they might prompt further suggestions.
I felt doubt over the likely benefit of extending the phasing lines. As
the phase relationship between the four verticals is determined by the
hybrid it feels like the relationship will be preserved so long as feed
lines to the verticals are of equal electrical length. So far as I am
aware 1/4 wave lines are used is by virtue of their current forcing
property which can to an extent compensate for differences in
characteristics of the four verticals. My four verticals appear remarkably
similar both physically and electrically. I did anyway extend the phasing
lines by 61cm to make them 1/4 wave at 3525kHz. This adjustment is
equivalent to 3 degrees of arc or 3.3%.
I discerned no difference in F/B or F/S as a consequence of the extended
phasing lines. Percentage power dumped to the 50 ohm load changed only
slightly as follows:
Frequency (kHz)
Original phasing lines
+61cm per line
3500
5%
5%
3580
3.5%
3.6%
3800
11.5%
13%
So far as I can tell the effect of adjusting phasing line length from mid
band to low is marginal. A mid band length appears fine.
This morning I have taken the hybrid back to my workshop for a more
detailed analysis to see if I can spot anything which might explain my loss
of 10dB in F/B and F/S. Measurements were made using an N2PK VNA. All
hybrid ports were terminated in 50R. The unit is configured such that when
controller power is absent the array fires NW. All measurements were made
with this setting.
Phase angle relative to source.
Port
3500kHz
3650kHz
3800kHz
NW
-128.0
-130.5
-133.0
SW
-49.0
-50.7
-52.1
SE
62.6
60.5
58.6
NE
-47.5
-48.9
-50.3
Normalised to SE (rear) port
Port
3500kHz
3650kHz
3800kHz
NW
-190.6
-191.0
-191.6
SW
-111.6
-111.2
-110.7
SE
0
0
0
NE
-110.1
-109.4
-108.9
Port amplitude relative to source (dB)
Port
3500kHz
3650kHz
3800kHz
NW
-4.75
-5.03
-5.25
SW
-5.66
-5.82
-5.96
SE
-6.74
-6.95
-7.15
NE
-5.85
-6.15
-6.35
In theory the hybrid coupler provides outputs at 0, -90 and -180 degrees
with equal amplitude 1/4 of input or -6dB. In practice mine isn't quite
like that but port amplitudes are within +/- 0.25dB between 3500 kHz and
3800 kHz and phase relationships within +/- 0.6 degrees. I see nothing
here to clearly account for my loss of 10dB F/B and F/S.
Perhaps it is simply the signals I have heard since I low banded the
array? This doesn't feel too likely as I found plenty of signals to
demonstrate 20dB at the top of the band and none so far at the bottom. I
didn't have many ideas on the cause of my problem at the start and I am
fast running out.
I have rigged a small battery powered beacon TX which I will deposit in a
hedge approximately 1km SW of the array. This will at least give me a test
signal during daylight hours when adjustments can be made. Here in Cyprus
80m has nothing but noise on the daylight side of grey line so on air
checks can only be made when adjustments cannot.
If any reader spots something untoward in my findings please let me know.
Additional suggestions on where I might look next will also be appreciated.
Many thanks.
73 Bob, 5B4AGN
On 28 October 2015 at 08:14, Bob Henderson <bob.5b4agn@gmail.com> wrote:
> In the weeks before CQWW SSB I installed my first 80m 4SQ. I am mostly a
> CW guy and want the antenna to work well at the bottom of the band.
>
> The vertical elements are Titanex V80S verticals which I bought having an
> 80m 4SQ in mind some 15 years ago. It takes me a while to get around to
> things at times! The V80S is set for mid EU band i.e. 3650kHz and without
> adding extra alloy or cutting some off there is no facility for
> adjustment. I started out with the idea I would rig the 4SQ for 3650 and
> see how effective it turned out to be at 3500 and 3800. The sides of the
> square are 20.54m. The four 1/4 wave phasing lines have been cut for 3650
> and the array is fed via a home built hybrid coupler.
>
> 4SQ experts will likely have already spotted my mistake. I overlooked the
> inevitable rise in resonant frequency of the vertical elements due to
> mutual coupling. Anyway in some respects this was kind of lucky or would
> have been had we not had serious electrical storms last weekend.
>
> First tests were very promising but with best results at 3800 not
> 3650kHz. F/B was a good 20dB and F/S not much short of that. Power dumped
> to the 50R load was around 2% but rising to around 15% at 3500 where both
> F/B and F/S were significantly less impressive at around 10-12dB.
>
> With CQWW SSB visible now only in the rear view mirror I have turned my
> attention to shifting the array down band. I reasoned I would need to pull
> the individual vertical resonance down by around 250kHZ to 3400kHz for
> optimum operation in the CW band. Taking down each vertical then adding
> further alloy would be a far from easy option so I have instead added a
> small amount of base loading. In fact 6 turns of wire around the bottom
> 30mm diameter insulator on each vertical did the job. The effect of the
> adjustment is as follows:
>
> Power dumped
>
> 3500 = 5.0%
> 3580 = 3.5% (minimum)
> 3800 = 11.5%
>
> The above looks acceptable but....
>
> F/B and F/S are around 10dB across the band so far as I have been able to
> determine. So to my question..........
>
> What step do I best take to improve the F/B and F/S in the CW part of the
> band. My options appear limited to adjusting the L&C in the hybrid and
> adding length to the 1/4 wave phasing lines. My gut feel tells me to hit
> the hybrid first. Does this make sense or is there something else I am
> overlooking?
>
> Advice from the experienced will be much appreciated.
>
> Thanks.
>
> 73 Bob, 5B4AGN
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|