achieving more gain by stacking/baying two large UHF aerials

Today I installed two Vision 76 element aerials, vertically polarized,
side by side and about a metre apart. I needed the extra directivity
this gives, but I was also interested in the gain. Vision sells a
'combiner/balun' as part of their stacked log periodic kit. This
contains nothing but PCB tracks commoning the two inputs and the
output, so whether it has any function as a matcher I don't know. I
tried this device with the two 76 element aerials, which of course is
not what it's sold for.

I measured all 11 signals from Crosspool, and averaged the results.
There no were no big deviations or peculiarities in each sequence of
figures.

Taking the average of the signal from each of the two aerials as 0,
the gain achieved by stacking two was as follows:

Taylor 2 way splitter: 1.9dB
No combiner/matcher*: 2.1dB
Vision combiner: 1.3dB

*No combiner/matcher: the two feeders were connected together with a
soldered joint. It's interesting that even with a 2:1 mismatch the
gain is slightly better than with a splitter. I expected very strange
results with this method of combining, but the gain was fairly
consistent across the channels.

When I’ve stacked Blake 18 elements and various logs I’ve usually
managed to get a bit more gain – around 2.7dB on average. I don’t know
why these figures are lower than that. I've never stacked two of these
big Vision aerials before. I'll experiment further.

The actual result of this installation was pretty impressive. These
big Vision wideband aerials do seem to work very well (for a wideband)
and stacking two of them gave a really worthwhile improvement on the
previous aerial, which was an 18 element wideband. Signal levels were
8 to 13dB better, BER on the difficult muxes went from marginal to
perfect, and there was no visible ghosting on the analogue channels.
The last was a real surprise.

Bill

Well, back in the old days I used to find directly soldered harnesses were
much better than so called custom harnesses.
Of course in the older than that days of band LLL the matching could be
done with Y matched systems, which matched the impedences but had the
disadvantage that the whole array had to be bought as such, as the driven
elements were non standard.

It does make one wonder though, just how much they bother to match current
systems driven arrays impedence wise in any case.
Over a wide band I suspect its not that possible to be very accurate.

Brian

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" wrote in message
...
Today I installed two Vision 76 element aerials, vertically polarized,
side by side and about a metre apart. I needed the extra directivity
this gives, but I was also interested in the gain. Vision sells a
'combiner/balun' as part of their stacked log periodic kit. This
contains nothing but PCB tracks commoning the two inputs and the
output, so whether it has any function as a matcher I don't know. I
tried this device with the two 76 element aerials, which of course is
not what it's sold for.

I measured all 11 signals from Crosspool, and averaged the results.
There no were no big deviations or peculiarities in each sequence of
figures.

Taking the average of the signal from each of the two aerials as 0,
the gain achieved by stacking two was as follows:

Taylor 2 way splitter: 1.9dB
No combiner/matcher*: 2.1dB
Vision combiner: 1.3dB

*No combiner/matcher: the two feeders were connected together with a
soldered joint. It's interesting that even with a 2:1 mismatch the
gain is slightly better than with a splitter. I expected very strange
results with this method of combining, but the gain was fairly
consistent across the channels.

When I’ve stacked Blake 18 elements and various logs I’ve usually
managed to get a bit more gain – around 2.7dB on average. I don’t know
why these figures are lower than that. I've never stacked two of these
big Vision aerials before. I'll experiment further.

The actual result of this installation was pretty impressive. These
big Vision wideband aerials do seem to work very well (for a wideband)
and stacking two of them gave a really worthwhile improvement on the
previous aerial, which was an 18 element wideband. Signal levels were
8 to 13dB better, BER on the difficult muxes went from marginal to
perfect, and there was no visible ghosting on the analogue channels.
The last was a real surprise.

Bill

In article
,
wrote:
Today I installed two Vision 76 element aerials, vertically polarized,
side by side and about a metre apart.

Taking the average of the signal from each of the two aerials as 0, the
gain achieved by stacking two was as follows:

Taylor 2 way splitter:
1.9dB No combiner/matcher*:
2.1dB Vision combiner: 1.3dB

*No combiner/matcher: the two feeders were connected together with a
soldered joint. It's interesting that even with a 2:1 mismatch the gain
is slightly better than with a splitter. I expected very strange results
with this method of combining, but the gain was fairly consistent across
the channels.

Three questions spring to my mind.

First to clarify one practical point. Did you wire them 'in parallel'? -
i.e. connecting inners together and screens together? Feeds of the same
length?

Second, any idea what the actual feed impedance of the individual antennas
are? The above implies they are well above 75Ohm if you used a 'parallel'
connection.

Finally, a point I can't recall being seen covered. How large the degree of
near-field interaction between the antennas would have been - thus altering
their individual characters. Any experiments with the effect of varying the
spacing between them?

Slainte,

Jim

--
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On May 6, 8:22*am, Jim Lesurf wrote:
In article
,
* wrote:

Today I installed two Vision 76 element aerials, vertically polarized,
side by side and about a metre apart.
Taking the average of the signal from each of the two aerials as 0, the
gain achieved by stacking two was as follows:
Taylor 2 way splitter:
1.9dB No combiner/matcher*:
2.1dB Vision combiner: 1.3dB
*No combiner/matcher: the two feeders were connected together with a
soldered joint. It's interesting that even with a 2:1 mismatch the gain
is slightly better than with a splitter. I expected very strange results
with this method of combining, but the gain was fairly consistent across
the channels.

Three questions spring to my mind.

First to clarify one practical point. Did you wire them 'in parallel'? -
i.e. connecting inners together and screens together?
Yes. The joint was physically very small and fully screened.

Feeds of the same
length?
Yes


Second, any idea what the actual feed impedance of the individual antennas
are? The above implies they are well above 75Ohm if you used a 'parallel'
connection.
The aerials are 'off the shelf' ones intended for 75ohm cable, so I
have to assume they match it reasonably. Basically what I had was two
nominally 75ohm aerials, with two lengths of 75 ohm cable connected to
a single 75 ohm downlead. It might be worth trying 50ohm cable from
the aerials to the combining point.


Finally, a point I can't recall being seen covered. How large the degree of
near-field interaction between the antennas would have been - thus altering
their individual characters. Any experiments with the effect of varying the
spacing between them?
On this occasion no, but I've experimented before and always seem to
find that the 'capture area' of logs and conventional yagis is smaller
than I expect. This is based on the simple method of measuring the
gain of one aerial whilst positioning another one at different
distances from it. I've settled generally for a spacing of about
800mm. The problem with wider spacing is that the nulls created by the
phasing get very sharp and if the aerial is twisted around in a gale
the signal levels can flutter. Very wide spacing gives nulls close to
the midline of the forward lobe, which therefore becomes too narrow.

I intend to repeat this installation, and I'll experiment with inter-
aerial spacing. It could be that these large aerials need more space
around them. Note however that the measurements of gain achieved by
phasing was measured relative to the average outputs of the two
aerials individually, whilst they were mounted side by side, so if the
inter-aerial spacing was affecting them this didn't affect the phasing-
derived gain figures.

Subjectively, the final results were remarkably good, so I don't think
the performance of each aerial was much compromised by the proximity
of the other.

Another odd thing. Following the measurements etc I connected a
Proception 9dB masthead amp about 300mm of cable below the splitter (I
settled for a sceened inductive splitter for the final job). Testing
the amp in situ the gain seemed a bit better than it should be, and
now as I look at the 'bottom of the the downlead figures' I seem to
have acquired an unexplained 2 or 3dB from somewhere. The Proception
amps are normally pretty accurately 9 or 10dB. Maybe I had one that
was out of spec, or maybe something funny was happening.

Next time I do this I will also try a resistive splitter as a
combiner.
.
Slainte,

Jim

Bill

In message
,
" writes
On May 6, 8:22*am, Jim Lesurf wrote:

Second, any idea what the actual feed impedance of the individual antennas
are? The above implies they are well above 75Ohm if you used a 'parallel'
connection.
The aerials are 'off the shelf' ones intended for 75ohm cable, so I
have to assume they match it reasonably. Basically what I had was two
nominally 75ohm aerials, with two lengths of 75 ohm cable connected to
a single 75 ohm downlead. It might be worth trying 50ohm cable from
the aerials to the combining point.

You could match the system by inserting a quarterwave of 53 (= 50) ohm
cable between the feeder and the junction of the feeds from the two
aerials. If it's a wideband allocation, the 'quarterwave' is a problem.

Alternatively, you could use equal odd multiple of quarterwaves of 106
ohm coax (!) between the aerials and the junction with the 75 ohm. I
suppose the closest is 91 ohm. But there is still the 'wideband
quarterwave' problem.
--
Ian

On May 6, 5:02*pm, Ian Jackson
wrote:
In message
,
" writes

Alternatively, you could use equal odd multiple of quarterwaves of 106
ohm coax (!) between the aerials and the junction with the 75 ohm. I
suppose the closest is 91 ohm. But there is still the 'wideband
quarterwave' problem.
--
Ian

Yes, and this is ch21 to ch67!

Bill

In message
,
" writes
On May 6, 5:02*pm, Ian Jackson
wrote:
In message
,
" writes

Alternatively, you could use equal odd multiple of quarterwaves of 106
ohm coax (!) between the aerials and the junction with the 75 ohm. I
suppose the closest is 91 ohm. But there is still the 'wideband
quarterwave' problem.
--
Ian

Yes, and this is ch21 to ch67!

So 'quarterwave' would not be very accurate, even if cut for the centre
(geometric mean) frequency.

Obviously the only way to cover this bandwidth would be to use something
which is not frequency-sensitive. The 'splitter-as-a-combiner' method is
an example, but you suffer the inherent losses in the ferrite
transformers. You might get slightly less loss (0.5dB?) if you remove
the splitter transformer (the centre-tapped one with the 150 or 180 ohms
across it), leaving only the 2:1 impedance transformer (70 to 37.5
ohms). The correct turns ratio is sqrt 2 (1.414) to 1. Typically it is a
practical 4:3 (4 tapped at 3) or 3:2 (3 tapped at 2). The tap needs to
be connected directly to both of the 'output' ports (which will now be
in parallel). There will be no isolation between the 'outputs'.

This modification will almost certainly cause the matching of the unit
to be no longer optimised. It should re-optimised (all ports) using a
sweep and an RLR bridge. [It may need some 'fiddle-capacitors' on the
ports, or the values of any exiting fiddle-capacitors to be changed.]
This all seems a lot of hassle, but it might just put this method
slightly ahead of simply using a T-piece.
--
Ian

On May 6, 9:40*pm, Ian Jackson
wrote:
In message
,
" writesOn May 6, 5:02*pm, Ian Jackson
wrote:
In message
,
" writes

Alternatively, you could use equal odd multiple of quarterwaves of 106
ohm coax (!) between the aerials and the junction with the 75 ohm. I
suppose the closest is 91 ohm. But there is still the 'wideband
quarterwave' problem.
--
Ian

Yes, and this is ch21 to ch67!

So 'quarterwave' would not be very accurate, even if cut for the centre
(geometric mean) frequency.

Obviously the only way to cover this bandwidth would be to use something
which is not frequency-sensitive. The 'splitter-as-a-combiner' method is
an example, but you suffer the inherent losses in the ferrite
transformers. You might get slightly less loss (0.5dB?) if you remove
the splitter transformer (the centre-tapped one with the 150 or 180 ohms
across it), leaving only the 2:1 impedance transformer (70 to 37.5
ohms). The correct turns ratio is sqrt 2 (1.414) to 1. Typically it is a
practical 4:3 (4 tapped at 3) or 3:2 (3 tapped at 2). The tap needs to
be connected directly to both of the 'output' ports (which will now be
in parallel). There will be no isolation between the 'outputs'.

This modification will almost certainly cause the matching of the unit
to be no longer optimised. It should re-optimised (all ports) using a
sweep and an RLR bridge. [It may need some 'fiddle-capacitors' on the
ports, or the values of any exiting fiddle-capacitors to be changed.]
This all seems a lot of hassle, but it might just put this method
slightly ahead of simply using a T-piece.
--
Ian

Ohhh! Much food for thought, and scope for experimentation! Thanks
dear boy.

Bill

All this reminds me of some back yard experiments I did with alternate
sections of co-ax wired outer to inner to make a 145Mhz colinier aerial.

Apparently the design was developed by no less a person than Alan Blumlein,
but in my hands, it was a total failure.

--
Graham.

%Profound_observation%

Second, any idea what the actual feed impedance of the individual antennas
are? The above implies they are well above 75Ohm if you used a 'parallel'
connection.
The aerials are 'off the shelf' ones intended for 75ohm cable, so I
have to assume they match it reasonably. Basically what I had was two
nominally 75ohm aerials, with two lengths of 75 ohm cable connected to
a single 75 ohm downlead. It might be worth trying 50ohm cable from
the aerials to the combining point.

Just to clarify are you parrelling all 75 Ohm feeders together?...

--
Tony Sayer