Best aerial to combat cross-polar and rear-direction interference?

lovinthetelly

lovinthetelly

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#1
Hi - I'm looking for a bit of advice on aerials that will best reject signals coming from the rear, and the opposite polarisation.

The aerial will be mounted horizontally and pointing towards Sutton Coldfield. Until the 700MHz frequency changes earlier this year, our current aerial could not receive COM7, COM8 or the Local mux due to co-channel interference from Mendip (also using UHF33, 35 and 51 for these muxes). Mendip is almost 180 degrees relative to Sutton Coldfield, so its signals were hitting the rear of the aerial and knocking out Sutton Coldfield.

I can now receive COM7 and COM8 as they are on different frequencies to Mendip, but when Mendip's frequencies change to 55 and 56 next year, I expect to lose these with the current aerial. I'd also like to receive the Local mux (on UHF48) too - this briefly appeared but was lost after Bromsgrove's ARQB moved to channel 48 (vertically polarised). Bromsgrove is almost in-line with Sutton Coldfield from my location (5degrees difference).

Am I best going for a longer aerial with more elements, a high gain figure, and/or a high front-to-back ratio? I've been looking at the following:

Fracarro Omega 8
Gain 16
Front-to-back 32
Beam width 22

Antiference XG10
Gain 14.9
Front-to-back 26
Beam width 16

Antiference XT48F
Gain 16
Front-to-back 29
Beam width 26

Antiference TCX18T
Gain 15.7
Front-to-back 22
Beam width 19

Vision V1048LF
Gain 13.8
Front-to-back 26
Beam width 15

Triax 108451
Gain 14.5
Front-to-back 25
Beam width 15


Any suggestions gratefully received!
 
Terryl

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#2
The more forward director elements the antenna has the tighter the front beamwidth, however if you want a better front to back rejection (F/R ratio) a simple modification to the antenna can be done.

If you look at some of the antennas you have listed you will see the back reflector, it looks like a flattened "V", this can be modified by adding a simple aluminum window screen (no larger then 12mm mesh and without the frame) to it, make the screen about 15 cm larger on all sides then the back reflector, attach with aluminum wire and your set.

This will improve the F/B ratio, how much is anyone's guess.

A more complicated setup is a Faraday shield, this consists of two screens separated by an insulator, (wood, PVC pipe or fiberglass) this shield is not touching or directly connected to the existing antenna but is mounted directly behind it, it is Earth grounded at one single point, (both screens) this shield is 3 to 4 times the size and shape of the antenna it is shielding, something like this can totally block most signals from the back side of the antenna it is protecting.
 
aceb

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#3
Phased four element grids? I used to use four bayed four element grids for DXing and the rear rejection was fantastic.
 
PaulR

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#4
Can you position your aerial so that most of the building is shielding the rear? An aerial attached to the chimney or apex of the roof has the worst rear rejection of all.
 
Channel Hopper

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#5
Phased four element grids? I used to use four bayed four element grids for DXing and the rear rejection was fantastic.
I kept a couple back when everything went digital, they were called 'bow-tie' aerials, not much gain but excellent rejection.


You could use a parabolic dish if you can't make something up, with an appropriate feed assembly.

Gabriel Antennas used to supply similar, but it was the last decade.

Dependable, Lightweight Parabolic Grid Antennas
 
Adam792

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#6
Presumably you're round the Worcester area like me then? :D

I used to live on the North East side of the city, and we could get COM7 and COM8 from Sutton Coldfield fairly nicely there because of the shielding provided by the high ground round Elbury Mount and the old Tolladine golf course in the Mendip kind of direction (even though I could just about pick up Mendip BBC-A on an indoor aerial in the South facing upstairs bedroom!) We used to use a group B XB10 aerial there, but that was before the use of 55/56. The Birmingham local mux on 51 used to come in fine though.

Ridge Hill is already using 55 and 56 co-channel with Sutton Coldfield. It's worth bearing in mind that since the whole 55/56 SFN plan has started, they've lengthened the guard interval of these multiplexes to make them slightly less susceptible to interference between the transmitters, so you may find they work a bit better than they used to once Mendip moves over.

The problem round here often seems to be that Sutton Coldfield isn't massively strong even in the parts of the area that use it. I think the Lickey Hills block the signal a bit.

Now I live lower down in the centre there's no chance of getting COM7 or 8 because we have to use Malvern... Ridge Hill is possible for some people nearby but it's too poor here.

It's a pity neither Bromsgrove or Malvern carry the two extra muxes.
 
aceb

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#8
Channel Hopper

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#9
The Channel Master 4251 was another, 7ft reflector with a two element bowtie phased feed. Never got to use one but very popular amongst DXers in the States.

Channel Master 4251 Tribute Page

Why is the 4251 so powerful compared to other UHF antennas?
The key to the power of the 4251 is the parabolic screen: it acts like a giant "scoop" that rakes in signals. When it comes right down to it, the 4251's design is fairly simple. All it really is is a basic UHF bow-tie antenna placed at the focus of a parabolic screen.......




So all anyone needs to get similar performance is one of the discarded 2.4m Fortecstar C-band reflectors, a bow tie aerial, some way or getting zero elevation from the mount (and put it on their chimney).

Sorted !
 
lovinthetelly

lovinthetelly

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#10
Thanks for all the suggestions.

I think I might be able to lower the aerial slightly (to shield it from Mendip), but I suspect I'll end up losing COM7, COM8 and the Local mux from Sutton Coldfield - there's one tall building in the way. Annoyingly, we're on fairly high ground and can receive most multiplexes from Mendip indoors - including the local Bristol multiplex! We used to pick up everything from Ridge Hill @Adam792, but since the frequency changes this year it's been hopeless - I suspect the new Worcester relay is the cause of that. Malvern signals are equally poor for some reason.

Does a grid antenna better reject cross-polarised signals than a 'standard' aerial? I've been trying out a spare 36-element log periodic today, but I'm not having much luck with the local mux on UHF48 - just noise from Bromsgrove ARQB on the same frequency (albeit vertical).
 
Adam792

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#11
Thanks for all the suggestions.

I think I might be able to lower the aerial slightly (to shield it from Mendip), but I suspect I'll end up losing COM7, COM8 and the Local mux from Sutton Coldfield - there's one tall building in the way. Annoyingly, we're on fairly high ground and can receive most multiplexes from Mendip indoors - including the local Bristol multiplex! We used to pick up everything from Ridge Hill @Adam792, but since the frequency changes this year it's been hopeless - I suspect the new Worcester relay is the cause of that. Malvern signals are equally poor for some reason.

Does a grid antenna better reject cross-polarised signals than a 'standard' aerial? I've been trying out a spare 36-element log periodic today, but I'm not having much luck with the local mux on UHF48 - just noise from Bromsgrove ARQB on the same frequency (albeit vertical).
Ah yes of course. I think at the moment the new transmitter on Elbury Mount is relaying Sutton Coldfield. It’s supposed to be line fed at some point so it can be properly part of an SFN with Ridge Hill as they carry separate versions of the multiplexes at the moment so must be interfering with each other quite badly around the outskirts of Worcester.

I’m less than 2 miles from the new transmitter but I can’t receive it at all, presumably because the Ridge Hill signal is fairly strong around here and it just causes mush, as in your case.
 
Channel Hopper

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#12
Does a grid antenna better reject cross-polarised signals than a 'standard' aerial? I've been trying out a spare 36-element log periodic today, but I'm not having much luck with the local mux on UHF48 - just noise from Bromsgrove ARQB on the same frequency (albeit vertical).
Of course

Y u no rissen ?
 
lovinthetelly

lovinthetelly

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#13
Channel Hopper

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#14
A shield is a shield, though if you find signal does break through on the opposite polarity, you can always replace the grid for a wok.
 
lovinthetelly

lovinthetelly

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#15
Ah yes of course. I think at the moment the new transmitter on Elbury Mount is relaying Sutton Coldfield. It’s supposed to be line fed at some point so it can be properly part of an SFN with Ridge Hill as they carry separate versions of the multiplexes at the moment so must be interfering with each other quite badly around the outskirts of Worcester.

I’m less than 2 miles from the new transmitter but I can’t receive it at all, presumably because the Ridge Hill signal is fairly strong around here and it just causes mush, as in your case.
According to DigitalUK the Worcester mast is a relay of Ridge Hill, and is scheduled for a power increase on 31st October. Maybe that's when it switches to being line-fed?

The same DigitalUK page states that Bromsgrove started carrying the Birmingham local mux on 19th September (UHF29), though this doesn't appear to be the case - so probably best to take their information with a pinch of salt!
 
RustySpoons

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#16
This is doing a sterling job here picking up Wenvoe (Not line of sight and weak here due to my elevation) and rejecting the much stronger and closer Mendip
Log Periodic TV Aerial 22 Element
 
Adam792

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#17
According to DigitalUK the Worcester mast is a relay of Ridge Hill, and is scheduled for a power increase on 31st October. Maybe that's when it switches to being line-fed?

The same DigitalUK page states that Bromsgrove started carrying the Birmingham local mux on 19th September (UHF29), though this doesn't appear to be the case - so probably best to take their information with a pinch of salt!
Ahh interesting, thanks! Well I’ll give it another go after then and see if I can receive anything.

Yes it was always intended to be an SFN with Ridge Hill but since it was installed it’s had a horizontal receive aerial pointing at Sutton Coldfield and the multiplexes carried are identical to Sutton Coldfield rather than Ridge Hill, or certainly were the last time I checked it (e.g BBC-A carrying Radio Stoke and Radio Derby and not Radio Gloucestershire and Radio Shropshire).

According to someone in the know over on the MB21 transmitter gallery, this is meant to be stopgap until the line feed from Ridge Hill is sorted. Maybe this will be what the power increase is to do with as you say!
 
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#18
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