Group A or Group C/D

Mossy Monk

i have 2 aerials on my roof. Group A and Group C/D. how do i tell which is which. i forget and i'm after confusing myself

watty

A is lowest frequency so longer rods
D is highest frequency, rods much shorter

(Not talking about length of boom the rods are on)

The flat aerials with

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four sets of little aerials, often as "V" shaped pairs above are Wideband (Group A to D often called Group W or A/B-W)

lawhec

By Colour Coding:

Group A Aerials use Red as their colour, C/D aerials use Green.

By physical examination:

Group A aerials have broader elements which are spaced further apart than C/D aerials.

Mossy Monk

thanks lads

Galway

hey, i have a triax unix 100 but cant tell if it is WB or C/D deffo not A or B. There are no markings on aerial box or colour codes.

Any ideas?

watty

Measure distance from centre boom to edge of the folded dipole section that the cable connects to.

Galway

once i have done that how do i then know if it is c/d or wb?

watty

You'd know if it was A, B or C/D

Looking at the model on Web you are correct, they sell a Group W

This usually means "Wideband"

The adjective "wideband" can't be applied to 100 element Yagi (or indeed any Yagi with more than 10 elements). So I'm a bit confused.

THIS AERIAL


(apologies nicked from http://www.brymar.co.uk/acatalog/Online_Catalogue_Aerials_12.html who seem to think that disable a right click is useful )

Is a wideband aerial, intriniscally.

Can't see how the one above it on page can be wideband
100 Element Triax Unix Aerial Triax Unix 100 element aerial, extra high power, digital ready.


here is the Triax site:
http://www.triax.dk/ifs/files/triax/is/presentation/home/Satellite_x_terrestrial/dth_(direct_to_home)/sitemap.jsp#Aerials,%20dishes%20and%20accessories

They have these 3 aerials:

UNIX 100 (108755)
Channels 21-50
Elements 100
Gain 17.0 dB
Front To Back Ratio 27 dB
Beam Width Hor +/- 11 degrees

UNIX 100 (108754)
Channels 21-40
Elements 100
Gain 17.0 dB
Front To Back Ratio 27 dB
Beam Width Hor +/- 11 degrees

UNIX 100 Digital (108756)
Channels 21-69
Elements 100
Gain 17.0 dB
Front To Back Ratio 27 dB
Beam Width Hor +/- 11 degrees


The digits in brackets = part number = last part of EAN BARCODE price label

Can't say I believe the Gain / Bandwidth (channels) stated. Esp on the 108756 model so called group W


This I do believe:
Log-periodic UHF (108680)
Channels 21-69
Elements 30
Gain 10.0 dB
Front To Back Ratio > 30 dB
Beam Width Hor +/- 25 UHF degrees

BTW they have a funny way of counting "elements" they seem to regard each piece of metal as an "element"
I count 15 elements on the 30 element Logperiodic

I think they count each element (director, driven element or reflector) on UNIX 100/ 52 as FOUR elements. So it is really a 25 element aerial.

The difference between a regular yagi and the UNIX is that the X shape elements instead of -- shape elements means that polarisation shifts due to path or weather over long distance are not a problem. It does slightly widen bandwidth but at expense of gain (there are no free lunches in aerial design).

Galway

there are no markings at all on aerial or packaging. I think it must be WB as it will pick up group a from castlebar but seem most effective on C/d channels. Any other way of ascertaining group? Deffo not A or B as I have both of these to compare. The Triax is supposed to be more effective overall to the anitiference xG type- do u think??

Also i have a hirschmann Wesa (I think) round elements then interspaced with single rods,has an orange and black dipole box and huge mesh tye reflector. Any view on how effective these are.I have found it worse than antiference XG 18 for BAlla deflector reception. It is a 21-37 as it is German.

watty

If it is most effective on C/D then it is a C/D. Thats what Group CD means.

Any aerial will pickup outside its group.

The smaller the aerial the less difference between a well out of group signal and the "in band".

The bigger (more elements, longer), the less bandwidth the aerial has so more difference between in group and out of group. But it will still give more signal than a "rabbit ear" or any other settop aerial on *ANY* channel.

The "UNIX" or any "pure" yagi can't by definition be really wideband (W or WB) as they get "narrower" in bandwidth as the aerial gets longer (boom length compared with wavelength is what counts, not the actual number of bits of metal).

600MHz (groupA /B area) is 0.5m for a wavelength.

There is a diminishing return on gain over 2 wavelength, so any UHF aerial about 1m long (3ft) is fairly good. A 4ft long (1.2m) is about as good as it gets. After that more gain is easier to acheive by stacking 2, 4 or 8 aerials in an array.

Stacking vertically slightly improves bandwidth, which is why the flat style wide band arrays as in picture, stack four dipoles vertically, (obviosly I'm referring to Horizontal polarised view, for vertical polarisation, rotate the WHOLE array by 90 degrees).

A "quad array" rather than four yagi stacked vertically, is easier to wire up and gives a good gain/bandwidth compromise.

It isn't easy to wire, but two sets of four "colinear" mounted yagi is about the limit.

Only dipole arrays (like the one in the picture) Log Periodic or "Discone" aerials are wideband. A "UNIX" (really a sepcialised yagi varient) or yagi aerial CAN NOT be wide band if it has more than 3 or 4 elements.

Discone antenna are very wide band and have more gain than a halo, but since they are omni directional, any UHF aerial is better for TV. They are only kind that will work from say 30Mhz to 3000MHz (good for a scanner).

A "colinear vertical" made from dipoles or 5/8th wave sections like a "Watson 300" or a WiFi 10dB OMNI only works on 1 or 2 bands (The 10dB Omnis use about 8 dipoles in a fibre glass tube parasitic on one coax for 2.4GHz only, the Watson uses wierd metal loops in a plastic/fibre pole that are 5/8th at 145MHz and resonate also at 430MHz)

A log periodic for 30Mhz to 3000Mhz is possible, but would be huge!

If you want one group or channel then 1 or more Yagi about 4ft long. If you want the whole UHF band from one direction then a Logperiodic.

If you want DX reception of say group A and D from same direction then use two yagi well separated (about 2meter apart) and a regular combiner or better still a professional "diplexer " for Group A and D. Why? Because true wideband aerials do not have anything like as much gain as a same size cheaper yagi.

There is no free lunch. You either have Wide Band or Max Gain, not both (except with a HUGE Logperiodic about 3meter long for UHF, never seen one).

Galway

it gives perfect reception on group a whereas the group a one is poor for B and useless for c/d.

watty

The Group A is it a "UNIX" or some other design?

There are all kinds of reasons why aerials are different, but you don't compare an aerial with another one to see which band/group/channel it is, you provide it with the same signal on different channels at same strength (not easy without and aerial testing range and special gear).

Either way it sounds a better aerial than the Group A model.

Even length or route of coax, a bent or cracked element or a corroded joint can have a huge effect on performance.
Or distance from another object or aerial, or how it is mounted on pole. I have seen length of pole make a dramatic effect, apart from overall height of aerial.

Galway

The group A & the B are both Triax Unix 100 - the gr A is massive in length. The a and B are clearly marked. So i guess its a K or Wb (sort of as u say).I find the grid aerials most unsatisfactory from the gain point of view - ok if signal is clean and strong.

The triax unix are supposed to be superior ro XG antiference, the blakes, Wisi, Alcad, is this so?

RTE1 ch 29 from Castlebar is suffering co-channel tonight is can only be from Holywell Hill I would say -I doubt is 3 ROCK. BBC 1 NI from Balla deflector is suffering co channel from Mullaghanish TG4 both ch 31 andalso BBC 2NI on 27 with TV3.

watty

Originally posted by Galway :
I find the grid aerials most unsatisfactory from the gain point of view - ok if signal is clean and strong.

Yes but they *ARE* wideband. The TRIAX arn't really. Can't be.
No free lunch

For DX, an aerial that just does a few channels is best and reduces interference and intermodulation from other channels nearby.

Originally posted by Galway :

The triax unix are supposed to be superior ro XG antiference, the blakes, Wisi, Alcad, is this so?

No idea.


Our TV3 has "cleaned up" on its own.

Mike 1972

BTW when you see a "Gain" figure quoted check if its dBi or dBd (Theres a 2.16 dB difference)

An aerial with a 12.2 dBi gain only has a dBd gain of 10dB

Although Grid/colourking/fireguard/antighost/stacked bowtie aerials are supposedly wideband they exhibit a marked fall off in the lower end of Band 4 (group A ) especially the cheaper versions

Galway

yes, they seem on in gr c/d but pretty crap in group A esp 29-37.I am NOT a fan and prefer a groupised high gain yagi anyday.

watty

Depends what you are trying to do:
For 10Mhz and up

Low gain but any channel: *Properly* made "bowtie" aerial. Many are inferior quality.

High gain / Max gain on a few channels: Yagi (but unpredictable side lobes depending on model)

Very low side lobes/ghost, wide band, medium gain: Log Periodic.

Wide band and High Gain, low side lobes: Darned gaint sized Log Periodic.

Omni-directional Horizontal polariasation: Halo. (But has NEGATIVE gain!).

Omni-directional Vertical polarisation: Colinear Array

Vertical or Horizontal, nearly omni if vertical, wide band: Dipole (but no gain).

Reasonable bandwidth, slight gain: Dipole with corner reflector

Ultra wide band, very little or no gain: Discone (1Mhz to 3000MHz possible).

Wide band, but only gain/diectional at higher frequencies: Dish (Gain x4 for every x2 of Frequency). Can use dipole or horn feed.

High frequency narrow band: Horn on rectangular waveguide

Yagis, log periodic, discone are problematic above 2GHz due to tolerances required. Colinear perhaps useable as narrow band aerial up to 5GHz. Dishes and Horns useful from 2Ghz up.

A very big open mesh dish can be used 600MHz .. 2000MHz if large enough for more than 18dBi gain. (It is hard to get more than 13dBi from a single yagi). It would be HUGE.

"i" means relative gain to a theortical "Isotropic" radiator
"d" means relative gain to a perfect dipole in a vacuum

Since a dipole has some gain compared with Isotropic (2.1db) this is why the two figures vary. The dipole has less gain along the axis of the rod, which is why a vertical dipole is omnidirectional and horizontal dipole is not omnidirectional. An isotropic radiator would radiate a spherical pattern. Any aerial that raditates (receives is same thing) more one direction than another is likely to have gain. (A halo would be the only exception I know)

There is also circular polarisation used by C-Band Satellite and some UHF specialist transmissions (weather fax). It can be Left hand or Right hand.

A ordinary aerial in vertical or horizontal polaristed mode will receive exactly 1/2 signal of LH or RH circular polaraised signal.

For wide band circular polarised you use a helical aerial.
For normal "yagi" bandwidth you can buy a yagi made out of two yagis at rightangles on a single boom with two feed cables:
Depending on how you connect the feeds to a switch box with some lengths of coax in it cut for channels you want:
o Vertical
o Horizontal
o Slant -45 deg
o Slant +45 deg
o Circular LH
o Circular RH

These yagi are handy not just for circular but for "ordinary" DX work on VHF or UHF as you can mount it on a rotator (£60 .. £200) to point at desired distant Radio, TV or Amatuer transmitter and then select vertical or horizontal without going up on roof and rotating the rods vertical or horizontal.

They arn't cheap. Also for Circular polarisations the fitted coax length in the switch box only does a couple of channels (not a problem for the UHF Satellite weather fax).