VHF aerial in Attic

Zardoz

I am looking for a bit of advice on setting up a vhf aerial for RTE in an attic.
I have a 4 way distribution amp in the attic ,a Proception one ,its pretty good.
I have a Contract band UHF aerial outside on a pole and its wired into the amp.
The analogue and digital signal from it are excellent in all 4 rooms I have connected.
The vhf aerial is in the attic,its an old aerial and its a temperamental thing.
I have it set at the correct polarity and point it in the direction of the transmitter but the signal is pretty good for RTE2 but poor for RTE1.
I tried moving it slightly but its total hit and miss ,one minute the signal is ok ,the next its crap.The wiring is good and there are no shorts.
Its a pretty confined attic and there is a plastic water tank ,and an enclosure for a skylight in front of the aerial.
Would this effect the signal ?
I was thinking of moving the aerial to the front of the attic in front of all the stuff in the hope of getting a better signal or perhaps the aerial is too old,a newer model might be better ?

Possibly a silly question but are you sure the television is tuned to the correct transmitter for RTÉ 1?

Zardoz

Karsini wrote: »

Possibly a silly question but are you sure the television is tuned to the correct transmitter for RTÉ 1?

Yes ,I have it tuned it correctly ,I am not a fool.:)
Its a reception issue ,one which I need to deduce is caused by either the aerial,the attic or a combination of the 2.

Antenna

Zardoz wrote: »

The vhf aerial is in the attic,its an old aerial and its a temperamental thing.
I have it set at the correct polarity and point it in the direction of the transmitter but the signal is pretty good for RTE2 but poor for RTE1.

Some old VHF 'Yagi' Band 3 aerials sold in Ireland - predating the launch of RTE2 (1978), were a narrow band design and made for just a section of Band III. This was fine with just a single RTE channel, but it is said after RTE2 launched aerials often had to be replaced because the new channel was poor. I heard stories in my area that some people twisted the existing aerial around 180 degrees (pointing the opposite direction)! RTE2 (Ch G) picture then became better but RTE1 (Ch D) deteriorated.

I think I remember seeing an old aerial in an attic marked Channels D and E. - This was for RTE1 but didn't include RTE2 (Ch G) in that area

Could the aerial be that old?

Antenna

Antenna wrote: »

Some old VHF 'Yagi' Band 3 aerials sold in Ireland - predating the launch of RTE2 (1978), were a narrow band design and made for just a section of Band III. This was fine with just a single RTE channel, but it is said after RTE2 started aerials often had to be replaced because the second channel was poor. I heard stories that some people twisted the existing aerial around 180 degrees (pointing the opposite direction)! RTE2 (Ch G) picture then became better but RTE1 (Ch D) deteriorated.

Could the aerial be that old?

That's correct, hadn't thought of it though. I remember in my parents' house the original VHF aerial was still in the attic, it only worked on channel D (RTÉ 1 Mullaghanish). Clearly had CH.D written in blue permanent marker on it.

Zardoz

Thanks for the feedback guys.
I moved the aerial to the front of the attic this morning so its in an open area,a few feet inside the roof.
The signal was fair on both RTE1 and RTE2 ,bit of snow and some vertical banding ,when pointed it in the direction of the transmitter (Mullaghanish)
Out of curiosity I turned the aerial 180 degrees around so as the black connector where the wire is attached was at the front.
The picture on RTE1 and 2 became very good ,no snow,no lines but there was ghosting .
Its a very good picture on both channels but for the ghosting.
I have the aerial resting on a wooden joist .
The aerial is very old,it is at least 30 years old so it could indeed be a narrow band one.Its a 7 element yagi and the connector is right at the end.Its symmetrical too bar the connector.
I must try to reduce the ghosting now or would I just be better off buying a newer aerial ?

Sam Russell

You could try to 'improve' the aerial. By reducing the length of the directors, you will increase the bandwidth of the aerial. Now it is a black art, but you have a duff aerial at the moment. I would try 5-10% off the length of the shorter elements. A yagi goes Reflector, dipole, director, director, director .... There is only one reflector. The reflector is longer than the dipole, the directors shorter. You could also try other orientations.

But you would be better with a new aerial designed for the job.

Zardoz

Sam Russell wrote: »

You could try to 'improve' the aerial. By reducing the length of the directors, you will increase the bandwidth of the aerial. Now it is a black art, but you have a duff aerial at the moment. I would try 5-10% off the length of the shorter elements. A yagi goes Reflector, dipole, director, director, director .... There is only one reflector. The reflector is longer than the dipole, the directors shorter. You could also try other orientations.

But you would be better with a new aerial designed for the job.

The aerial I have is very symmetrical.
All the elements are the same length and width and there is no reflector on it.Doesnt seem to be a dipole either !
Its also pretty flimsy.
I think I'll just buy a new aerial ,the aerial I have is obviously a dud.

Rippy

Rte 1 on VHF from Mulla often gives me problems. It's always 1 never 2. If you must keep the aerial in the attic, antiferance Lp7 log periodic is your best bet. Some amplification through a low noise amp may help too. Does your proception Amp have an active or passive VHF input?
Much experimentation with position in Attic will help.
Put it outside if you can

Zardoz

Rippy wrote: »

Rte 1 on VHF from Mulla often gives me problems. It's always 1 never 2. If you must keep the aerial in the attic, antiferance Lp7 log periodic is your best bet. Some amplification through a low noise amp may help too. Does your proception Amp have an active or passive VHF input?
Much experimentation with position in Attic will help.
Put it outside if you can

Thanks Rippy.
I looked up that antiference Lp7 log periodic aerial and oddly enough that is the aerial that I have !
Its the one in the middle here http://www.wrightsaerials.tv/aerialphotography/aerialingus/004.shtml
I think the problem I had was that I had it pointed 180 degrees out of line as the bit where the wire is attached goes at the front to the aerial not at the back as normal.
I know the error of my ways now.
I guess I will just have to vary the positioning in the attic .
The Proception amp is this one
http://www.tvtrade.ie/proception-4-way-tv-amplifier-with-bypass.html
It seems pretty good .

Sam Russell

The log-periodic aerial feeds from the shortest elements end, and the elements act as directors until they exceed the resonant length when they become reflectors. This type of aerial is naturally wide band and should work for you. Does it have a balun? (a small circuit that transforms the cable feed to balanced feed for the aerial- usually a printed circuit)

Zardoz

Sam Russell wrote: »

Does it have a balun? (a small circuit that transforms the cable feed to balanced feed for the aerial- usually a printed circuit)

It doesnt look like a printed circuit.
Inside the black connector are two small metal plates ,one for the signal and one for the ground .It has a saddle clamp for the ground and a screw for the signal.
The new contract UHF aerial I bought had a pcb but this one doesnt.

Sam Russell

Zardoz wrote: »

It doesnt look like a printed circuit.
Inside the black connector are two small metal plates ,one for the signal and one for the ground .It has a saddle clamp for the ground and a screw for the signal.
The new contract UHF aerial I bought had a pcb but this one doesnt.

Well, there you go, a balun could have saved you. Well, given you a few more dBs gain, anyway, it should work better if you point the sharp end towards the transmitter.

Zardoz

Sam Russell wrote: »

Well, there you go, a balun could have saved you. Well, given you a few more dBs gain, anyway, it should work better if you point the sharp end towards the transmitter.

By the sharp end you mean the end where the wire is connected ?
Could I get a Balun in a shop and attach it ?

Sam Russell

Zardoz wrote: »

By the sharp end you mean the end where the wire is connected ?
Could I get a Balun in a shop and attach it ?

The sharp end is the end with the shortest elements. The wire is connected at that end, but some manufacturers run the wire inside the boom and it may look as if it is connected at the other end. The lements get longer and further spaced as they go from front to back.

The balun is part of the design, and it is specific to the frequency. It is not worth it, the aerial should function as is. If the signal is not enough, mount it outside.

Zardoz

Sam Russell wrote: »

The sharp end is the end with the shortest elements. The wire is connected at that end, but some manufacturers run the wire inside the boom and it may look as if it is connected at the other end. The lements get longer and further spaced as they go from front to back.

The balun is part of the design, and it is specific to the frequency. It is not worth it, the aerial should function as is. If the signal is not enough, mount it outside.

Thanks Sam,I will fine tune the aerial by moving it around a bit in the attic .
My main problem was that it was 180 degrees out of position .
Is it ok to rest the aerial across joists or should I hang it off string from the joists ?

Sam Russell

Zardoz wrote: »

Is it ok to rest the aerial across joists or should I hang it off string from the joists ?

Only if you are trying to receive horizontal polariazed signals. Which transmitter are you trying for?

lucernarian

I don't think a balun adds to gain, but rather it helps with impulse noise?

I have a similar problem from Kippure, I get a perfect RTE2 signal in my attic but a very distorted RTE1. And I had to search a long time for a spot in the attic which even gave me interference-free RTE2. It's pointing through a gable wall in the attic. The aerial even works better in the rooms below, pointing through the same wall!!

Sam Russell

To_be_confirmed wrote: »

I don't think a balun adds to gain, but rather it helps with impulse noise?

A balun is a transformer that goes from BALanced to UNbalanced signals. If you do not have one, you lose half the signal, hence the 3dB. It is not gain, it is loss. If you use a monopole aerial on a ground plain, it works surprisingly well, because it does not require a balun to work properly.

Simples.:)

lucernarian

But that's just it, it doesn't add to the gain of an aerial. It just reduces losses and in any case, I thought the worst case scenario would be a 3dB loss. I usually haven't noticed a difference between using balanced and unbalanced aerials for TV, and a graph I've seen on aerialsandtv.com would also suggest the difference can be small.

http://www.aerialsandtv.com/_wp_generated/wp5e723b3f_0f.jpg

The aerials I compared were grid aerials so perhaps the impedance of it was different to a folded dipole.

Sam Russell

I got better results from a bent coat hanger than from a grid array in a recent test on DTT. Aerial design is a black art, and a small difference can have a large effect. Baluns are frequency dependant and can be well designed or not, as can aerials.

Try it and see. If it works, then great.

Zardoz

Sam Russell wrote: »

Only if you are trying to receive horizontal polariazed signals. Which transmitter are you trying for?

Mullagnanish UHF ,which is Vertical polarity.
Is it ok to rest it on Horizontal joists so ?

Sam Russell

Zardoz wrote: »

Mullagnanish UHF ,which is Vertical polarity.
Is it ok to rest it on Horizontal joists so ?

If the polarity is vertical, the elements have to be vertical. You could hang it from a bit of string, if that will point it in the right direction.

Zardoz

Sam Russell wrote: »

If the polarity is vertical, the elements have to be vertical. You could hang it from a bit of string, if that will point it in the right direction.

I can rest it on horizontal cross beams and have it point vertically,I have it like that at the moment.
Good signal on RTE1 and 2 bar the ghosting.
The joists donts affect the signal I'd imagine ?