Connecting 2 aerials together

[andrewpreece]

This isn't a straightforward one, but here's my first thoughts: you must ensure the phase of the signal from each aerial ADDS together rather than any other way. In the worst case the signals will completely cancel each other out. The phase is both dependent on the position of the aerials and the lengths of cables used to connect them together. The simplest strategy is to have the two aerials parallel and exactly at the same distance from the transmitter, and have the connecting cables exactly the same length, i.e the aerials will be side by side. As to the distance apart, this is governed by something called effective aperture, which you would need to work out, but as a rule of thumb if they are separated from each other by the same distance as their length I doubt you would suffer much loss. Finally you are joining two aerials together which will result, I believe in an impedance mismatch, which is not as big a problem as if you were trying to transmit high power with these aerials, but I think it will result in a reflection and some signal loss etc if you do not use a matching device or a ferrite combiner thingy. Others on this newsgroup probably know more of the theory than I,

Andy.

PS Check if your transmitter is vertically or horizontally polarised and orientate the aerials suitably, if you didn't already know.

[Gel]

I intend adding a 2nd aerial in loft to be placed underneath/connected into existing; this to give me increased signal strength.

Is there a minimum distance they should be apart to prevent ghosting effects?

Is there any special way to wire together?

Regards the various aerial groups available, what actually is the difference in their actual construction; if you installed one of incorrect group for your area would it work at all?

[Dave Plowman (News)]

This won't necessarily work. You might only change the pickup pattern - and may just end up with less signal, due to a mismatch.

Best way is to change for a single higher gain aerial - and better still, mount it outside.

[norm]

Don't do it. It's a waste of time and will probably make things worse. It is possible to splice together the two lengths of cable but ;

a) the short lengths of cable will probably need to be of a non standard impedance

b) the lengths are critical

c) a given length is only optimal at one frequency. The range of frequenices used by TV is too wide for this method to be effective.

Try googling for the term 'phasing harness'.

By a better aerial.

norm

[barclayhomes]

Thanks all; I do have main aerial on roof pointing at Crystal Palace [am in W Berkshire] and internal one is aimed at Hannington[I thnk] for Meridian etc; these go via [non powered] splitter box into TV sockets, which are electronically boosted.

Are the new aerials advertised as 'digital suitable' really any different, or is it just the packaging.=AC!!=AC

Like others, I find C5 is worst performer on analogue, but remarkably better on digital, though some picture distortion occasionally.

[Richard Sterry]

Not a good approach. The aerials will need to be identical, and you need a phasing harness or matching coupler to end up back with the correct 75 Ohm impedance instead of 37.5 Ohm (2 x 75 Ohm in parallel). Such things are common enough in other applications, but I don't know if they are readily available for TV aerials. In any case, the improvement in gain is only 3dB in theory, and in practice you will throw some of that away in connector losses etc., so not really worth the effort.

You would be better off using a single, good-quality higher-gain aerial, and maybe fitting a decent distribution amplifier. If you can mount the aerial outside, clear of obstructions, this would also help, and might avoid the need for an amplifier or a larger aerial.

An aerial for the wrong group will not work as effectively as the correct one, but it will work to a degree.

Rick

[John Borman]

I think I was lucky, when back in the days of OnDigital we were not getting the best reception so I added another aerial in the loft quite crudely put together with a booster downstairs and a booster in the loft space. Although we are in a poor signal area we have a great picture with no ghosting (On normal TV) and all the available channels on Digi.

John

[Dave Plowman (News)]

An aerial has to be suitable to receive all wanted signals in your area and reject others. If the 'digital' frequencies lie outside the analogue ones, then an aerial designed to cope with these should work better.

[The Natural Philosopher]

Ah. If you are not poinnting them at the same targets than a booster each and aimple splitter used backwards, or a resistive pad combiner, may work wonders. I would not advise combining without pre-amplification. Too lossy noise wise.

Broader bandwih, since there are typ[ically a wider sperad of frequencies to pickup.

Having said that, I put in a decent analog system and its great on digital.

Dittro, - sparks from teh rooms stats bugger analog and digital, but C5 has lost its noise and interference stripes from La Continent stations.

[andrewpreece]

Thinking on it a bit more, the real problem is the fact that connecting two aerials and two coax's together halves the impedance as compared to the 75 Ohm of the existing TV coax, and also that any signal from one of the aerials may travel not only down the coax to the TV, but up the interconnection coax of the other aerial, thus potentially reradiating the back in the direction it came! I'm thinking you need a direcional coupler, almost certain to contain a ferrite device called ( IIRC ) a circulator. This allows energy to flow in one direction only, so neither of the two aerials should be aware of the presence of the other. I know nothing about the avalability of such devices off the top of my head, but you could try Maplin or rADIOSPARES ONLINE IF NOONE ON THE HIGH STREET WELLS THEM.

Certainly, although one poster pointed out the pitfalls of connecting two or more aerials together, it is theoretically and practically possible, and aerial arrays are nothing new in the telecomms world.

Andy.

[nog]

It'll be worth posting this question in uk.tech.digital-tv. There's a very friendly, and knowledgable, aerial specialist who posts there.

[Dave Plowman (News)]

But he's already got the bones of the answer here.

[Dave Plowman (News)]

Yes - and twin TV aerials were available too. IIRC, J-Beam Explorer? But designing a twin or multiple aerial setup from scratch is a different matter from simply trying to add a second one.

[andrewpreece]

Take your points, but if he buys two identical aerials that'll solve problem

1, and if he sets up the aerials exactly side by side and pointing at the transmitter, surely that'll be enough? These aerials do not have a very tight beam pattern so alignment within a few degrees should be ok, and I'd have thought that setting them side by side accurately enough could be done by some measuring? ( I'll bow to experience here ). The VSWR of wideband aerials is a red-herring, surely? Two aerials will still provide ( theoretically ), twice the power of one, whatever the VSWR might be at any particular frequency?

As for the baying distance, if he gets them too close there'll be nasty side effects, but beyond a certain distance they can be treated as non-interacting, shurely?

As to a few mm of length matching necessary for the cables, that would imply a wavelength of something like 24cm in free space for the tv signal, which is, ummm, 1.2 Ghz. Yes alright, depends what you mean by a few mm, I'd have thought it would be possible to achieve a very decent match with 5mm accuracy. What about this 9.5dB loss though? Surely a second aerial can add only

3dB power max to the original signal? Surely a loss as big as 9.5 dB is much bigger than the maximum theoretical gain?

Quarter wave transformer, OK, I'm with you, easy enough to do with a bit of coax though.

You're right, I certainly was talking loblocks: I had forgotten the signals were in phase. Presumably then the signal from each aerial will treat the junction with the other as a node of infinite impedance, so we can forget about the imaginary problem I conjured up :-) Not entirely sure what the reference to radio shadows is about ...?

Doubtless you are right, it would seem as though the OP needs a fair knowledge of RF to achieve his aims, plus a disposition to accurate work. I love it when Usenet descends into academic discussion, that's the most interesting bit ( not necessarily for the OP though! )

Andy. :-)

PS Not familiar with RF stuff but trying to learn a bit more.

[Andy Wade]

No, bigger problems are ensuring that the aerials are identical, that the stacking or baying distance is correct, and that the cables from the combiner to the aerials are phase-matched (identical type of cable and equal lengths to within a few mm). A 2:1 mismatch (VSWR = 2, return loss = 9.5 dB), although undesirable, is not going to lead to any great disaster. The VSWR of wideband TV aerials is often not much better than

3 (6 dB return loss) in any case.

To achieve a better match a 2-way phasing harness simply uses a quarter-wavelength section of 50 ohm coax between the T-junction and the main 75 ohm downlead.

Now you've degenerated into talking utter loblocks. You don't need a circulator here, just an in-phase parallel connection! Any antenna (& in fact any large-enough metal or dielectric object) will disturb the local field around it, and create a shadow behind.

If the OP is still reading: this has become an academic discussion in best Usenet tradition. Your original notion that you can just couple in A. N. Other random aerial is fundamentally flawed and to attempt it would only lead to grief. Getting the aerial out of the loft is where you should direct your efforts.

[Dave Plowman (News)]

If this works half decently - and I've never come across a VHF radio aerial/UHF TV combination that gives as good results on FM as a direct one

- then the 'labgeaar box' will have filters in it to prevent unwanted signals from one aerial interfering with the other. In other words, it's very likely that the FM aerial will have a degree of UHF signals on it and vice versa. Perhaps the most difficult part of good aerial design is not picking up the wanted signals, but rejecting the unwanted ones, as anyone who has played with a set top TV aerial will realise.

[The Natural Philosopher]

Seems pretty good to me.

Its possible.

But anyway, I am pretty sure the OP wanted to get signals from a second transmitter patched into his system.

Short of working ouyt waht signals these mght be, and tuning for them specifically, I see no way that you could combine them other than the way I suggested.

If teh anteannae are accurately posutioned, and false pickups on te sidelobes should be small enough to not be an issue when combined with the main lobe on the other antenna.

I accepte a little tweaking of each antenna direction would improve quality on the wanted channels.

>

[Alan]

In message , Gel wrote

See

The article assumes that the aerials are roof mounted. Loft mounting has an additional set of problems.

If possible, mounting the aerial on the roof rather than in the loft would probably give you the extra signal you require.

The article comes from Bill Wrights web site Bill regularly posts (daily) to the uk.tech.digital-tv newsgroup.

It may be worth checking the news archives at

search for loft aerial in uk.tech.digital-tv and limit the search to articles in 2004

[Dave Plowman (News)]

Not what he says at all. Read it again.

[Andy Wade]

You started this by arguing that the mismatch was the main problem; now you're trying to say it doesn't matter! To generalise a bit I'd say that mismatch is never a red herring, but that how much it matters and how good the VSWR needs to be is very dependent on the circumstances of a particular application. You could have attacked my previous argument by saying that if the VSWR of TV aerials is that bad then all the reason not to make it worse with a mismatched combiner (especially since the input match of TV tuners also tends to be fairly poor).

The trade-off is between forward gain and the cleanliness of the radiation pattern. Stack too close, so the capture areas overlap, and you don't see the full 3 dB gain increase. Too far apart leads to a pattern with excessively high sidelobes.

That's about the order of accuracy needed, yes. Don't forget that the wavelength in the cable is shorter, because of the dielectric. A typical foam-dielectric TV cable has a velocity factor of about 0.85.

You're getting confused between return loss and insertion loss. Return loss is a way of quantifying the amount of mismatch - it's just the amplitude of the reflected wave from a termination. E.g. if you've got a 75 ohm cable terminated in a 100 ohm load the reflection coefficient will be (100-75)/(100+75) = 1/7 = 0.14. 20 times the common log of this gives the reflected wave amplitude in dB: -16.9 dB. Thus we say that the termination exhibits 16.9 dB return loss (a pretty good match for many purposes). The VSWR in this case is 100/75 or 1.33. The mismatch loss between a perfectly matched source and this load would be -10*log(1

- 0.14^2) = 0.09 dB, which is not a lot. If you do the same calculations with a 2:1 mismatch (reflection coefficient modulus of 1/3) you'll find that the return loss is 9.5 dB and the mismatch loss is only a shade over 0.5 dB.

That's the point precisely. This is well beyond most TV aerial riggers (but certainly not all of them).