Yagi style aerial + modification

anonprivate

Standard Member
Hi,

I get very good reception from a local forward transmitter.

I would like to sacrifice some of this gain in order to increase the gain from the rear.

Is there a quick way of masking (not permanent removal) a few of the directors so that the gain from the rear will be increased.

The aerial has about nine directors, a dipole (encased in plastic), and a four pole reflector system, The reflectors can be slid out with effort. When I moved them down to expose more of the dipole (upper portion), it made no difference to the reception.

Before anyone asks. No, the aerial cannot be moved; and Yes, I have considered establishing a second aerial.

Thanks
 
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Chris Muriel

Distinguished Member
There will be a loss from the rear, an amount dependent on the front-to-back ratio of the aerial.
To decrease this loss, you would have to remove the reflectors at the rear altogether.
 

anonprivate

Standard Member
Thank you for responding.

I am not sure what you are saying!

Are you saying that if I remove the reflectors completely that I will increase the gain to the rear of the aerial, and lose a little from the front.

Best wishes.

A
 

Tight Git

Distinguished Member
Basically yes, otherwise you'd be conjuring up something for nothing!

But are you aware that what you are suggesting is far more complex than it might sound?

Any tampering with the Yagi design is bound to introduce polar diagram anomalies, not to mention possible impedance matching problems.
 

Gavtech

Administrator
The aerial has about nine directors, a dipole (encased in plastic), and a four pole reflector system, The reflectors can be slid out with effort. When I moved them down to expose more of the dipole (upper portion), it made no difference to the reception.

Why not simply suck it and see.

Remove the reflectors and see if it achieves your objective or not.
 

anonprivate

Standard Member
Why not simply suck it and see.

Remove the reflectors and see if it achieves your objective or not.


I do intend to try the idea. I was wondering if it would work in theory so that I know what to expect. Someone may have tried it and could relate their experience.

I can alway put the relectors back and restore the original signal pattern, hence, nothing is lost.

Best wishes
 

Chris Muriel

Distinguished Member
It wouldn't help a great deal if someone else had tried it unless at the very least they had the same make and model of aerial. As alluded to above, yagi design is actually quite critiical with a lot of interdependencies between number of elements, element spacing and size etc. ; the performance of the aerial is also affected by its position related to that of other metal or partially conductive objects, its height above ground or above a roof and so on.
You have nothing to lose by removing the elements and seeing if it then meets your objectives; please report back on the results.
 

lbear

Well-known Member
If you have a very strong signal from the front, have you had a look at the characteristics of the aerial? Often the "back" reception peaks in a similar but smaller way than the front (a sort of figure of 8).

That made me wonder if you would maintain a good or adequate signal to the TV from the main transmitter if you reversed the aerial. The increase in power after DSO at most transmitters should compensate for the lower back sensitivity. You mentioned it could not be moved but could the aerial be turned through 180 degrees?
 

Chris Muriel

Distinguished Member
That's a good bit of sideways thinking - and might just work :eek:
Well worth trying a 180º rotation as that should just mean slackening a couple of pole clamps, rotating and retightening.
 

anonprivate

Standard Member
If you have a very strong signal from the front, have you had a look at the characteristics of the aerial? Often the "back" reception peaks in a similar but smaller way than the front (a sort of figure of 8).

That made me wonder if you would maintain a good or adequate signal to the TV from the main transmitter if you reversed the aerial. The increase in power after DSO at most transmitters should compensate for the lower back sensitivity. You mentioned it could not be moved but could the aerial be turned through 180 degrees?

Unfortunately, it cannot be turned through 180 degree.
 

Chris Muriel

Distinguished Member
This is the 1st time I realised it was a loft aerial.
So, hoping your loft is boarded to make access easy and safe, remove the reflectors, test results and report results here.
 

anonprivate

Standard Member
This is the 1st time I realised it was a loft aerial.
So, hoping your loft is boarded to make access easy and safe, remove the reflectors, test results and report results here.


Access is difficult - no boards, loft packed, little headroom, no light - could not be much worse.

I will venture in shortly and see what I can achieve.

As amatter of interest, if the aerial has four reflectors, equally spaced, how many need to be removed to effectively disable the reflector system - my estimation is all of them.

The point of removing the relectors is in the hope of pushing the radiation pattern a little away from the front and towards the rear. I do realise that there is a complex relationship involved with all sorts of radiation patterns that will result from the adjustment and make the overall result difficult to predict - ignoring environmental factors.

Having said that, my prediction is that I will effectively convert a mono-directional aerial into a bi-directional aerial with a strong bias towards the front because of the directors and the position(back ) of the dipole.

Best wishes.

A
 

Chris Muriel

Distinguished Member
All 4 reflectors wil have some effect but the ones closest to the (I assume metal) boom will theoretically make more difference. You could try removing those 2 first, I suppose.
 
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anonprivate

Standard Member
All 4 reflectors wil have some effect but the ones closest to the boom will theoretically make more difference. You could try removing those 2 first, I suppose.

I think that you mean closest to the dipole. I don't believe that the boom has any effect. It could be made of wood - mine is metal.
 

Chris Muriel

Distinguished Member
I'm assuming both a metal boom and a V-shaped reflector; that would make the reflector elements nearest to the boom also nearest to the driven element (dipole).
 

Rodders53

Distinguished Member
:confused:
OK am I the only one thinking 'which transmitters?' and 'why?/what programme services?' is this poster trying to get off the back of an aerial in a confined space??

Knowing the approximate location of the home may help us provide sound technical advice on whether / how to achieve the aims... (Of course an outside aerial would likely receive some 10x the signal of that in the loft. But a room / set-top aerial may even provide the other wanted transmitter?)
 

grahamlthompson

In memoriam
Puzzled me why the OP didn't simply diplex a log periodic with the existing aerial. The basic log is tiny and surely will outperform a crippled yagi.
 

anonprivate

Standard Member
I am only looking for specific advice (the expectations of others in given circumstances) - before I try a few things out.

I am quite familiar with the transmitters in my vacinity, the mutiplexes, aerial polariasation and types, bandwiths, the websites, etc.

Hope this explains the lack of detail.
 

Rodders53

Distinguished Member
I am only looking for specific advice

Hope this explains the lack of detail.

No it doesn't explain anything. Mostly there's little to gain from receiving from another Tx in programme terms. Often the other is available via catchup or live streaming more simply than doing what you propose which is likely to upset your main reception.

However do whatever you want... I'll butt out.
 

anonprivate

Standard Member
No it doesn't explain anything. Mostly there's little to gain from receiving from another Tx in programme terms. Often the other is available via catchup or live streaming more simply than doing what you propose which is likely to upset your main reception.

However do whatever you want... I'll butt out.

There can be an advantage in gaining some signals from another Tx.

If I upset the reception, I can simply reverse the procedure.

I will of course do what I choose.

You can butt out - I don't mind.
 
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