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Topic Title: Digital TV
Topic Summary: Filtered diplexer
Created On: 06 May 2013 06:56 AM
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 15 May 2013 05:58 PM
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kengreen

Posts: 400
Joined: 15 April 2013

You write of " ... the best front-to-back antennas ...". The best known is the YAGI and there is no bigger fraud than the much applauded Yagi. I once took part in making an all-brass 3-element (folded dipole) for use practically on the Beach where electrolytic corrosion was wrecking normal aerials in 3 months. It proved to be a shocking experience.

The only time I have encountered anything so tricky was when I was called on to restore the rf filters in a Racal RA17 communications receiver; an idiot had tried to "improve" performance by "peaking" the strings of mini adjustable capacitors which were parts of e-m coupled line of LC resonant circuits.

And that is exactly what a Yagi is. The performance is CRITICALLY dependent on the exact tuning of each element and of their exact separation (in terms of wavelength). The majority of users visualise directional aerials as offerring a whopping forward lobe and a negligible rear lobe and that could not be further from the truth. To begin with the Yagi has a multiplicity of side lobes some of which can be bigger than a rear lobe - if indeed there is a rear-lobe at all. A rear-lobe can be greater than a forward lobe and both rear and
forward lobes are seldom in line with the Boom.

The best use for a Yagi is to use it to suppress an interfering signal by getting it into one of the sharp nulls between side-lobes.

btw are you comfortable with the difference between "forward gain" and "back-to-front ratio" ?


Ken Green
 15 May 2013 07:36 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Yes, Alan, And welcome to the senile citizens' association:-)

Wat you say is perfectly understandable but you did not say what you were using for aerials. From what you described I'm inclined to think I would be using a single dipole (to cut down those very strong signals and prevent waving in the breeze) and sort them electronically with your diplexers.

Ken
 15 May 2013 07:56 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Alan, Been rereading your Post. Sorry but my eyes are of less use than my legs.

For sure you ought not to be using Belling Lee. They look awful on a Bridge. And if you generate standing waves on your feeders (mismatch between feeder and i/p to receiver) it will do nothing for the performance of your aerials.

I don't know anywhere you might get those diplexers. They are sort of one-off ? Run the clock back some 10-years and I would have had a go at building them for you but ... Time Machine also is on the blink.

Ken
 15 May 2013 08:07 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Alan,

Me again. The best way to make the sort of diplexer you require would be by cutting lengths of coax to form stubs- some to be open-circuited and some s/c.

PLEASE don't ask me to design the network? I have passed my sell-by date.

Ken
 15 May 2013 08:25 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Yes, Alan, And welcome to the senile citizens' association:-)

Wat you say is perfectly understandable but you did not say what you were using for aerials. From what you described I'm inclined to think I would be using a single dipole (to cut down those very strong signals and prevent waving in the breeze) and sort them electronically with your diplexers.

Ken
 15 May 2013 08:58 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Alan,

Ae you into mucking about. ?

Fit a T-connector at the i/p to Rx
Add short piece coax with end o/c
Watch tv RX and trim length of stub until interfering signal diminishes
(or use longer coax stub with end s/c)

Whether or no this works will depend on:
Luck
Tuning of i/p to RX
Quality of coax
Match AE to feeder up the pole
State of theTide in Aberdeen.

Ken
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