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DIY FM Dipole - Trials and Tribulations

M

Mr Ian

pfm Member
I have a very old 6 element FM aerial on the roof. The signal has deterioated so much that its next to useless. The local aerial guys are quoting silly money for a replacement or a rewire so I played about with DIY Dipoles.

Basic Design

  • 15mm copper pipe cut into 70cm lenghts give take
  • A 15mm plastic push fit equal tee.
  • Two brass Munsen
  • CT100 75 Ohm aerial cable or similar copper core copper shield coaxial cable
  • 1 aerial plug to fit tuner aerial socket (check is the tuner a male or female connector)
Push the pipe into the each opposite side on the equal tee
Strip back the aerial cable about 15 cm
wrap the earth/shield round one munsen and screw it onto one of the pipes
Wrap the solid centre core round the other munsen and screw it to the other pipe
Fit the aerial plug to the cable jobe done.

Mount it horiziontal as high as you can get it.

Comparison from signal strengh meter on tuner
Old 6 element aerial of roof - 8d
omni directional circular bought dipole aerial in loft - 34db
DIY dipole on chair in bed room - 29db
DIY dipole in loft with cheap cable and a couple of connectors to fix it to the tuner - 39db
DIY dipole in loft with Belden 1694A as a down lead and a new connector - 40db

Belden 1694A
Looking for the best possible downlead I though Belden 1694A was the eay to go. Its an ultra high quality RG6 video cable designed for the most demanding of video uses.

Foolishly decided to rip out the 3 1694A out of a lenghly component cable bundle. I can only say it took nearly 2 hours just to get the cable out - I cant remember what the product code is for the bundle but its construction was serious. The 1694A's exemplary construction pales in comparison. I can honestly say I have never worked with such un forgiving cables before. PS they dont bend easily either

But it delivered an extra 1db on a 6m down cable.
 
Mr Ian said:
I have a very old 6 element FM aerial on the roof. The signal has deterioated so much that its next to useless. The local aerial guys are quoting silly money for a replacement or a rewire so I played about with DIY Dipoles.

Basic Design

  • 15mm copper pipe cut into 70cm lenghts give take
  • A 15mm plastic push fit equal tee.
  • Two brass Munsen
  • CT100 75 Ohm aerial cable or similar copper core copper shield coaxial cable
  • 1 aerial plug to fit tuner aerial socket (check is the tuner a male or female connector)
Push the pipe into the each opposite side on the equal tee
Strip back the aerial cable about 15 cm
wrap the earth/shield round one munsen and screw it onto one of the pipes
Wrap the solid centre core round the other munsen and screw it to the other pipe
Fit the aerial plug to the cable jobe done.

Mount it horiziontal as high as you can get it.

Comparison from signal strengh meter on tuner
Old 6 element aerial of roof - 8d
omni directional circular bought dipole aerial in loft - 34db
DIY dipole on chair in bed room - 29db
DIY dipole in loft with cheap cable and a couple of connectors to fix it to the tuner - 39db
DIY dipole in loft with Belden 1694A as a down lead and a new connector - 40db

Belden 1694A
Looking for the best possible downlead I though Belden 1694A was the eay to go. Its an ultra high quality RG6 video cable designed for the most demanding of video uses.

Foolishly decided to rip out the 3 1694A out of a lenghly component cable bundle. I can only say it took nearly 2 hours just to get the cable out - I cant remember what the product code is for the bundle but its construction was serious. The 1694A's exemplary construction pales in comparison. I can honestly say I have never worked with such un forgiving cables before. PS they dont bend easily either

But it delivered an extra 1db on a 6m down cable.
Click to expand...

Very interesting. I recently had a good Ron Smith aerial mounted high on the roof stolen in the middle of the night. Silently.

However, the bastards merely cut the low loss coax at the aerial. They left the entire run to my flat intact. As an experiment, I thought to try it on my Leak Troughline. It is a vintage tuner. Insensitive. I was amazed that there was almost no subjective difference.

Have you thought about a signal booster?
 
Mr Ian said:
I have a very old 6 element FM aerial on the roof. The signal has deterioated so much that its next to useless. The local aerial guys are quoting silly money for a replacement or a rewire so I played about .
Click to expand...

To me, unless your 6 ele antenna is now either

a) pointing in the wrong direction
B) literally fallen to bits

I would be super surprised a standard dipole would be any better.

my guess is your coax is buggered due to water ingress/broken/shorted.

what do I base this on…many years titting about with aerials as a Radio Ham.

your dipole may be perfectly fine but will not be better than a properly designed and fed with decent coax functioning 6 element beam…ya can’t buck da laws of physics
 
If it's been a gradual deterioration, I would suspect the cable junction into the aerial.

I had this happen to a TV aerial many years ago. The co-axial cable had rusted where it was screwed into the aerial junction box and the signal slowly disappeared. Cutting off the last three inches and rejoining them solved it.
I would borrow some proper roof ladders if you want to do it yourself!

Andy
 
I built a two element FM aerial from an old ‘H’ type BBC TV aerial.
The design and construction details were in a popular electronics magazine.
It worked well in my area, which had a good FM signal.
 
I concur that the very old rg59 has deteriorated so much that it ruins the signal but my rewire costs are way above multiple diy dipoles.

I can self fit dipoles but the roof aerial needs something more specialist/braver than me
 
^ Yup, I have a loop hidden in plain sight in a one of my book cupboards - made out of 1.2mm solid copper wire, cut 3.2m long and simply wrapped around four panel pins in the edges of the shelves - for a square of 800mm/side. Worked better given I don't have access to the roof / outside mounting than a dipole built almost exactly as described in the first post
 
eguth said:
Very interesting.

Have you thought about a signal booster?
Click to expand...

Here is the good news, I placed a second lengh of copper tube/equal tee, that was not connected to anything, parrallel about 50cm away from the dipole and got an extra 2db. now up to 42db.

Now the bad news, Signal Amplifiers

Labgear: 14db gain 3.5db noise = 15db on the signal strengh/quality meter - a drop of 27db at the tuner
Masterplug: 11db gain ? noise = 10db on the signal strengh/quality meter - a drop of 32db at the tuner

so endeth the amplifier experiment. they may boost the signal level but .............
 
Eyebroughty said:
Hi,

This should help with building a Yagi antenna.

https://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.php

The correct distance between the reflector, the dipole and the driven elements makes the Yagi antenna work to it's best.

At FM frequencies the coax quality going from regular to expensive is more than offset by adding more driven elements, in fact regular coax is fine.

Cheers

John
Click to expand...

I gotta disagree with this..cheap crappy coax is a false economy.

Not only may you lose whatever gain you may get from the additional elements but you are more susceptible to interference by the crappy brad picking up additional noise...the higher the frequency the greater the loss so with FM based aerials use good screened coax - the difference between good and shite is so little you may as well.

Note also gain of an antenna is dependent on boom length not number of elements.
 
This venture was never about getting the best signal, the easy albeit expensive, route would hsve been ti get an aerial installer to run a new cable to the roof mount.

The goal was to see if a very cheap, easy to build, using readily available parts, fm aerial would work.

It does.

For a few pounds a relatively easy to hide aerial can be built with no more than a screw driver and a pair of scissors. One that works well too

The used belden component cable cost me less per metre than aerial cable. Selling the excess should recoup all my costs.
 
Just get a new aerial fitted with new cabling and done by a proper aerial installation company who will configure it to your local transmitter. You are never going to better that for radio reception. After all, all the programme material received is free, so think of it that way...
 
G T Audio said:
Just get a new aerial fitted with new cabling and done by a proper aerial installation company who will configure it to your local transmitter. You are never going to better that for radio reception. After all, all the programme material received is free, so think of it that way...
Click to expand...
The object of the Mr. Ian's exercise is to avoid the "silly money" being quoted. I applaud his effort. Being ripped off is not fun.
 
eguth said:
The object of the Mr. Ian's exercise is to avoid the "silly money" being quoted. I applaud his effort. Being ripped off is not fun.
Click to expand...

Getting a few quotes for an aerial installation would be sensible to make sure you are not being ripped off. It might be that the price quoted is the going rate these days and it might not be a rip off! I would imagine that aerial installers these days must be a dying breed...

Just to clarify, no way on earth will a DIY or indoor aerial solution be as good as a proper set up roof mounted aerial.
 
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