Ideal boom size for FM aerial - UK Vintage Radio Repair and Restoration Discussion Forum

stevehertz · 1st Apr 2018, 10:10 am

What is the ideal boom size for an FM aerial that is 1) practical and 2) will have least effect as a reflector or a director? My aerial is single dipole, vertically orientated on a roof mounted mast. My thoughts being that it has to be a size that is not too long as to create an unwieldy, lopsided downforce on the mast, and also not too short as to be to be 'blocked' by its proximity to the main mast. There has to be some kind of optimum position. No, I can't experiment with the length, it will be a one off job performed by a rigger.

In case my terminology is wrong, by 'boom' I mean the horizontal tube that comes off the main mast to hold the aerial.

Can I just say before we go off course, I have no interest in adding reflectors, directors, going horizontal and/or using a rotator. Been there, done all that, I know what I want, I just want a simple answer to a physics question please!

Thanks.

G8HQP Dave · 1st Apr 2018, 4:14 pm

The boom itself will not act as a director or reflector as it has the wrong orientation for that, but the spacing between the main mast (set by the boom length) will affect how the mast affects the vertical dipole. Roughly speaking, the main mast will act as a reflector so you will get maximum pickup in the direction away from it. This effect may be maximised by a spacing of around a quarter wavelength (so 75cm). If you want to avoid this then a somewhat longer boom is needed.

stevehertz · 1st Apr 2018, 4:41 pm

Indeed Dave, my poor wording, thanks for pointing that out. The critical distance being that between the antenna and the mast, as affected by the length of the boom.

Now, a boom length longer than 75cm is starting to get a problem in terms of it 'hanging' on the one side of the mast, and also how it may be affected by high winds. So what would be a suitable shorter distance? presumably somewhere between 37.5 and 75cm?

paulsherwin · 1st Apr 2018, 5:02 pm

Presumably it doesn't matter much so long as you don't want to listen to a station which is nulled out by the mast.

Dual Standard · 1st Apr 2018, 6:02 pm

Quote:

Originally Posted by G8HQP Dave

The boom itself will not act as a director or reflector as it has the wrong orientation for that, but the spacing between the main mast (set by the boom length) will affect how the mast affects the vertical dipole. Roughly speaking, the main mast will act as a reflector so you will get maximum pickup in the direction away from it. This effect may be maximised by a spacing of around a quarter wavelength (so 75cm). If you want to avoid this then a somewhat longer boom is needed.

Alternatively use a fibreglass mast. I suspect that your rigger may not have these but they are available from amateur radio dealers, usually in 2 metre lengths and 1.5" or 2" diameter.
Mike

stevehertz · 1st Apr 2018, 6:10 pm

Have you any experience of these as my rigger says "they are too bendy". But in the words of many Rice-Davies he would say that wouldn't he?

Dual Standard · 1st Apr 2018, 9:14 pm

Sorry no, I don't have personal experience of the fibreglass masts but I suspect your rigger is thinking of the extending bendy 'roach' type poles which certainly are too bendy, the ones I have in mind are strengthened fixed length poles.
Radioworld in Walsall sell them (no connection etc):-
https://www.radioworld.co.uk/amateur...eglass-antenna
These are often used to for mounting short (2 to 5 element) vertically polarised 2 metre band antennas so should cope easily with a single element Band 2 antenna. You may find them cheaper by searching arround.

Mike

G8HQP Dave · 1st Apr 2018, 10:42 pm

Putting the dipole nearer the mast may reduce somewhat the directionality, but it will also increase the coupling between them so the mast plus 'antenna' becomes the real antenna. For broadcast reception you may get away with this. You can reduce the coupling by ensuring that the top part of the dipole is above the top end of the mast i.e. mount the boom near the top of the mast. Better would be a non-conductive mast if you can find one which is mechanically suitable.

Herald1360 · 2nd Apr 2018, 12:06 am

Why not use a quarter wave vertical with four radials sticking out from the bottom to provide some sort of ground plane?

https://m0ukd.com/calculators/quarte...na-calculator/

stevehertz · 2nd Apr 2018, 9:56 am

Quote:

Originally Posted by G8HQP Dave

Putting the dipole nearer the mast may reduce somewhat the directionality, but it will also increase the coupling between them so the mast plus 'antenna' becomes the real antenna. For broadcast reception you may get away with this. You can reduce the coupling by ensuring that the top part of the dipole is above the top end of the mast i.e. mount the boom near the top of the mast. Better would be a non-conductive mast if you can find one which is mechanically suitable.

Not sure where you're coming from when you say that "putting the dipole nearer the mast may reduce somewhat the directionality.." because a vertical dipole is totally non-directional to start off with, so how can you make it less directional than 'none'?

Nuvistor · 2nd Apr 2018, 11:29 am

Is there going to be anything else on the mast or close by? I would go for sturdiness, boom length say 37cm and put the dipole on the side of the mast nearest the transmitter.

The comment about having the top of the dipole above the top of the mast is good but it depends if that is possible.

Cannot help with the physics.

stevehertz · 2nd Apr 2018, 12:06 pm

The present mast supports the TV aerial and the FM one. At the moment the TV aerial is at the top, and the FM one lower down. I don't have a problem with my TV signal, it's fine. I'm thinking about exchanging the mast for a strengthened glass fibre one and putting the FM aerial at the top, as far as it will mount, with the TV aerial below it. That will overcome the effect of the existing steel mast acting like a radiator/director and the extra height should help a tad too. Just the price of the mast plus the rigger's time.

G8HQP Dave · 2nd Apr 2018, 12:17 pm

Quote:

Originally Posted by stevehertz

Not sure where you're coming from when you say that "putting the dipole nearer the mast may reduce somewhat the directionality.." because a vertical dipole is totally non-directional to start off with, so how can you make it less directional than 'none'?

Putting a vertical antenna next to a conducting vertical mast causes directionality, which peak around a quarterwave distance. Putting them closer reduces the directionality but the mast begins to act as a short so reduces the effectiveness of the antenna. To get no directionality you need a big distance or a non-conducting mast.

If you are happy to lower the TV antenna a little then putting an FM groundplane on the top of the mast could work well.

bikerhifinut · 2nd Apr 2018, 12:39 pm

Daft question, does it need to be a vertical dipole? In the old place, with a reasonable signal strength from a vertically polarised BBC transmitter I used one of those circular omnidirectional folded dipole aerials mounted horizontally on the mast with the TV aerial and beside the freesat dish. I could get reasonable reception not only from the local Bilsdale transmitter but good mono reception from Pontop Pike and Holme Moss or whatever radio transmitter is on that side of Yorkshire.
Ah sorry I forgot your initial stipulation in post #1.
Andy

mark2collection · 2nd Apr 2018, 1:18 pm

Although different topology, the idea's there:-

When playing with my homemade vertical DAB antenna, I too struggled with the boom idea, until I discovered the very 'pole' I'd made may DAB antenna from (22mm diameter reinforced overflow pipe) was a very snug fit in the top of my then TV antenna mast. Lowering the TV antenna by an advised 18" and running some long screws with nylock nuts through the assembly, I'm happy to report, it worked a treat.

Using adhesive-lined heatshrink over the 'coupling' & running the DAB cable inside the TV mast made for an almost professional looking installation.

Mark

stevehertz · 2nd Apr 2018, 3:20 pm

Sorry, there's a typo in there somewhere, I'm not making out what you say?

mark2collection · 2nd Apr 2018, 4:09 pm

So there is, blimey. All thumbs, what!

It should say 'my' not 'may'.

Mark

Nymrod121 · 2nd Apr 2018, 4:16 pm

If it were me, my draft specification would probably be along the lines of the following:

50 mm (2") diameter vertical mast; overall length not exceeding 4570 mm (15'); aluminium / galvanised steel (preferred); material thickness not less than 2 mm;

Heavy-duty stand-off wall-brackets (not 'T&K'), galvanised; fixed to solid brickwork (not mortar joints) using heavy-duty wall-bolts (not coach-screws / plastic fixings);

Horizontal boom, 30-37mm overall diameter, overall length not exceeding 1100 mm, wall thickness not exceeding 2 mm; positioned as near to the top of the main support mast as prevailing conditions will allow; industry-approved boom-to-mast mounting clamp ...

... maybe.

However, it may be best for this to be handled by your local Confederation of Aerial Industries-registered installer:

https://www.getmeviewing.org.uk/

Best wishes

Guy

tealandsilver · 2nd Apr 2018, 5:36 pm

Quote:

Originally Posted by stevehertz

The present mast supports the TV aerial and the FM one. At the moment the TV aerial is at the top, and the FM one lower down. I don't have a problem with my TV signal, it's fine. I'm thinking about exchanging the mast for a strengthened glass fibre one and putting the FM aerial at the top, as far as it will mount, with the TV aerial below it. That will overcome the effect of the existing steel mast acting like a radiator/director and the extra height should help a tad too. Just the price of the mast plus the rigger's time.

Your proposed solution will certainly work from a radio/tv perspective. I would have doubts using a fibreglass pole if the site location is prone to high winds as there will be more flex than a similar length steel pole. If the pole is short (2m) then you should be ok. I have used one successfully in the past but was eventually forced to replace it with a steel pole as the fibreglass degraded to such an extent that fibre slivers were coming off the pole and coating the roof and paths.

G6Tanuki · 2nd Apr 2018, 5:55 pm

I've used fibreglass poles for quick-and-dirty antennas [old windsurfer masts: windsurfers are paranoid about inspecting their masts for any hint of incipient failure. Their castoffs are a good source of 18-footers though these days most of them are Carbon Fibre/Kevlar and so conductive...].

If you break a windsurfer-mast with the load of anything less than a 9-over-9-element stacked Band-II arrayi in a Force-10, you deserve my deepest admiration!

If you're using a metallic mast, boom-length can be important: the 'mast image' can easily act as a reflector or director to your dipole and mess-up the polar response. Whether it acts as a director or a reflector is a matter of luck: the mast may help or hinder reception of your preferred stations. Done sensibly you can use the mast-image to your advantage (for example to null-out a murderously-strong BBC local transmitter).

1st Apr 2018, 4:14 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: I deal boom size for FM aerial The boom itself will not act as a director or reflector as it has the wrong orientation for that, but the spacing between the main mast (set by the boom length) will affect how the mast affects the vertical dipole. Roughly speaking, the main mast will act as a reflector so you will get maximum pickup in the direction away from it. This effect may be maximised by a spacing of around a quarter wavelength (so 75cm). If you want to avoid this then a somewhat longer boom is needed.

1st Apr 2018, 4:41 pm	#3
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,833	Re: I deal boom size for FM aerial Indeed Dave, my poor wording, thanks for pointing that out. The critical distance being that between the antenna and the mast, as affected by the length of the boom. Now, a boom length longer than 75cm is starting to get a problem in terms of it 'hanging' on the one side of the mast, and also how it may be affected by high winds. So what would be a suitable shorter distance? presumably somewhere between 37.5 and 75cm? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

1st Apr 2018, 5:02 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Ideal boom size for FM aerial Presumably it doesn't matter much so long as you don't want to listen to a station which is nulled out by the mast.

1st Apr 2018, 6:10 pm	#6
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,833	Re: I deal boom size for FM aerial Have you any experience of these as my rigger says "they are too bendy". But in the words of many Rice-Davies he would say that wouldn't he? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

1st Apr 2018, 9:14 pm	#7
Dual Standard Hexode Join Date: Aug 2005 Location: Cheshire, UK. Posts: 263	Re: Ideal boom size for FM aerial Sorry no, I don't have personal experience of the fibreglass masts but I suspect your rigger is thinking of the extending bendy 'roach' type poles which certainly are too bendy, the ones I have in mind are strengthened fixed length poles. Radioworld in Walsall sell them (no connection etc):- https://www.radioworld.co.uk/amateur...eglass-antenna These are often used to for mounting short (2 to 5 element) vertically polarised 2 metre band antennas so should cope easily with a single element Band 2 antenna. You may find them cheaper by searching arround. Mike

1st Apr 2018, 10:10 am	#1
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,833	Ideal boom size for FM aerial What is the ideal boom size for an FM aerial that is 1) practical and 2) will have least effect as a reflector or a director? My aerial is single dipole, vertically orientated on a roof mounted mast. My thoughts being that it has to be a size that is not too long as to create an unwieldy, lopsided downforce on the mast, and also not too short as to be to be 'blocked' by its proximity to the main mast. There has to be some kind of optimum position. No, I can't experiment with the length, it will be a one off job performed by a rigger. In case my terminology is wrong, by 'boom' I mean the horizontal tube that comes off the main mast to hold the aerial. Can I just say before we go off course, I have no interest in adding reflectors, directors, going horizontal and/or using a rotator. Been there, done all that, I know what I want, I just want a simple answer to a physics question please! Thanks. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

1st Apr 2018, 10:42 pm	#8
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Ideal boom size for FM aerial Putting the dipole nearer the mast may reduce somewhat the directionality, but it will also increase the coupling between them so the mast plus 'antenna' becomes the real antenna. For broadcast reception you may get away with this. You can reduce the coupling by ensuring that the top part of the dipole is above the top end of the mast i.e. mount the boom near the top of the mast. Better would be a non-conductive mast if you can find one which is mechanically suitable.

2nd Apr 2018, 12:06 am	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Ideal boom size for FM aerial Why not use a quarter wave vertical with four radials sticking out from the bottom to provide some sort of ground plane? https://m0ukd.com/calculators/quarte...na-calculator/ __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

2nd Apr 2018, 11:29 am	#11
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Ideal boom size for FM aerial Is there going to be anything else on the mast or close by? I would go for sturdiness, boom length say 37cm and put the dipole on the side of the mast nearest the transmitter. The comment about having the top of the dipole above the top of the mast is good but it depends if that is possible. Cannot help with the physics. __________________ Frank

2nd Apr 2018, 12:06 pm	#12
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,833	Re: Ideal boom size for FM aerial The present mast supports the TV aerial and the FM one. At the moment the TV aerial is at the top, and the FM one lower down. I don't have a problem with my TV signal, it's fine. I'm thinking about exchanging the mast for a strengthened glass fibre one and putting the FM aerial at the top, as far as it will mount, with the TV aerial below it. That will overcome the effect of the existing steel mast acting like a radiator/director and the extra height should help a tad too. Just the price of the mast plus the rigger's time. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

2nd Apr 2018, 12:39 pm	#14
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Ideal boom size for FM aerial Daft question, does it need to be a vertical dipole? In the old place, with a reasonable signal strength from a vertically polarised BBC transmitter I used one of those circular omnidirectional folded dipole aerials mounted horizontally on the mast with the TV aerial and beside the freesat dish. I could get reasonable reception not only from the local Bilsdale transmitter but good mono reception from Pontop Pike and Holme Moss or whatever radio transmitter is on that side of Yorkshire. Ah sorry I forgot your initial stipulation in post #1. Andy Last edited by bikerhifinut; 2nd Apr 2018 at 12:50 pm.

2nd Apr 2018, 1:18 pm	#15
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: Ideal boom size for FM aerial Although different topology, the idea's there:- When playing with my homemade vertical DAB antenna, I too struggled with the boom idea, until I discovered the very 'pole' I'd made may DAB antenna from (22mm diameter reinforced overflow pipe) was a very snug fit in the top of my then TV antenna mast. Lowering the TV antenna by an advised 18" and running some long screws with nylock nuts through the assembly, I'm happy to report, it worked a treat. Using adhesive-lined heatshrink over the 'coupling' & running the DAB cable inside the TV mast made for an almost professional looking installation. Mark

2nd Apr 2018, 3:20 pm	#16
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,833	Re: Ideal boom size for FM aerial Sorry, there's a typo in there somewhere, I'm not making out what you say? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

2nd Apr 2018, 4:09 pm	#17
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: Ideal boom size for FM aerial So there is, blimey. All thumbs, what! It should say 'my' not 'may'. Mark

2nd Apr 2018, 4:16 pm	#18
Nymrod121 Nonode Join Date: Apr 2012 Location: Worcestershire, UK. Posts: 2,052	Re: Ideal boom size for FM aerial If it were me, my draft specification would probably be along the lines of the following: 50 mm (2") diameter vertical mast; overall length not exceeding 4570 mm (15'); aluminium / galvanised steel (preferred); material thickness not less than 2 mm; Heavy-duty stand-off wall-brackets (not 'T&K'), galvanised; fixed to solid brickwork (not mortar joints) using heavy-duty wall-bolts (not coach-screws / plastic fixings); Horizontal boom, 30-37mm overall diameter, overall length not exceeding 1100 mm, wall thickness not exceeding 2 mm; positioned as near to the top of the main support mast as prevailing conditions will allow; industry-approved boom-to-mast mounting clamp ... ... maybe. However, it may be best for this to be handled by your local Confederation of Aerial Industries-registered installer: https://www.getmeviewing.org.uk/ Best wishes Guy

2nd Apr 2018, 5:55 pm	#20
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Ideal boom size for FM aerial I've used fibreglass poles for quick-and-dirty antennas [old windsurfer masts: windsurfers are paranoid about inspecting their masts for any hint of incipient failure. Their castoffs are a good source of 18-footers though these days most of them are Carbon Fibre/Kevlar and so conductive...]. If you break a windsurfer-mast with the load of anything less than a 9-over-9-element stacked Band-II arrayi in a Force-10, you deserve my deepest admiration! If you're using a metallic mast, boom-length can be important: the 'mast image' can easily act as a reflector or director to your dipole and mess-up the polar response. Whether it acts as a director or a reflector is a matter of luck: the mast may help or hinder reception of your preferred stations. Done sensibly you can use the mast-image to your advantage (for example to null-out a murderously-strong BBC local transmitter).