advertisement


FM antenna - clearance *around* the antenna?

Jim Audiomisc

Jim Audiomisc

pfm Member
A discussion has set me wondering about a factor which people tend to be unaware of when installing an FM antenna. So thought it worth looking at and raising here...

For obvious reasons people want to keep the 'line of sight' clear of any obstructions. But for correct performance you may also need some clearance between the antenna and anything to either side of it or above, below, and even behind. And just as antennas with higher 'gain' will be bigger, so will these clearances if you want the gain you paid for. i.e. Not enough to be able to just physically fit the antenna into your loft *if* you want the full results.

Not sure I did the maths right, but a simple example can indicate what you may be up against. (Pun alert!)

http://jcgl.orpheusweb.co.uk/temp/AEat100MHz.png

The above assumes a 'perfect' antenna which collected all the arriving RF field within a square whose width is as shown. The bigger the square the higher it's effective 'gain' plotted here in dBi. i.e. how much more you'd get than using an 'isotropic' collector. (IIRC a simple dipole scores about "3dBi".)

Thus you need to keep that square area free of any other items that may upset behaviour.

Snag being in practice that typical VHF FM antennas are *not* perfect collectors of this simplicity. They tend to gather in a less uniformed way across the volume around them. Result being that they can be affected by objects *beyond* a sort of 'square tunnel' around them.

Bottom line is that you need clearances *bigger* that indicated by the plot.

I can't recall offhand anyone measuring or calculating this elbow room' requirement for the common FM antennas. Maybe Kraus has. Anyone know?

Anyway, be interested to see any reactions. :)
 
If someone can point me at an antenna pattern for the 'Ron Smith' monster I could try to transform it to show the related field in the plane around the antenna normal to the axis. (The snag being that most plots show the power pattern, which means having to guess the phases of sidelobes.)
 
All too technical for me, but if you're going to erect antennae in optimum domestic positions, it'll prob. be on a chimney mast. Most house chimneys will be some distance apart and there is unlikely to be any reflective surface in the immediate vicinity. Of course, anything substantial in the line of sight is a different matter and I had birdies once on an upmarket Pioneer tuner with 8 element antenna way above the 5 storey house as a student in Canterbury, but this could've been in a dip geologically. Never had birdies since in any of my many domiciles and on my various tuners. The odd thing was that the Revox and Leak tuners employed at the same house but adjacent rooms had exc. reception. The tuner, maybe?
 
I recently lost my massive ron smith antennae as neighbours kindly built their 8 bedroom monstrosity about 8cm from it !!! Grrrr
 
Jim Audiomisc said:
A discussion has set me wondering about a factor which people tend to be unaware of when installing an FM antenna. So thought it worth looking at and raising here...

For obvious reasons people want to keep the 'line of sight' clear of any obstructions. But for correct performance you may also need some clearance between the antenna and anything to either side of it or above, below, and even behind. And just as antennas with higher 'gain' will be bigger, so will these clearances if you want the gain you paid for. i.e. Not enough to be able to just physically fit the antenna into your loft *if* you want the full results.

Not sure I did the maths right, but a simple example can indicate what you may be up against. (Pun alert!)

http://jcgl.orpheusweb.co.uk/temp/AEat100MHz.png

The above assumes a 'perfect' antenna which collected all the arriving RF field within a square whose width is as shown. The bigger the square the higher it's effective 'gain' plotted here in dBi. i.e. how much more you'd get than using an 'isotropic' collector. (IIRC a simple dipole scores about "3dBi".)

Thus you need to keep that square area free of any other items that may upset behaviour.

Snag being in practice that typical VHF FM antennas are *not* perfect collectors of this simplicity. They tend to gather in a less uniformed way across the volume around them. Result being that they can be affected by objects *beyond* a sort of 'square tunnel' around them.

Bottom line is that you need clearances *bigger* that indicated by the plot.

I can't recall offhand anyone measuring or calculating this elbow room' requirement for the common FM antennas. Maybe Kraus has. Anyone know?

Anyway, be interested to see any reactions. :)
Click to expand...
Certainly anything metalic will be a problem, but I'm not clear on how any material that has no mechanism in physics to interact with radio waves could be problematic. I suppose the question is whether anything that may be around is truly transparent in reality. e.g. brick, wood, roof tiles etc
 
hifinutt said:
I recently lost my massive ron smith antennae as neighbours kindly built their 8 bedroom monstrosity about 8cm from it !!! Grrrr
Click to expand...
I remember that. G17 I think but surely it still worked but maybe the adjacent monstrosity was in direct line of reception. Guess your array changed into a row (with the neighbour !)
 
gez said:
Certainly anything metalic will be a problem, but I'm not clear on how any material that has no mechanism in physics to interact with radio waves could be problematic. I suppose the question is whether anything that may be around is truly transparent in reality. e.g. brick, wood, roof tiles etc
Click to expand...
Damp brick and wood is pretty lossy at RF. The challenges of avoiding messing your antenna pattern is one reason for the use of a horizontal polarisation on a vertical metal pole, even though the transmitter is mixed.
I suppose that you could model a Yagi in 4nec2, with additional elements some distance away to see what i does to the simulation.

4nec2 antenna modeler and optimizer

NEC2/4 based Antenna modeler, analyzer and optimizer.
www.qsl.net
 
gez said:
Certainly anything metalic will be a problem, but I'm not clear on how any material that has no mechanism in physics to interact with radio waves could be problematic. I suppose the question is whether anything that may be around is truly transparent in reality. e.g. brick, wood, roof tiles etc
Click to expand...

Erm, dielectric materials will often absorb, scatter, and have an internal value for light velocity that isn't 'c'. So, yes, the can have an effect. And the more 'gain' an antenna made using 'dipole array' methos has, the more it may be affected by them.
 
  • Like
Reactions: gez
You can work out the field pertubation pattern around the antenna from knowing the far field antenna pattern - but knowing the relative phase matters. So a power vs angle plot needs interpretation.
 
davidsrsb said:
Damp brick and wood is pretty lossy at RF. The challenges of avoiding messing your antenna pattern is one reason for the use of a horizontal polarisation on a vertical metal pole, even though the transmitter is mixed.
I suppose that you could model a Yagi in 4nec2, with additional elements some distance away to see what i does to the simulation.

4nec2 antenna modeler and optimizer

NEC2/4 based Antenna modeler, analyzer and optimizer.
www.qsl.net
Click to expand...

The 4nec2 page says its a 'doze' program. My house has no windows. 8-} But as I say, a measured (or accurate analytical) far field pattern for a beamed antenna can suffice. Usually people trot out Fourier for this. But Gaussian Beam Mode in my experience is faster and more accurate. Even did it using ye olde HP41CV back in the day. 8-] So should be improveable now. However I don't (yet) have patterns to start from. When I get a chance I'll look in Kraus's books and an old ARRL handbook I have to see if I can find some coconuts. Main drag these days for me is getting 'round tuits'...
 
You must log in or register to reply here.


advertisement


Back
Top