[cma-l] Community Radio on AM

[Alan Coote]

I’m going back a bit, but I recall Longley Rice only being useful for VHF between 70Mhz - 250MHz. Either side, other models need to kick in. I also remember there being a lot of fudge factors.

 

Anything that is line of sight you could always use; Free Space Loss  = 20log(d km) +20log(f MHz) +32.4. Then subtract a bit for buildings and trees, and a bit if the signal path is over water or damp ground, subtract a bit more for cabling and connectors and then multiply by the antenna gain… or just use software to do it for you!          

 

The above is useless for AM band propagation of course.  

 

Kind Regards

Alan

 

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From: cma-l-bounces at mailman.commedia.org.uk [mailto:cma-l-bounces at mailman.commedia.org.uk] On Behalf Of Two Lochs Radio
Sent: 24 September 2014 11:32
To: The Community Media Association Discussion List; Martin Steers
Cc: Ian Hickling
Subject: Re: [cma-l] Community Radio on AM

 

The only Ofcom community AM coverage plot I have happened to look at in the past simply drew a nominal 5km radius circle - despite it being over a hilly city with a river through it! 

 

The Canadian CRC's LRCOV online propagation calculator covers 20 MHz to 40 GHz‎ using the standard Longley Rice propagation model, but as you say analyzing the antenna propagation pattern is a prerequisite for sensible results. The standard NEC tools will cover the AM frequencies quite happily, but at such long wavelengths the effects of ground proximity and nature and nearby metalwork/support guys make it a fairly uncertain process unless you are dealing with high wires in large open fields as used by the major broadcasters.

 

Also AM propagation varies dramatically between day and night, though this is generally of concern only to wide area broadcasters, not community stations received from the surface wave unless they are interfered with at night by a far distant sky wave.


Alex

 

 

(----- Original Message ----- 

From: Ian Hickling <mailto:transplanfm at hotmail.com>  

To: Martin Steers <mailto:martin at martinsteers.co.uk>  ; The Community Media AssociationDiscussion List <mailto:cma-l at mailman.commedia.org.uk>  

Sent: Tuesday, September 23, 2014 4:02 PM

Subject: [cma-l] Community Radio on AM

 

I haven't done any real appraisals apart for the Farnborough station which manages about a 2-mile radius in daytime hours which is be barely adequate but does at least cover the core population. 

I'm not aware of any computer coverage software that will cope with MF - and of course there's the question of exactly the EMRP value - and even more indeterminate - the antenna propagation pattern.
I'm sure other members will  have first-hand information.

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