[cma-l] AM v FM

[roger at thornesonline.com]

Having had the "pleasure" of removing an AM aerial when our CR closed  
down I have a hands on but layman's appreciation of what was involved  
in erecting it in the first place.

I think the mast was something over 20 meters high with a "hat" on the  
top, a large concrete foundation with a copper mat around it, several  
long copper rods and copper wire stretching in all directions plus a  
hut required to house the transmitter and a sturdy 8ft high fence  
around to keep out the horses in the field.



Quoting Ian Hickling <transplanfm at hotmail.com>:

> I do sincerely agree
> But with the amount of land required, plus the structure and its  
> erection, plus the earth mat and ATU - the construction of an aerial  
> which is anything like naturally resonant is a major project and  
> expense.
> Hence they are nowhere near a quarter wavelength and need added  
> capacitance and inductance in order to achieve a reasonable match.
> This also introduces a huge question mark over the shape of the  
> propagation envelope and removes any hope of effective modelling.
> So overall efficiency is about 8 to 10%
>
> Ian Hickling
> Partner
>
> Office: 01635 578435  (7am-11pm UK time)Carphone: 07530 980115 (only  
> responds when driving)6 Horn Street, Compton, NEWBURY, RG20 6QS
>
>> From: tlr at gairloch.co.uk
>> To: cma-l at mailman.commedia.org.uk
>> Date: Mon, 14 Dec 2015 17:11:10 +0000
>> Subject: Re: [cma-l] AM V's FM
>>
>> >
>> > If you need to generate 800W of RF for an EMRP of 70W I'd say you need to
>> > improve your antenna system efficiency :-)
>> >
>>
>> As you know, easier said than done for a small station David! With a typical
>> wavelength of 200-300 metres, an efficient omnidirectional radiator can be a
>> mighty big thing. A traditional quarter-wave vertical mast is not something
>> you can just pop on a roof pole like an FM dipole! Loading coils can shorten
>> it, but nonetheless efficiency rapidly falls off as you make the antenna
>> more compact.
>>
>> A more surprising figure is Ian's implied 28% electrical efficiency for the
>> AM transmitter - Nautel's J1000 offers 60%-70% efficiency (ie ~1300W
>> consumption for 800W modulated output). The Nautel TX costs around £10k, but
>> it would save around £1,800 electricity a year in Ian's example, so would
>> probably pay back the extra cost in a little over 2 years.
>>
>> Alex
>>
>> ----- Original Message -----
>> From: "Info" <info at transmittersrus.com>
>> To: <cma-l at commedia.org.uk>
>> Sent: Monday, December 14, 2015 4:03 PM
>> Subject: [cma-l] AM V's FM
>>
>>
>> > Ian,
>> >
>> > If you need to generate 800W of RF for an EMRP of 70W I'd say you need to
>> > improve your antenna system efficiency :-)
>> >
>> > David
>> > Transmitters 'R' Us
>> >
>> > _______________
>>
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