LF: ERP estimation

[Andy Talbot <andy.g4jnt@gmail.com>]

You cannot calculate ERP directly from Transmit power without also obtaining an estimate of the additional losses, mainly caused by ground resistance.   You must have either the RMS RF current in the vertical, OR teh total series loss resistance of the actual vertical system, before any Tx matching that may be in place.

 

If you can measure RF current in the vertical, that will give you ERP directly from Ohms law on the Radiation resistance (which is directly related to the effective height of the antenna).

     ERP = I^2 * 3200 * (Heff / lambda) ^ 2     

For a vertical with no top hat, Heff = half the effective height.  

If you were to have an infinite top hat, Heff = actual height.   

For most practical antennas use a value about half way, so a resonable approximation is given by :

     ERP = I^2 * 1600 * (H / lambda)^2

 

Another way is to measure your total load resistance in series with the antenna,  = Rloss

Once you have this value, the efficiency is then given by Radiation resistance / Rloss . 

Rrad ~= 1600 * (H/lambda)^2 as shown above.  So ERP =   Tx power * Rrad / Rloss

 

Andy

www.g4jnt.com

 

On 30 April 2011 20:32, Luis EA3WX <shekina1@orange.es> wrote:

Hi Dennis,

 

A question, please. I am beaconing on 502,25 KHz. My transmitter puts 35 W into a T antenna, high 20 m and an horizontal capacitor top of 10 m.

 

With others ham fellows we are discussing what ERP I am sending, but we have no results. Can you tell me how measure or are you calculating or estimating this ERP?

 

Thanks, 73

 

Luis, EA3WX

 

From: DENNIS EASTERLING

Sent: Saturday, April 30, 2011 8:55 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: Re: GB4FPR 502 KHZ INTERNATIONAL MARCONI DAY 30/4/2011

 

Hi Graham

 

I tried sending to you 1630-1635 and 1940-1945 local times today Saturday 30 04 2011, but no reply.   I guess you could not hear me, or I you.   I send CW 25watts into a 'T' aerial 20m top and 10m vertical.   Estimated erp 250mW max.   Anyway, I hope you had a good day with plenty of DX.

 

73 Dennis M0JXM