[Boatanchors] Short Wave Broadcast Folks:

[Sheldon Daitch]

Glenn,

re the ERP aspect you discussed below, that is much more the norm in the FM and TV world, but on the MW and HF broadcasting side, a 100kW transmitter should be capable of having an RF output of 100kW.

Yes, there is gain in the antenna systems, especially the HF antenna systems, and it depends on the configuration of the actual antenna and the frequency involved.  Typical gains  for a 4 x 4 dipole array is in the 15-20 dB range and for the 4 x 6 dipole arrays can be approaching 25dB.

Losses in the transmission lines are part of the "ERP" aspect, as you note, and 9-inch rigid coax helps to reduce those losses.

Back to transmitter output power, many HF and international MW broadcasters are using one form or another of dynamic carrier control - essentially the instantaneous carrier output level is somewhat related to the audio level - far more easily done with the solid state modulators (SSM) which have been replacing the far more typical push-pull plate modulation systems found years back.  Continental Electronics calls it CCM - Controlled Carrier-Level Modulation, AEG-Telefunken called it DAM - Dynamic Amplitude Modulation, Thomcast/ABB - DCC - Dynamic Carrier Control and the BBC uses AMC - Amplitude Modulation Compounding.    In my reference on CCM, written by J. Fred Riley, I see he doesn't mention the Harris version, but Harris also called it DCC.

These modulation techniques are designed to save money in transmitter operation.  The concepts of these techniques can be found easily in web searches, but with the exception of AMC, the power output of the transmitter with no modulation is cut back, perhaps as much as 6 dB.  At full modulation, 100%, the carrier output is full rated power.  

AMC is a bit the opposite, at zero modulation levels, the carrier power is full rated power, and as the modulation level increases, the power output is reduced - relying on the modulation to cover any decreases in signal to noise ratio because of the lower TX output power.  

A concise discussion is at:  http://www.radioworld.com/article/international-broadcasters-know-dcc/24576

I could be wrong, but the only classic push-pull modulators in the HF transmitters for the IBB network are in the three GE 250kW transmitters at Greenville B Site, oh, yea, and in the three 50kW Gates HF-50s at the Tinang Philippines transportable site.   Can't remember if all of the 250kW transmitters at Tinang main site are upgraded with SSMs.  Continental Electronics has been our primary supplier of SSMs for converted transmitters.

Commercial power at high power HF transmitter sites is a bit different, too, because of the larger power consumptions.  At my current site, the incoming commercial power is 11kV three phase and all lower voltages for the station are developed  from transformers in our building.


73
Sheldon
WA4MZZ
________________________________________
From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of Glenn Little <glennmaillist at bellsouth.net>
Sent: Wednesday, April 16, 2014 9:03 PM
To: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Short Wave Broadcast Folks:

That 100 KW station is not putting out 100KW transmitter power, but, ERP.
When I was engineer at a TV station, we installed the required
digital transmitter.
The output tube generated 23KW of RF.
The gain of the antenna, the loss of the 2000 feet of 7 1/4" (or
close) rigid line and the loss of the combiner gave us an ERP of 630 KW.

For HF, the gain will not be as high, but, the principal is the same.
They have a target audience and will concentrate their energy to that target.

When we had one of the two PAs off line due to a failure, we did get
a call from the power company because our consumption was below the
calculated lower threshold.


Also the power to a transmitter site should be 480V 3 phase or
208/227V 3 phase.
Much more efficient for the rotating components ( blowers, pumps, etc.).

73
Glenn
WB4UIV