LF: Re: Power of Donebach 153kHz TX

["Talbot Andrew" <ACTALBOT@mail.dstl.gov.uk>]

I've just made a check of both Droitwich and Donebach transmissions using
the WatKins Johnson WJ-8711 receiver here at work.  Using the narrowest IF
bandwidth of 56Hz, setting manual gain control, and monitoring the output
tone using Gram on its widest setting I cannot detect any variation in
carrier level with modulation on either of the two transmitters.  The level
meter, which indicates actual signal level, accurately calibrated in dBm,
also shows no significant variation with modulation. There is perhaps 1dB of
variation on Donebach ( due to fading ?)  and nothing on Droitwich.

I'm very sure (read 99.9% certain) Droitwich has no amplitude modulation -
viewing it on a vectorscope in the past shows only the phase variation.  So
perhaps you are seeing some secondary effects introduced by the receiver ?

Andy  G4JNT




> I am pretty sure Donebach uses some form of controlled carrier modulation, 
> apart from the phase modulation mentioned by Walter.
> Our Dutch MF transmittters at 747kHz and 1008kHz are also made by 
> Telefunken (400kW at daytime).  The 747kHz transmitter uses controlled 
> carrier to save power and expenses; the 1008kHz one does not. The carrier 
> at 747kHz, as measured by the SPM-12 at 25Hz bandwidth, shows the same kind
> of fluctuations as the Donebach one, the 1008kHz carrier is steady.
R>emarkable is Droitwich at 198kHz. There the carrier goes downward during 
> modulation!

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Dear Walter & the LF-Group,

Today I tried to measure the field strength of Donebach at 153kHz
again.

I am pretty sure Donebach uses some form of controlled carrier
modulation, apart from the phase modulation mentioned by 
Walter.

Our Dutch MF transmittters at 747kHz and 1008kHz are also made by
Telefunken (400kW at daytime).  The 747kHz transmitter uses
controlled carrier to save power and expenses; the 1008kHz one does not.
The carrier at 747kHz, as measured by the SPM-12 at 25Hz bandwidth, shows
the same kind of fluctuations as the Donebach one, the 1008kHz carrier is
steady.

Remarkable is Droitwich at 198kHz. There the carrier goes downward during
modulation!

The varying carrier strength of Donebach makes it difficult to read the
meter of the SPM-12. But the instrument has at the back a "DC
output"  that is proportional to the measured value (so it is
not  DC a purist will remark). I fed that voltage to an oscilloscope
and noted the maximum deflection, which also cannot be read 
accurately because of the wild jumps of  the deflection with the
modulation. I then replaced the loop aerial by a HP606B signal generator,
tuned it to 153kHz and adjusted the output to the same deflection. The
generator voltage was then 20 microvolt. With a 1 x 1m single turn
untuned loop this means the field strength of Donebach is 6.25
microvolt/m. Using the CCIR groundwave curve for "Land"
(conductivity 30mS/m; dielectric constant 40) the power of Donebach
follows as 200kW EMRP.
The aerial efficiency is given by  Walter as 88%.  This
indicates a transmitter power of 200/0.88 =
227kW.
Perhaps, as suggested by Walter, there is only one
250kW transmitter in operation. What with the uncertainties involved and
the fact the aerial system is not omnidirectional the 200kw EMRP looks to
me like a plausible result, taking into account the uncertainties
involved in the process.

As a check I inserted in G4GFQ's groundwave propagation computer program
P = 227kW,  efficiency of the transmit aerial 88%, pastoral ground
(number 5) and distance 435km. The field strength came out as 5.33
microvolt/m, which differs 1.4dB with my measured value.

73, Dick, PA0SE
 

At 09:10 28-7-02 +0200, you wrote:

Dick, what I forgot yesterday - the efficiency of the Donebach-antenna is quoted as 88% for 153KHz.
73 Walter
 

-----Ursprüngliche Nachricht-----

Von: Walter

An: rsgb_lf_group@blacksheep.org

Gesendet: Samstag, 27. Juli 2002 18:06

Betreff: LF: Re: Power of Donebach 153kHz TX

Dear Dick and All,

    about DLF. During daylight time normal transmitting power is 500KW from 2 transmitters by Telefunken, each 250KW. These power is fed to the northwestern one of the two masts. The southeastern one is coupled by radiation and has a phase shift. This generates a 3dB attenuation to southeast - because of Radio Brasov, Romania, on the same frequency. During the night power is reduced to 250KW, fed to both masts, also with phase shift. So the attenuation increases to 11dB to protect the Brasov-area. I think you get the full radiation. But probably today is only one transmitter in use because of maintenance. Please try next week again. BTW: The carrier is also modulated in phase shift with GPS-correction-data.

73 Walter DJ2LF

 

 

 

-----Ursprüngliche Nachricht-----

Von: Dick Rollema

An: LF-Group

Gesendet: Samstag, 27. Juli 2002 11:03

Betreff: LF: Power of Donebach 153kHz TX

To All from PA0SE,

Sorry to bother you again with a field strength story. But the delete key is within easy reach...

The Deutschlandfunk Donebach transmitter at 153kHz is on again. At 0840 UTC today I measured its field strength as 5.6millivolt/metre.

Using the CCIR groundwave propagation curve that produces plausible results on DCF39 and M0BMU the Effective Monopole Radiated Power (EMRP) comes out as 157kW.

On the Internet I found transmitter power is 500kW. But there are two 363m high aerial masts. This suggests a directive array. As the transmissions are probably not meant for the Netherlands we may get only part of the full blast.

Measuring with the SPM-12 in 25Hz bandwidth the reading moved up and down a bit with the modulation. Perhaps some power (and money) saving controlled carrier system is used. It could also be that some of the lowest modulation frequency components are passed by the 25Hz  I.F. filter. But other BC stations on LF do not show the effect.

73, Dick, PA0SE