dcf77 loopstick tuning.

[dr pepper]

I have a dcf77 rx circuit, the loopstick that came with it is tiny, I'd like to replace it with a larger one, I have some salvaged loopsticks from old broadcast receivers, I can pad the lw coil with caps easy enough, but till now I've been using my sig gen via a 100k to measure the resonant freq of the coil/cap as my dip meter doesnt go low ebough.
The freq adjust on my sig gen is very insensitive, so I was thinking of breadboarding a simple oscillator and use the coil/cap as the tuned element and just measure the freq with my counter, if I do this I'm thinking the dcf signal would mess things up, would I get away putting the circuit in a screened box, or is there a better way to do this.
Q might still be an issue as I cant easily measure it, however the b/w of the aerial must be 3kc min to receive audio in its intended application, so 1 hz modulation from dcf shouldnt be an issue.

 

[JimB]

if I do this I'm thinking the dcf signal would mess things up

Dont worry, the signal from DCF77 will be nowhere near strong enough to cause any problems.

JimB

 

[dr pepper]

Ok I'll try it.
With a 100k inline I thought dcf might fight it, that said I'm a long way off mainflingen.

 

[dr pepper]

DCF on 77.5 kc doesnt mess things up.

Something else does.

I was confused to see 81kc on my 'scope direct off the loopstick, I thought my 'scope was out of calib, it isnt I googled 81 khz, a few miles away from me is a naval transmitter on 81kc, that kinda spoils things.
I have a cpmmercial dcf clock, that does actually work so reception is possible, I pulled the clock apart theres just a black blob on the rx board, however there is a 77.5 kc xtal right next to it, so maybe an xtal filter would give me enough discrimination.

 

[JimB]

Ah, that will be GYN2 at Inskip near Blackpool.

I have GYW1 at Crimond, about 15 miles away, always a very strong signal when I am trying to play with VLF stuff.

What are you trying to do with DCF77 ?
MSF on 60kHz is not too far from you at Anthorn in Cumbria (?)

JimB

 

[koolguy]

Hello Guys,
What is this
dcf77?

 

[JimB]

A standard time/frequency transmitter, located in Germany.
Operates on 77.5kHz.

Look here:
https://en.wikipedia.org/wiki/DCF77

JimB

 

[koolguy]

what is the use of it there?

 

[JimB]

what is the use of it there?

Did you even bother to look at, let alone read, the link which I provided?

JimB

 

[dr pepper]

OK then.

81khz from gyn2 is 130 degrees direction wise to mainflingen frankfurt, can I make my loopstick sensistive from just one direction instead of 2?, ie deafen one of the lobes.
I dont fancy buiding a yagi for 77khz.

 

[RCinFLA]

An A.M. radio loop stick is approximately 250 uH so tuning it at 77.5 KHz will take a parallel cap in the range of 17 nF to tune it. The Q will likely be higher at 77.5 KHz then at 550 KHz. Starting guess Q of 125 would yield an Rp of 15k. To not degrade the unload coil Q you should not load more then 150k ohms.

You can do this in two ways. Two 330K resistors to parallel resonate take. Generator into one, sense circuit in other. Another way is to couple in and out via small value capacitors. To project 300K from a 50 ohm generator source and sense would be 500 pF coupling cap. Two caps, one for gen, one for sense. Take off 2 x 500 pF = 1000 pF from the total parallel resonating cap. 17000 pF - 1000 pF = 16,000 pF approx. resonating cap. Measure peaking freq for resonate freq determination, tweek as necessary to get to 77.5 KHz then measure 3 db down points above and below to calculate coil unloaded Q.

You could use a scope for sense directly on coil with /10 probe which would parallel 1 meg and about 11 to 15 pF.

With this value capacitor you have to be concerned about the Q of the selected tuning cap. Use mylar or silver mica if you can find one of size. Avoid metal film caps.

Another thing that can go wrong with ferrite loop antennas is degradation if they have been exposed to magnetic fields sometime in their past life. Magnetic field will permanently bias the core resulting is very severe Q degradation and lowering of inductance. Just waving a magnet by them will degrade them.

 

[dr pepper]

Thats a interesting approach I'll try that.

I was coupling the loopstick to the sig gen with a piece of 3/4 tube with 20 turns on it being fed by the gen.

 

[koolguy]

Please ignore my question , i was unaware of hijacking threads.
thanks