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High impedance antenna co-ax or not

dr pepper

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Thread starter #1
I've put together a little receiver for Msf (Wwvb).
The 24" wire antenna input goes direct to hte gate of a fet for high impedance as the antenna is short compared to the wavelength.
In the final design the rx pcb is 8" away from the board, what is a good way to get the ant input to the rx?, obviously co-ax would present a low impedance, would twisted pair with one grounded do the trick?
 

JimB

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#3
Not sure of your exact layout, but as Nigel suggested, running a single wire within the equipment box is a good solution provided that it is not running next to any noisy signals within the receiver.

How about moving the FET input circuit so that it is near to the antenna socket?
Or,
Make an "Active Antenna" with an FET input stage and something with a small amount of gain and a low impedance output, so that you can mount the antenna remotely in a quiet location and use coax to connect from the remote antenna to the receiver.
Power for the active antenna can come from the receiver using "Bias T" techniques.

would twisted pair with one grounded do the trick?
There is still going to be a fair amount of capacitance in that. But maybe not quite as much as a length of coax.

JimB
 

dr pepper

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Thread starter #4
I'd thought of a 'miniwhip' style ant with a 50r o/p, and use a transformer coupled input on the Rx, I was trying to avoid the complexity.
I'll bung the wire through the side and see, if it doesnt work then the active ant is the way.
 

JimB

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#5
If I remember correctly from previous discussions, you are located not too far from Anthorn, there should be plenty of signal.

JimB
 

unclejed613

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#6
for runs of cable shorter than about 1/10 wavelength, the characteristic impedance of the cable really doesn't matter much because at such small lengths, a piece of coax is just a piece of wire with a shield. same with twisted pair. the shield and the twisted pair do minimize coupling from nearby noise sources.
 

dr pepper

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So I'd be ok with around 500 meters then!

Anthorn should be strong signal, however a military Vlf transmitter is within sight of the house.
 

unclejed613

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#8
So I'd be ok with around 500 meters then!

Anthorn should be strong signal, however a military Vlf transmitter is within sight of the house.
i'll bet thats a lot of fun, and i'll bet that the stations signal probably permeates a lot of electronics in your house...
 

dr pepper

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Thread starter #9
Back in the days when we played vinyl the pickup received morse from the transmitter nearly as well as the recording & Am radio was useless.
They dotn seem to transmit morse now, cant get it on anything, but there is a very strong signal indeed on 81kc.
 

unclejed613

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#10
Back in the days when we played vinyl the pickup received morse from the transmitter nearly as well as the recording & Am radio was useless.
They dotn seem to transmit morse now, cant get it on anything, but there is a very strong signal indeed on 81kc.
they are using MSK (minimum shift keying) which is very similar to FSK and QPSK. there isn't a change in amplitude, so it will look like a lot like a dead carrier on an oscope input, and there won't be much audio when this is tuned by a radio in AM mode.
 

dr pepper

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85hz shift according to wikipedia, looks like there might be some phase keying looking at it with sdr#.
 

unclejed613

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#12
MSK is very similar to QPSK because it has 4 states in it's constellation. 85hz shift seems to be popular in military communications. when i was in the Army, our RTTY systems used 85hz shift (it was, however, not MSK, but FSK. http://www.prc68.com/I/GRC106.shtml#MD-522 this is the modem we used, and we used it in 85hz mode.
 

dr pepper

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Thread starter #13
Thats interesting.
Doesnt 802.11b use 4 quadrant qpsk, -n using a dynamic number of shifts.
 

unclejed613

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#14
802.11b uses DSSS (direct sequence spread spectrum) which adds noise to the signal to widen the bandwidth of the signal and reduce the effects of interference. in the 2.4Ghz band, channel widths are very wide (22Mhz for 802.11b). in the VLF band however, the signal needs to have a very narrow bandwidth. a synchronous detection receiver improves the signal to noise ratio. remember that these signals are intended to be received while submerged. they are very low data rate signals, and are also encrypted.
 

dr pepper

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DSSS is new to me.
I dare say demodulating vlf is probably simple, decoding on the other hand not so simple.
 

unclejed613

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#16
if i were to make a guess, i would say the data format is a) synchronous (no start or stop bits), and b) protected by a very strong stream cipher. simple to demodulate, maybe somewhat simple to decode (from the demodulated signal to 1's and 0's), but without the crypto gear, impossible to decrypt.
 

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