[time-nuts] NIST

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 1 02:30:48 EDT 2018



On 08/30/2018 11:20 PM, Brooke Clarke wrote:
> Hi Bob:
> 
> I would disagree in that ease of jamming/spoofing is strongly related to
> wavelength.  That's because antenna efficiency goes down as the size of
> the antenna gets smaller than 1/4 wave.
> So, it's easy to make a GPS jammer (1,100 to 1,600MHz) since a 1/4
> wavelength is a few inches, something that  you can hold in your hand.
> It's harder to make a WWV jammer (.5, 5, 10, 15, 20 MHz) since a 1/4
> wavelength in in the range of  500 to 12 feet, something that can be
> mounted on a vehicle for the higher frequencies.
> But it's extremely hard to make a jammer for WWVB (60 kHz) where a
> 1/4wavelength is over 4,000 feet.  This means an antenna that can be
> vehicle mounted will be very inefficient. Note this also means that it's
> extremely hard to make a Loran-C jammer.  Note that the WWVB and LORAN-C
> transmitters run very high power and the antennas are massive.

Locally you can transmit with a much smaller antenna. It's been shown
and works.

Sweden used to have a network of 212 m towers to jam and spoof
Loran-C/Chayka. It was a top secret network.

> This also means that if someone makes a WWVB simulator for their house
> the signal at the next door neighbor's house is probably going to be too
> small to effect their clocks.

Magnetic loop works, non-resonant suffice. Magnetic lopp is used for
hearing aid, and it doesn't take much. Enough for the house. Not getting
you very far though.

> PS. Some decades ago I maintained a beacon transmitter "LAH" on 175 kHz
> where the rules for unlicensed operation limited the input power to 1
> Watt and total antenna length to 50 feet.  Under these conditions the
> effective radiated power might be 2 milliwatts, orders of magnitude less
> if a portable system.
> http://www.auroralchorus.com/pli/1750meter_antennas.pdf
> 

The 137 kHz band for radio amateurs is limited, but with radiated power,
and getting up to that power is a great success and considering the band
and that one use CW it should be fun.

Cheers,
Magnus



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