[Lowfer] Tacamo 17.8 kHz

Mon Jan 24 18:26:26 EST 2011

The recent postings re Tacamo on 17.8 KHz have got me thinking.

In particular the fact that quite often wide band transmissions are
mentioned.

With ground based VLF systems, generally one big problem is the Q of the
antenna causing it to have a narrow bandwidth. This would have been no big
problem for CW Morse keying, but for FSK, some stations used the keying to
adjust the resonant frequency of the antenna to the Mark and Space
frequencies. Hence the trend towards narrow shift and MSK.

So with a 1.7 KHz bandwidth, something has to be different.

Let's look at the antenna. An electrically short antenna will have a narrow
bandwidth. I know only too well from my Merchant Navy R/O days on MF (410 -
512 KHz) Change frequency and you had to retune/reload the transmitter.
Similarly with T and inverted L antennas, if slack the tuning would be all
over the place. No such problem on HF, I could jump from Calling frequency
to Working frequency without having to consider retuning.

Now, trailing a wire from an aircraft. Tends to suggest that the antenna
circuit is between the aircraft fuselage and the wire; whereas for ground
based VLF it is between the antenna and perhaps just as much wire buried as
a ground plane. So for the airborne version we have a lot of antenna wire,
the link for the Boeing E-4 gives 5 miles and another link
http://en.wikipedia.org/wiki/TACAMO said two antenna reels - 5 miles each? I
do not know.

So, 5 miles = 26,400 feet = 8046 metres. A wavelength of 8,046 metres = a
frequency of 37.28 KHz Put it another way a quarter-wave length of 8,046
metres = a frequency of 9.32 KHz and a half-wave length of 8,046 metres = a
frequency of 18.6 KHz - Now, that's more like it. I have not allowed for
Velocity Factor in the above, to keep it simple.

So, some options. A quarter wave aerial (half a reel) but has to be tuned
against the aircraft fuselage/wings only. Should have a low impedance and
reasonable bandwidth. I have read of Tacamo aircraft flying a tight orbit so
that the aerial hangs down near vertically. 

Or how about using two half reels of antenna with a "device" on the two wire
ends to make them "fly" at an angle to the aircrafts track? We now have a
half-wave V-dipole. Again lowish impedance and reasonable bandwidth.

Or finally, how about a half-wave sleeve dipole? For 200 KW from
http://en.wikipedia.org/wiki/TACAMO with a 50 ohm antenna would be 63 amp.
Again should have a reasonable bandwidth. 200 KW is the same as SAQ the
other week. The temperature at 30,000 feet is around -45 to -50 degrees
centigrade, so I'm sure an expensive lightweight coaxial antenna could
handle the power..

The above are just a few thoughts. My time at sea was all commercial, My
flying (PA-38 PA-28) is all pleasure.

Cheers!

73's

Clive

GW4EYO

Ex GW8AOC

-----Original Message-----
From: lowfer-bounces at mailman.qth.net [mailto:lowfer-bounces at mailman.qth.net]
On Behalf Of Dexter McIntyre W4DEX
Sent: 24 January 2011 01:54
To: Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands
Subject: Re: [Lowfer] Tacamo 17.8 kHz

ToddRoberts2001 at aol.com wrote:

> I am  sure they

> are probably using different planes nowadays?

>   

Just found on wikipedia the same planes are still in use:

http://en.wikipedia.org/wiki/Boeing_E-4

I'm feel sure though most of the electronic gear has been replaced. 

It's obvious a lot of info has been declassified.

Dex

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