So far as the less robust reception I got this afternoon here in the
middle of the continent, there are a couple of other factors to
consider in addition to propagation.
Probably the biggest one is antenna efficiency. The frequency ratio
from 74 kHz to 30 kHz is 2.46:1. Unless your antenna is significantly
taller than for the other bands, that means the radiation resistance
has diminished by the square of that ratio (a factor af about 6.08).
At the very best, your loading coil loss and ground system resistance
might remain about the same, but could very well increase (such as if
the increased skin depth takes return currents lower into the earth,
into a less conductive rock layer). So, I would expect overall antenna
loss has increased significantly--perhaps by 15 or 16 dB.
Second, I'm using the receiver outside its rated tuning range, by means
of lower sideband mode. My 250 Hz IF filter helps minimize the static
levels seen by the receiver, but seems to have a few dB additional loss
when switched in the SSB chain. I discovered this a couple of months
ago when I was testing various ways of copying your WSPR-2 signal at
LF. Decoding was more effective when I used the narrow filter with
upper sideband rather than the 1.8 kHz SSB filter, but was even _more_
effective when the narrow filter was in the CW chain with its normal
BFO offset. Therefore, my receiving setup is likely to be a few dB
less sensitive as well when working in this non-standard fashion.
On the positive side, just before I returned to town to check e-mail,
it appeared that somewhat improved nighttime propagation might be
kicking in. We'll know more in a couple of hours.
John
-----Original Message-----
From: Bob Raide <[email protected]>
To: rsgb_lf_group <[email protected]>
Sent: Sat, Mar 1, 2014 3:45 pm
Subject: RE: LF: Re: U.S. VLF License WH2XBA
Alan;
I am noticing exactly that-groundwave not that good at 29 kHz. Much
better at 137 or even 74. Just got report from Kansas where I always
put good daytime sigs in on 137-74. Got lot to learn about this VLF
stuff-Bob
- - - - -
From: [email protected]
To: [email protected]
Date: Sat, 1 Mar 2014 21:15:41 +0000
Subject: Re: LF: Re: U.S. VLF License WH2XBA
Hi Bob if you mean "skywave" or ionospheric returns? that occurs day
and night at LF and VLF the skywave is weaker in daytime at LF due to a
portion of absorbing D-lyer below the apparent reflection height, also
at VLF but less so. At the powers we use the ground wave probably does
not go very far. I think the two are about the same strength at around
400miles then the skywave predominates. It may be less with amateir
antennas because a lot more of the signal is lanuched at higher angles
that would be the case for big antenna on Mil sites.
I think there is some confusion about VLF being "all groundwave". Many
of these ideas were formed in the days before Heaviside and Appleton.
Ionospheric return is fom the D-layer in daytime (with some absorption)
and by the lower E-layer at night.
Paul Nicholson has front-ended some Mil propagtion software for VLF
called LWPC you can tell its the "real stuff it was written in Fortran
:-))
Single shot strengths are calculated for the date and time entered so
you can see diurnal effects. I have not tried it at very short range
but it certainly models he day-night transition quite well
It is at http://abelian.org/lwpc/
Best Wishes for Good VLF DX
Alan
G3NYK
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