|
Yes. I sneaked a separate ground wire down the chimney to the
basement, where PE and heating system are connected to the
metal water inlet pipe. On a cold and
dry winter day, the antenna resistance at LF (including the coil) is as low
as 15 ohms. This probably is probably aided by a Faraday
shielding effect from the thermal insulation under the roof, which is
covered by a vapor-stop layer consisting of very thin aluminium foil
strips - seems the roof is acting as a sort of ground screen.
The telescopic rod consists of several sliding
sections separated by insulating plastic rings, which I thought was
good. But I was plagued by erratic partial discharge problems between
segments, which sometimes came and went away with wind-induced
bending. Strong noise in my neighbour's FM radio, and strange krrrk krrrrrk
sounds coming out of the chimney doorlet. In a recent rework,
I have connected all segments with little bolts, and tied
the bottom of the mast to ground potential.
73, Markus
Sent: Saturday, January 11, 2014 6:44 PM
Subject: RE: VLF: 8270
Markus; That is an amazing antenna! That short to work so
well at VLF. Evan for LF but it really works for you. I see no
provision for ground system and would guess you use house plumbing for ground
system?
From: [email protected]To: [email protected]Date:
Sat, 11 Jan 2014 18:26:30 +0100 Subject: Re: VLF: 8270
Bob,
it's the same antenna that I use for LF. The
mast is a fishing rod on an aluminium telescopic pole, pushed out of an unused
chimney. It carries three topload legs (N, W and S), about 42 m wire in
total (230 pF). The apex is 20 m AGL or 11 m above the roof, but the
effective height is only 9 m (calibrated at 138 kHz). On VLF frequencies
this is probably even a bit lower due to increased shunting effect from trees.
Assuming heff = 0.8 m, radiation resistance on 8.3 kHz would be 0.08
milliohm. With 60 Watts input, I got 0.3 A and 25 kV rms,
and radiated power (EMRP) would be around 7 microwatts.
Here's a sketch of the wire layout and the
auxiliary H- and E-field receive antennas:
One could actually see the antenna in Google
Map's slant view, but the imagery may have changed since:
Best 73,
Markus
Sent: Saturday, January 11, 2014 5:37
PM
Subject: RE: VLF:
8270
Markus;
What are you using for antenna
on 8270?
Hello Markus!
Nice signal here,
congratulations!
Lubos, OK2BVG
Took the carrier off air at 12:48. Looks
like a good trace at OK2BVG, and a dash with marginal SNR in Todmorden UK.
73, Markus
Sent: Saturday, January 11,
2014 11:33 AM
Subject: VLF:
8270
At 10:23 I have started to transmit a
straight carrier on 8270.000 Hz, estimated ERP around 14 uW. If all stays well,
I'll leave it on for a couple of hours, trying to leave a dash on Lubos'
and perhaps Paul's spectrograms.
73, Markus
(DF6NM) |
