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From: "Dick" <knaap159@zonnet.nl>
To: <rsgb_lf_group@blacksheep.org>
Date: Sun, 11 Jun 2006 22:51:21 +0200
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Subject: LF: Re:PA4VHF/p TX problems
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Status: O

First off all thanks to Mal and James for picking up my signals.
Thanks Alan for your comments.

Well, I did tune the system with the help of a low power TX before running
high power.

I think the problem might be caused by the step up transformer I used to go
from 50 to 150 Ohm,
before coupling into the vario meter.

So, this was the set-up , maybe someone point out a possible trouble-source:


TX--about 2m of aircell 7 coax, followed by an step up transformer on a
large ring core N30 material,
(epcos core N30, R58, with 2mm CU-wire)
one end of the transformer is grounded to the groundstakes/some radials,
there is a fixed tap at 16 turns which
is the 50 Ohm input.
Then the turns continious, and I can choose output taps at 20, 23, 26, 29,
32 and 36 turns.

>From this point the transformer feeds the variometer (1.2-2.3mH) and on top
of the variometer is the large coil
with several taps 2-8mH.


Antenna: 8meter multi strand wire 4mm diameter, then a spiral coil, diameter
1meter, 400uH, followed
by a top load: 16 wires, each about 1 meter long, and 4 toploading wires 10m
each.

There was certainly an  amount of RF on the 4 topload wires, one of them
touched an tree with some nice
sparks etc as a result, and a drop in RF-field (solved this problem later)
Luckely the tree didn't catch fire........

An practical problem is that the variometer and big loading coil are in a
sperate box, and it is
not possible to change their position.
(using the big loading coil as first coil,  tapping the coax on it......)

The problem might be core saturation, because when I switched the TX to more
output, the antenna current
didn't change much.
Doubling the output changed the antenna current from 0.9A to 1.2A or so.
I first thought there was something wrong with the meter ciruit (1A or 5A
full scale),
because wasn't able to push the meter above roughly 1A in the 5A scale (1A
scale was end-stop......)
Yesterday after 30 minutes CQ-ing in the evening I found out that the step
up transformer core was very hot
(couldn't touch it.......)

So, perhaps I should trow out this core and use an air wound coil to
transform 50 to 100-300 Ohm.
Any suggestions about size/turns etc, or the use of other core material?

73

Dick, PA4VHF