Re: LF: Re: Should I be aiming for a better match than this?

[Andy Talbot <andy.g4jnt@gmail.com>]

I have always intensely disliked tapped 'loading' coils such as that, for precisely the reason you mention - a tap at one turn.    This is the sort of thing you'd expect with a high Q system.  Tapped coils, mutual coupling is always a hit and miss affair you have to set up by trail and error.  A complete bodge - just because its simple

Much better is to measure your antenna's total input resistance when resonant, then make a ferrite cored matching transformer to transform that resistance to the 50R (or whatever) your Tx is designed to drive.

Ferrite cored transformers are near enough perfect at this (but remember V = 4.44 F.N.A.B for the minimum turns needed)  and a few taps there will allow for variation in antenna resistance changing with weather conditions.   

At 700 Watts I use an old ETD49 core with Ae = 200mm^2, ferrite type something like EC90 if memory serves.   Standard SMPSU type anyway.    At the 700 Watt level it barely gets warm

My 7m high Tee antenna at 137kHz when resonant varies from 90 ohms in dry summer conditions to as much as 130 or 140 ohms in wet soggy weather.  The transformer is incorporated into the Tx and serves dual purpose as matching and isolation for teh direct-from-mains switcher PA.   It has switchable output taps for a number of impedance including 50R and a range from 80 to 150R.   A 10m  run of UR43 coax is quite happy at the 3:1 ish SWR resulting

http://www.g4jnt.com/137tx.pdf

Andy  G4JNT

On 16 October 2015 at 09:48, Chris Wilson <chris@chriswilson.tv> wrote:

Hello Alan,

Friday, October 16, 2015


Things took a sorry turn late last night, when making a final TX I
noticed the meter in the G3YXM amp that switches between FWD / REF and
measuring DC volts was reading low. Waveform voltages from the
Scopematch looked much as always, but when changing the switch between
forward and reflected power the forward power was perhaps less than a
third on full power, and the reflected power was the converse and up a
lot. I tried into the dummy load, which uses a different co-ax and it
was the same. It was still putting a signal out into Germany, although
the WSPR reporter networks seemed to show a big drop in received
signal level.


I am wondering if this feedback or whatever sometimes happens has
damaged a FET, can one or more of the 4 fail and still leave it
operational, but with reduced power and more reflected power? The
failure I had with the output stage in the early stages of working
with this was spectacular, noisy, smoky and terminal, leaving no
output at all. For convenience the circuit is at:

http://www.chriswilson.tv/amp.doc


With regard to your advice below, I don't have a clamp on current
meter, but did pick up a suitable little meter movement at the Newbury
Rally to build one, so I will get one knocked together. I am
suspecting my toroidal auto transforming matching transformer, it
seems to need a bizarre feed tap, just one turn from the earthy end,
and get pretty warm. I may build a galvanically isolated one like John
Rabson has suggested. But first I need to find out what mischief I
have done to the amp!

I have now experienced the transmissions tripping the main house
distribution RCD several times, and have realised that the recent
random going off of the workshop alarm (panel shows a window knock
sensor has set it off, but I am sure it's RF) coincides with
transmission when the signal shows odd on the Scopematch. Thanks Alan.


> Hi Chris Im glad the tap idea helped. It is quite difficult at a distance to
> think of things that might cause the effects but it looks like we nailed
> one.

> Your suggestion of RF getting into the system is a good one. It is always
> difficult with a short antenna because you have a lot of radiation near the
> equipment.When I started using a 130ft end fed wire on HF with an FT-101 and
> its low-Z dynamic mic I got terrible problems not seen when I used a
> Heathkit HW100 with a crystal mic. The latter was a lot easier to by-pass
> for RF. Eventually a trial with a ferrite rod (ex MW aerial ferrite) pushed
> into the coils of the curly lead completely stopped the problem. I had no
> trouble on CW....only when the mic was in.

> What you need to find is where the RF is affecting, then how its getting in.
> It may well be that some of the chassis' is going live to RF ....if that is
> the case you may even feel it :-((

> Try to make sure that the RF stays outside the shack if you can. Ground the
> antenna end of the coax......try to run the coax at right angles to the
> aerial as it leaves the coil. Possibly put some beads on the the remote end
> of the coax or a couple of turns through a torroid. I think you have a
> ferrite coupled current meter?? put it over the coax when you are using the
> dummy load. You shoud see very little current indicated, do the same with
> the aerial/tuner connected and I guess you will see a lot more. That (if it
> is the case) will give you something to aim at. Get the readings  similar to
> those on the dummy load. I think the RF is probably comming back on the coax
> braid.

> That's my best guess at the moment.....my, you are learning a lot of
> practical radio in a short time :-))  I may be totally off target but it a
> good place to start.

> Best of Luck
> Alan
> G3NYK










--
Best regards,
 Chris                            mailto:chris@chriswilson.tv