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Message-ID: <9670125E608A4D63AE541048B2177A02@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4A181EEC.5050600@btconnect.com>
In-Reply-To: <4A181EEC.5050600@btconnect.com>
Date: Sun, 24 May 2009 11:12:58 +0100
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Subject: LF: Re: First Portable Trip
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Dear Richard, LF Group,


> Not a great success. Tried a 200ft inverted L with 7m vertical section fed 
> against 3 radials each between 50 and 100ft long. The final 100ft was very 
> low - within 1 metre of the ground.
>
(Mal seems to have started something here - I have always thought of an 
inverted L as having a roughly horizontal span, rather than being a sloping 
wire!) To get the maximum effective height, radiation resistance and 
therefore efficiency, the important dimension of the antenna is height - 
essentially the average height of all the wire making up the antenna. The 
long section at very low height is therefore counterproductive - it will 
reduce the effective height, and also increase the loss resistance. So it 
would be better to eliminate the last 100ft or more, and increase the 
inductance of the loading coil. If it is necessary to have that overall 
length to reach an available fixing point or something, use non-conducting 
string. Also, with a single span of wire and a single mast, you will 
probably get greater average height if you support the wire in the middle in 
an inverted-V shape.

I am not sure about the radials - there is a similar length of wire in the 
radials as there is in the antenna itself, so you could expect a significant 
RF voltage between the radials and the actual ground, which would not help 
matching or losses. Longer radials, more of them, or ground rods would 
probably be a good idea.

> Did not succeeded in tuning aerial up in link configuration. Tried tapping 
> the feeder at various points on the main coil but this reduced the Q 
> dramatically

The tapped loading coil type of antenna tuner with the TX connected to a 
low-impedance tap is one of those devices that looks appealingly simple, but 
isn't! The impedance match depends on the inductance of the complete loading 
coil, the inductance of the tapped section and the mutual inductance between 
the tapped section and the rest of the coil. These in turn depend on the 
shape, size and winding pitch of the coil as well as the number of turns. I 
made a spreadsheet that would calculate the impedance match for a tapped 
loading coil (downloadable from http://www.wireless.org.uk/features.htm), 
but suffice it to say that there are limits to the range of resistance that 
can be matched with a particular coil geometry. By "link configuration" I 
assume you mean the loading coil has a low impedance link winding over it, 
connected to the TX. This is similar to the tapped coil, but adds further 
variables into the mix, due to the different size and pitch of the link 
coil. I have not done the calculations for the link case, which will be 
similar to the tapped coil, but generally the mutual inductance will be 
less, which will reduce the maximum resistance that can be matched. What all 
this means is that it is quite possible that your coil has the correct 
inductance, but at the same time it is impossible to match the antenna 
resistance using it. This is likely if the antenna loss resistance is high; 
you need a "short, fat" loading coil (large diameter with fine winding 
pitch) to match a high resistance. I prefer to use a seperate loading coil 
and matching transformer; resistance and reactance are then almost 
independent of each other, and you only have to worry about the inductance 
of the coil and the turns ratio of the transformer.

> Band was still noisy even on relatively remote hilltop with horrible 
> "line-timebase" type burble drifting across band.

This is a problem most of us have had - several portable expeditions have 
put out big signals, but not been able to receive much. I have spent much 
more time trying to get good reception than setting up TX antennas, and 
often it is not possible to get good transmission and reception with the 
same antenna. There is always a temptation to try to get QRV as soon as 
possible, but a bit of reconnaisance with a receiver first is always a good 
idea.

Hope to hear you /P soon,

Cheers, Jim Moritz
73 de M0BMU