Message-ID: <001a01c9dc5e$b6bbb040$0301a8c0@mal769a60aa920>
From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <4A181EEC.5050600@btconnect.com> <9670125E608A4D63AE541048B2177A02@JimPC>
Date: Sun, 24 May 2009 11:59:29 +0100
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Subject: LF: Re: Re: First Portable Trip
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Jim
I did not say an inv L virtually lying on the ground as in Richards case(1 
METRE ABOVE GROUND), this is more like a beverage antenna.
I did say as much vertical as possible and let the so called horizontal 
taper towards earth. The far end  as high as possible tied via insulator to 
a bush or stake, say a few metres above ground.
Depending on the vert height you might not get much horizontal out before 
its too low, so it needs a sensible approach. Use the loading coil to 
resonate and match the TX.
Mal/g3kev


----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, May 24, 2009 11:12 AM
Subject: LF: Re: First Portable Trip


> 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
>
>


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