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To: "RSGB LF group" <rsgb_lf_group@blacksheep.org>
From: "Tom Tishken KD4WOV's blackberry device" <kd4wov@earthlink.net>
Date: Fri, 14 Aug 2015 22:23:42 +0000
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Subject: Re: LF: Re: RE: Folded monopole - Food for thought
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Question: what would the efficiency of a helical antenna be compared to a inverted L?

4 bamboo poles with a wire wound in a big helical let's say 20 feet across? 

Just a thought.

De kd4wov Tom
EL98 grand island, Florida

Sent via a mighty BlackBerry BOLD 9000 by AT&T (a NSA listening device)

-----Original Message-----
From: "Alan Melia" <alan.melia@btinternet.com>
Sender: owner-rsgb_lf_group@blacksheep.org
Date: Fri, 14 Aug 2015 22:31:14 
To: <rsgb_lf_group@blacksheep.org>
Reply-To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: RE: Folded monopole - Food for thought

The problem with a monopole at LF is that the effective height is only half 
(approx) the physical height. For an inverted L where the top length is at 
least twice the height, the effective height is close to the physical height 
.....so 3dB more ERP. It may not matter so much on 160m (I used a 16foot 
centre loaded "tank-whip" at one time :-))  )  but where ERP watts are 
difficult to come by, it IS important.

I believe one of the US "Lowfers" used a "gamma matched tower" but I cant 
remember who now.

One has to be careful with NEC at LF it only models the things that are 
built into it. There is a QEX article this month purely simulation based. It 
will be fine installed in a 2acre field but not for a vertical on a normal 
"back lot" There are two major effects (for LF) it doesnt seem to consider, 
ground strata/skin effect, enviromental losses, and the effect of 
top-capacity.  These are much more dominant at long wavelengths.

Alan
G3NYK

----- Original Message ----- 
From: "Clemens Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>; <rsgb_lf_group@yahoogroups.co.uk>
Sent: Friday, August 14, 2015 9:20 PM
Subject: LF: RE: Folded monopole - Food for thought


> Hello Andy,
>
> I seem to remember that NEC modelling done by a real expert in this area,
> W4RNL (sk), years ago revealed that a 'linear' (ungrounded) monopole of 
> the same height as a folded
> monopole has a significant higher gain if the height is below 60 degrees 
> (0.17 lambda)
> (excluding losses in the matching circuitry).
> But I've never modeled these antennas myself.
>
> 73
> Clemens
> DL4RAJ
>
>
>>-----Original Message-----
>>From: owner-rsgb_lf_group@blacksheep.org
>>[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Andy Talbot
>>Sent: Friday, August 14, 2015 5:52 PM
>>To: rsgb_lf_group@blacksheep.org; rsgb_lf_group@yahoogroups.co.uk
>>Subject: LF: Folded monopole - Food for thought
>>
>>On the RSGBTech group recently G3RZP described his folded
>>monopole topband antenna.  he runs an insulated wire up the
>>side of the lattice mast to which it is bonded at the top.
>>The bottom of this wire becomes the feed point and requires a
>>matching unit to tune out the residual inductance of the
>>resulting loop - plus impedance transformation..  The bottom
>>of the mast is grounded, and connected to a radial mat.
>>
>>The advantage of this arrangement is it allows a conventional
>>mast to be used as a radiator.  The feed is nominally
>>inductive, being a think squashed loop, but the height of the
>>mast stays what it would have been if fed conventionally.  An
>>HF beam and sundry other stuff on the top acts as a capacity hat.
>>
>>Disadvantages relate to isolation of cabling for equipment for
>>other bands' use, and needn't concern us with its use here.
>>
>>I wonder if anyone has tried this scheme on MF or LF over the
>>years/ (I know topband is MF, but you all know what I mean :-)
>>Being able to feed a short monopole using capacitive tuning
>>instead of a conventional bulky, complex-to-build adjustable
>>loading coil must have its advantages.
>>
>>It may also make for a quicker to put together portable of
>>fast up- fast down system, or ad-hoc antennas?
>>
>>Some time ago one of the well thought out more rational
>>arguments about magnetic loop antennas was that when they
>>become larger than "very very small" , the linear dimension
>>across the loop then starts to behave as a dipole with
>>appreciable addition to the true loop operation.    here we
>>have a loop, but deliberately elongated to enhance the dipole
>>(or here the monopole) behaviour.
>>
>>Consider the ultimate limit, a tall twin feeder, shorted at
>>the top and fed at the bottom, with one side grounded.
>>
>>An idle thought...
>>
>>Andy  G4JNT
>>
>>
>>
>>
>>
>
>