Re: LF: Re: Re: Re: Re: Loop TX antennas at VLF?

[Scott Tilley <sthed475@telus.net>]

Hi Mal and Group

The advantage we gain with loops on 137 is a unique balance based on the 
specific conditions of various QTHs vs. the site conditions impact on a 
vertical type antenna.  I'm glad to see Mal is 'kinda' accepting this as 
a possibility.  There may be hope for the old boy yet :-)    It's a 
workable engineering compromise on 137 but that same logic is lost on 
9KHz in my humble opinion.
I did some calculations on the viability of a loop on 9KHz some time ago 
and it's my opinion that it would not be a viable option due to the huge 
capacitance required and the efficiency of the loop would be very very 
very very low.  Consider the effect of Q on the tuner.  This would 
require a very stable cap.  Perhaps a Gyrator could be made to do this 
with an inductance but the complexity of the design is starting to 
eliminate the viability of the antenna when everything else is considered.
The input power needed to create a suitable ERP would cause considerable 
engineering problems as well for the TX design, tuner etc...
Having pushed the power levels into the realm of over 2KW input on 137 
on my large loop system scary temperatures emerge and a lot of careful 
testing and design needs to be done to ensure the tuner doesn't go 
'critical'.  The engineering needed to ensure stability on 9KHz in order 
to create a stable tuner that could handle the power to create a 
reasonable ERP would be mind boggling.
I find myself in the awkward position of agreeing will Mal's 
recommendation about the use of kite and balloon launched verticals at 
suitable portable sites for serious DX work on 9KHz.
PS - I truly hope someone proves me wrong !

73 Scott
VE7TIL


On 3/18/2011 12:19 PM, mal hamilton wrote:
> Jim
> Now you have introduced another element into the argument ENVIRONMENT,
> which is not applicable to the argument because each individual QTH will be
> different. In a discussion like this,   one has to assume a level playing
> field.
> Discuss each antenna in the ideal environment THEN consider the X factor
> depending on a specific QTH where a loop might have an advantage although I
> doubt it.
> also
> the loops to which you refer in the USA used on 137 are fairly large
> compared to what some intend to use on 9 khz in the UK, I doubt if they
> would radiate over any distance even on 137.
> Why are some reluctant to go for high power and bigger antennas, whereas in
> EU balloons and kites are acceptable as the norm for those trying to cover
> the maximum distance.
> In the UK there are wide open spaces in the countryside plus beaches for
> kite and balloon activities.
> 73 de mal/g3kev
>
>
>
>
> ----- Original Message -----
> From: "James Moritz"<james.moritz@btopenworld.com>
> To:<rsgb_lf_group@blacksheep.org>
> Sent: Friday, March 18, 2011 6:40 PM
> Subject: LF: Re: Re: Re: Loop TX antennas at VLF?
>
>
> > Dear Mal, LF Group,
> >
> > You are ignoring the fact that a number of amateurs have used loops quite
> > successfully in the LF range, particularly in the US for 136k and "Lowfer"
> > operation. The type of situation where loops can be more efficient than
> > verticals of a similar size seems to be where the antenna is in a location
> > with many tall trees. The trees can be used to support a large antenna,
> but
> > also bring about high losses due to dielectric losses in, and the
> screening
> > effect of, the poorly conducting wood. Loops, with generally reduced
> > electric fields, are less adversely affected, it would appear.
> >
> > In the VLF range, using the same size of antenna and current level, the
> > voltage on a vertical antenna is much higher, so one would expect extreme
> > levels of loss of this type. So while on paper, and in the middle of a
> nice
> > flat field, the vertical would be more efficient, the loop might actually
> be
> > better in a practical situation with less than ideal locations. Loading
> > coils are also a significant factor - it seems to be difficult to make a
> > reasonably sized VLF loading coil for a small antenna that does not add
> > significantly to the overall loss. The previous calculation shows that the
> > requirements for a low-loss loop capacitor should not be too difficult.
> >
> > Cheers, Jim Moritz
> > 73 de M0BMU