[OZAPRS] On loops v whips mobile

[Carlos Peco-Berrocal]

COMROD makes both a half loop and a full loop for NVIS (being sold by
others under their own name). They are expensive to acquire/repair, and
tests revealed that they are no better than a tilted whip. As a result,
sales are not happening in the numbers they hopped for:


http://www.hfindustry.com/meetings_presentations/presentation_materials/2010_feb_hfia/presentations/HFIA_loop_presentation.pdf


The COMROD loop was on display the other day at Eylex stand at MilCIS here
in Canberra. I have some hi-res photos I took at another show (Land
Forces?) and Owen Duffy found the tech description in COMROD's patents (in
case someone wants to replicate it at home).

73,
Carlos VK1EA






On Thursday, December 7, 2017, Glen English VK1XX <
glenlist at pacificmedia.com.au> wrote:
> I was doing some quick numbers of loops versus whips at 10 MHz and 7 MHz.
> At 10 MHz, the whip may be more suited as you get very little straight up
at high angle, compared to a vertically orientated mag loop. Most of the
work at 10 MHz will be < 45 degrees takeoff
>
> So, some numbers
> the Diamond 1.4m base loaded whip-
> - Rrad = 1 ohm. yes 1 ohm !
> - for Rground = 15 (pretty good)
> efficiency will be about 6%
>
> Loop, 3m circumference, inch wide Aluminium strip. Not bad, Q=1000
capacitor.. reasonable
> that's say a square loop 50cm tall, 1m long.
> likely efficiency =  9%.  maybe less depending on its height above the
steel roof. might improve with various 'shield' plates.
>
> But not that much better considering the hassle, and the overhead
performance of the loop is not useful.
>
> Whip wins.
>
> *7 MHz*
> similar sort of whips, same size loop :
> Whip efficiency (0.5 ohm Rrad) = 3%
> Loop efficiency same loop : ~ 3.3 %
>
> Now the difference at 7 MHz is that the high angle stuff is VERY useful,
because the ionosphere will provide plenty of reflection vertical in the
daytime, and the loop has plenty of output at 45 deg etc.
>
> The whip good for low angle stuff at night, but but really using the high
angle modes that 7 MHz is useful for,
>
> Loop wins.
>
> *3.5MHz *
> Whip Likely efficiency :  (0.1ohm Rrad) 0.6%
> Loop  0.4%
> The high angle stuff is important here, Loop wins.
>
> However this tells up the loop is too small
> Increase the loop circumference to 4m and the efficiency increases to 0.9%
> Increase the loop circumference to 5m and the efficiency increases to ~
1.3%
> 1% is not bad!
> I am taking into account the Rloss due to capacitor Q which varies with
loop size.
>
> The reality is that whips can be MUCH better than the base loaded example
>
> Pretty hard to go past a continuously helically loaded whip on a tapered
fibreglass blank.
> Think 2x the efficiency as the base loaded stinger.
>
> of course whips with capacitive hats are the trick, but that's another
story.
>
> **For more up radiation from a whip, consider taking a wire to the top of
a whip and pulling it backwards in a curve, sort of an inverted L. There is
very little current in the top (end) of the antenna , so it wont do much
but it will do something. Good time to have a 'loading coil' 2/3rd up the
total antenna in order to improve the current distribution (ideally
rectangular, usually triangular )
>
> -glen
>
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