[OZAPRS] On loops v whips mobile
Glen English VK1XX
glenlist at pacificmedia.com.au
Thu Dec 7 21:08:25 AEDT 2017
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
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.
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,
Whip Likely efficiency : (0.1ohm Rrad) 0.6%
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
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
Think 2x the efficiency as the base loaded stinger.
of course whips with capacitive hats are the trick, but that's another
**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 )
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