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Author Topic: Shortened 160 Meter Delta Loop  (Read 8955 times)

Posts: 22

« on: December 10, 2001, 11:55:41 PM »

Hey antenna Gurus!
I would like to build a shortened 160 meter delta loop wire antenna. Each leg will have a big loading coil in the center of it to lengthen it electrically. My yard will permit a delta loop with 60 ft legs (60x60x60).
Here's my question. How big do I make the coils? I would like to know diameter, length and number of turns of wire.
 Is it possible to build this antenna? I realize it will be lossy and narrow banded.... How would I go about calculating such a coil?
 Please email replies to:

Thanks.......  Mark

Posts: 21764

« Reply #1 on: December 11, 2001, 01:31:27 PM »

Not sure why you'd want to use three loading coils, one in each side.  The coils don't radiate, they just add inductance, and one lump would work as well as three lumps in this regard.  For that matter, bearing in mind that the intended purpose of the coils is to bring the antenna into electrical resonance, which will help provide a reasonable match point for feeding it, and the coils serve no other purpose since they truly do not radiate, a 4:1 balun, with the "high impedance" side connected to your transmission line and "low impedance" side connected to the loop's feedpoint, will do about the same job with less work and less windloading.

Even better than that would be a 9:1 balanced:balanced isolation transformer (like a bifilar toroid) which would be fed with 450 Ohm ladder line and then tuned in the shack with a balanced tuner.  Great step forwards in efficiency, and no loading coils needed.

73 de Steve WB2WIK/6

Posts: 17479

« Reply #2 on: December 11, 2001, 01:21:40 PM »

First, I would NOT recommend that you use a loading coil at the
point opposite the feedpoint.  This is the point of maximum current,
and you want as much of that radiating as possible.

Since the overall loop circumference is less than a half wavelength,
the reactance at the feedpoint will probably be inductive already,
with a fairly high impedance value (around 1000 ohms?)  I would do
your matching at the feedpoint.  Depending on the exact feedpoint
impedance, you may be able to match it by putting a series
capacitor in each leg to cancel out the inductance, then feeding
it through a 3 : 1 turns ratio transformer (50 to 450 ohms).  Or use
a balanced "L" network to step the impedance down to, say, 200
ohms, which would match the coax with a 4 : 1 impedance balun.
This would consist of a shunt capacitor between the two ends of
the loop, and a series coil between each end of the capacitor and
the leads from the balun.  A starting point for the coils would be
perhaps 20 turns on a 2" diameter form, set up so you can tap any
turn.  This allows you to vary the inductance in each leg (keeping
them approximately equal) and readjusting the variable capacitor
each time for resonance.  Do your testing at low power, then, when
you find a matching network that works, measure the values and
replace the components with ones designed for long term weather
resistance and your output power.  (A transmitting mica capacitor
would probably be a good choice to replace the variable cap.)

You can feed such an antenna with open wire line or 300 ohm
twinlead to a tuner.  However, many of the commercial "T"-net
tuners will not be very efficient because of the limited maximum
capacitance of the variable capacitors they use.  The load
presented to the tuner will depend on the length of the feedline.
I put up a similar antenna for a friend and he had great reports on
160m until his tuner melted - the coil got so hot it unsoldered itself
when running just 100 watts (well below the power the tuner was
rated to handle.)  So, if you take this approach, take the cover off
your tuner and feel the coil after operating for a while to make sure
it is not overheating.  Often it works better to build a fixed matching
network with larger capacitors:  I had to add 2200pf to my tuner
one time to tune an antenna on 160.  Variable capacitors this
large are quite expensive, so find the right setting at low power
using a mixture of fixed and variable capacitors, then replace them
with heavy duty RF caps once you find the right values.  It may
be easier to use fixed capacitors and a variable coil (which can be
tapped at any turn) on this band.

Good luck! - Dale WB6BYU
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