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Author Topic: 80m Loop antenna; winter's over and time to work on the antenna  (Read 1723 times)

Posts: 184


« on: April 01, 2013, 01:35:30 PM »

Several years ago I put up an 80m loop that allowed me to work on every band from 80-10, a most excellent antenna.  Then I changed something coupled with a delay in testing and one thing lead to another then here I am years later and the loop basically stinks.  I know the right way to build a loop (Don Keith's, N4KC, basic design is where I need to get back to).  One thing I don't know is how close to a metal mast do I have to stay away from before I have to keep everything?  In the past I think I used one but in an ideal world non-metalic seems the way to go, yet I know there's lots of guys who hang their loops from towers so I think I might be overthinking this aspect of the thing.

So, I'm going back to 277 feet of wire, 450 ohm ladder to a 1:1 current balun right before I go through my copper pipe to the radio (coax feed method).

Glen - KE7FD

Posts: 15278

« Reply #1 on: April 01, 2013, 01:51:42 PM »

If the antenna is off to the side of the mast, then a foot or less of
spacing is adequate (unless you are running high power.)
Basically enough room for an insulator on the wires and a short
length of rope before it runs though the pulley and down to the
ground level.  (I use halyards with pulleys in all corners of my loops
to make it easy to take them down for repair, tuning, etc.)  Usually
I use the electric fence pulleys that are like a plastic "egg" insulator,
but allow you to slip the insulator around the wire rather than needing
to thread an end through the hole.

The exact details aren't critical - with ladderline feed the length
can vary +/- 10% or so with little change in overall performance.
The last one I put up I think I had the wire from each leg tied around
one of my tent poles for strain relief, then the last few feet of wire
dropped down to the table and connected to the back of the tuner.

The impedance at the tuner will vary with the length of ladder line,
or course.  As long as you keep the coax length short it should work.

Posts: 184


« Reply #2 on: April 01, 2013, 02:16:24 PM »

Is the length of ladder critical or required length?

Posts: 15278

« Reply #3 on: April 01, 2013, 02:56:50 PM »

The length of the ladder line affects the impedance at the balun and the SWR
on the coax, but this varies by band. It shouldn't be a problem with a wide-range
tuner connected directly to the ladder line.

However, if you are using a balun into coax with no tuner at that point, it will
affect the SWR on the coax, and a high SWR can increase the losses.

As an example, thefeedpoint impedance on 80m should be about 100 ohms or so.
With 50' of Wireman 551 ladder line we can use VK1OD's handy calculator:

to calculate the impedance at the balun as 630+j700 ohms at 3.5 MHz.  The
SWR on the coax will be around 30 : 1 across the band.  The same feedline
length on 40m (where the loop impedance is roughly 150 ohms) gives an SWR
of about 9 : 1 on the coax.  With 62 feet of ladder line the SWR on the coax
is about 3 : 1. 

On 20m the feedpoint will be around 200 ohms, rising to about 400 ohms on
10m.  Generally multiples of 1/2 wavelength (62' on 40m) will give the lowest
SWR on harmonic frequencies, but not necessarily on 80m or on the WARC
bands.  If the coax between the balun and the tuner is short then the losses
due to high SWR might be acceptable, but you may have to experiment to
find a line length that your tuner can match on all bands of interest.  (It
depends on the tuner design and the length of coax.)

One approach that I had good luck with (at least for CW operation on the
pre-WARC bands) is to cut the antenna for about 3.55 MHz and feed it with
a 4 : 1 balun directly to coax at the antenna feedpoint.  Typically the
feedpoint impedance at resonance varies between about 90 and 400 ohms,
giving a worst case SWR of just over 2 : 1 at the resonant points on 80, 40,
20, 15 and 10m.  (This works best for CW operation because it requires a
resonance low in the 80m band to get the harmonics aligned properly.)
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