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Author Topic: Reflectors and Horizontal Wire Loop questions  (Read 5552 times)
WALTERB
Member

Posts: 528




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« Reply #15 on: May 11, 2013, 06:48:06 AM »

Either a diamond or triangle loop of about 280' should give a low SWR on 20m
with a 4 : 1 balun at the feedpoint - probably good enough to cover the whole
20m band at under 2 : 1 SWR.  (Minor length adjustment may be needed to
center it in the 20m band, of course.)  Actual feedpoint impedance is around
250 ohms.  (If the 4 : 1 balun is too heavy to put at the feedpoint, then use a
balanced feedline that is a multiple of 1/2 wave to wherever you can mount
it, and you should still have a low enough SWR that you don't need a tuner. 
Note that this method narrows the SWR bandwidth.)

The pattern depends on the shape:  with a diamond the major lobes are off
the corners, with nulls in the directions of the sides.  The strongest lobe is
away from the feedpoint (assuming it is fed in one corner.)

With a triangle the strongest lobe is still away from the feedpoint, but there
are now 6 lobes rather than 4.  The nulls between them aren't as deep, so
it tends to be more omnidirectional, but the peak gain is about 2 dB down
from the diamond.

By contrast a 40m loop used on 20m still has a 4-lobbed pattern off the corners,
but broader lobes than the 80m loop and > 4dB less gain in the main lobe.

As long as you can aim the 4 lobes in useful directions, the 80m diamond will
probably give better performance for what you want.

I used Google earth last night, and actually I believe I can get a perfect square out of my loop. That would be about the best coverage next to a circle. All of it over open backyard and at least 75 feet away from power lines and other interference.

I'm thinking of using coax out of the shack (as it is now) to my  single ground point, then a 4:1 balun and ladder line (450 ohm) up to the feed point (50 feet).  So 50 feet of RG-213 properly grounded, and 50 feet of 450 ohm ladder line vertically oriented to the feed point.    How does that sound?
thanks
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WB6BYU
Member

Posts: 12976




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« Reply #16 on: May 11, 2013, 08:52:06 AM »

Quote from: WALTERB

...How does that sound?



Well, let's see...

The feedpoint impedance is about 240 ohms on 20m.

Using VK1OD's transmission line loss calculator here:
http://www.vk1od.net/calc/tl/tllc.php
 we can see what it looks like through 50' of Wireman 551 ladder line:  the
impedance at the balun would be 560-j175 ohms.  Total loss is about 0.1dB:
so far, so good.

Divide that by 4, assuming a perfect 4 : 1 balun (which might or might not
be the case.)  The coax now sees about 140-j45 ohms. 

We can then stick that back in the same calculator for the 50' of RG-213, we
get 36-j44 ohms at the shack end, for an SWR of 2.8 : 1, and a loss of 0.6dB.
The total loss of 0.7dB is probably acceptable, but you'll need a tuner to match
the coax.

That's the same loss you would have if you put the balun at the antenna feedpoint
and ran 100' of RG-213 to the shack, except that with direct coax feed the
SWR would only be 1.2 : 1.

By contrast, if you lengthened the Wireman 551 to 63 feet, the SWR in the
shack would be 1.2 : 1 and you wouldn't need a tuner on 20m.  The total
loss would be perhaps 0.2 to 0.3dB less, but that isn't significant in this case.


That's great if you ONLY work 20m.  The antenna itself, with the 4 : 1 balun
at the feedpoint, has resonances at 40m, 20m and 15m, as well as near 10.75
and 17.75 MHz.  SWR on 30m is somewhat high (perhaps 8 : 1), but losses
should still be low on 17m where the SWR is about 3 : 1.

If you add a length of ladder line, the resonances shift around.  Actually, 50'
isn't too bad, since the SWR on the ladder line is no more than about 2 : 1
on both bands, but by using the 63' length you keep the 40 / 20 / 15m resonances
and actually improve the match on 17m.  But the match on 80m, which was
about 2.5 : 1 (at the low end of the band), degrades and is now around 20 : 1.

(To save time I didn't look at 12m or 10m.)

In this case, for operation on 40m, 20m, 17m and 15m, 50' of ladder line to the
balun is usable with a tuner, but 63' gives a lower SWR.  On 80m, using the
ladder line between the antenna and the tuner makes the SWR much worse,
and actually increases losses in the coax compared to putting the balun
at the antenna feedpoint.


For multiband use I've had best results either using ladder line all the way to
the tuner in the shack, or putting the balun at the loop feedpoint and running
coax all the way.  Sticking the balun in the middle with random lengths of
ladder line and coax on each end can lead to some quirky impedances and
high losses due to SWR, even when it can be matched with a tuner in the shack.
Sometimes, such as in this case where the loop impedance is well behaved to
start with, you can get away with it if you choose the length of ladder line
carefully.


Oh, and you don't have to get the loop exactly square:  anything close to that
should work.  I use electric fence insulators that I can slip around the wire
(rather than having to thread an end through).  Typically I lay out the wire on
the ground, add the insulators and ropes to correspond with the available
supports, then hoist the corners in sequence.  The insulators can slip along
the wire and equalize the tension (at least somewhat).  If you are trying to
maintain a more exact shape, then tie a loop in the wire at each corner and
attach your support rope to that.
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KE3WD
Member

Posts: 5694




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« Reply #17 on: May 11, 2013, 09:07:54 AM »


I'm thinking of using coax out of the shack (as it is now) to my  single ground point, then a 4:1 balun and ladder line (450 ohm) up to the feed point (50 feet).  So 50 feet of RG-213 properly grounded, and 50 feet of 450 ohm ladder line vertically oriented to the feed point.    How does that sound?
thanks

I would use TWO coaxes as Balanced Feed from shack to antenna. 

All the advantages of balanced line with all the advantages of shielding. 


73
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AC5UP
Member

Posts: 3812




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« Reply #18 on: May 11, 2013, 09:31:08 AM »

.... and don't be afraid to consider a balanced & shielded feed made with plain vanilla RG-6 from your local handy guy store. Cheap, easy to find in 100' retail packs, low loss, center conductor is more than adequate at the 100 watt level, doesn't mind proximity to metal, and the difference between 50 and 75 Ohms is inconsequential when you know you'll be tuning through a matchbox.

Truth be told, there are plenty of Q's being made using RG-6. Especially when it's a dipole on a hybrid rig with a Pi matching section.....   Wink
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WALTERB
Member

Posts: 528




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« Reply #19 on: May 11, 2013, 01:48:37 PM »

Quote from: WALTERB

...How does that sound?



Well, let's see...

The feedpoint impedance is about 240 ohms on 20m.

Using VK1OD's transmission line loss calculator here:
http://www.vk1od.net/calc/tl/tllc.php
 we can see what it looks like through 50' of Wireman 551 ladder line:  the
impedance at the balun would be 560-j175 ohms.  Total loss is about 0.1dB:
so far, so good.

Divide that by 4, assuming a perfect 4 : 1 balun (which might or might not
be the case.)  The coax now sees about 140-j45 ohms.  

We can then stick that back in the same calculator for the 50' of RG-213, we
get 36-j44 ohms at the shack end, for an SWR of 2.8 : 1, and a loss of 0.6dB.
The total loss of 0.7dB is probably acceptable, but you'll need a tuner to match
the coax.

That's the same loss you would have if you put the balun at the antenna feedpoint
and ran 100' of RG-213 to the shack, except that with direct coax feed the
SWR would only be 1.2 : 1.

By contrast, if you lengthened the Wireman 551 to 63 feet, the SWR in the
shack would be 1.2 : 1 and you wouldn't need a tuner on 20m.  The total
loss would be perhaps 0.2 to 0.3dB less, but that isn't significant in this case.


That's great if you ONLY work 20m.  The antenna itself, with the 4 : 1 balun
at the feedpoint, has resonances at 40m, 20m and 15m, as well as near 10.75
and 17.75 MHz.  SWR on 30m is somewhat high (perhaps 8 : 1), but losses
should still be low on 17m where the SWR is about 3 : 1.

If you add a length of ladder line, the resonances shift around.  Actually, 50'
isn't too bad, since the SWR on the ladder line is no more than about 2 : 1
on both bands, but by using the 63' length you keep the 40 / 20 / 15m resonances
and actually improve the match on 17m.  But the match on 80m, which was
about 2.5 : 1 (at the low end of the band), degrades and is now around 20 : 1.

(To save time I didn't look at 12m or 10m.)

In this case, for operation on 40m, 20m, 17m and 15m, 50' of ladder line to the
balun is usable with a tuner, but 63' gives a lower SWR.  On 80m, using the
ladder line between the antenna and the tuner makes the SWR much worse,
and actually increases losses in the coax compared to putting the balun
at the antenna feedpoint.


For multiband use I've had best results either using ladder line all the way to
the tuner in the shack, or putting the balun at the loop feedpoint and running
coax all the way.  Sticking the balun in the middle with random lengths of
ladder line and coax on each end can lead to some quirky impedances and
high losses due to SWR, even when it can be matched with a tuner in the shack.
Sometimes, such as in this case where the loop impedance is well behaved to
start with, you can get away with it if you choose the length of ladder line
carefully.


Oh, and you don't have to get the loop exactly square:  anything close to that
should work.  I use electric fence insulators that I can slip around the wire
(rather than having to thread an end through).  Typically I lay out the wire on
the ground, add the insulators and ropes to correspond with the available
supports, then hoist the corners in sequence.  The insulators can slip along
the wire and equalize the tension (at least somewhat).  If you are trying to
maintain a more exact shape, then tie a loop in the wire at each corner and
attach your support rope to that.

thanks.  If the RG-213 would work better, then that's even easier because that is what I feed the OCF dipole currently in place.  could just drop the OCF, and pull up the Sky-wire cut at 280' with the OCF balun which is at the feedpoint at about 45 feet. its totally supported by a maple tree.  The total run of Coax is 100'.   25' out of the shack to the common grounding buss, and 75'  up to the Balun.  This will make the wife happy because a Robin perches (sideways) on the coax to get under our porch to feed the babies. :-)

Shorting up the coax and moving the balun directly overhead won't work because of the weight of the balun and coax with no support.  

I was planning on letting the skywire free float inside the dog bone insulators and just adjusting the shape of the loop by adding tension to the support lines.  currently I have a lot of play in them because of wind and storms. 

thanks.
« Last Edit: May 11, 2013, 01:51:17 PM by WALTERB » Logged
WALTERB
Member

Posts: 528




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« Reply #20 on: May 11, 2013, 07:13:07 PM »

If I'm going with RG-213,  and I have this balun, can I use it?
http://www.balundesigns.com/servlet/the-78/4-cln-1-dual-core-current/Detail

thanks.
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KE3WD
Member

Posts: 5694




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« Reply #21 on: May 12, 2013, 12:02:15 AM »

Balanced Line, made of two coaxes, you won't need an exterior balun.  You will need one in the shack, of course, mine is internal to the big manual tuner. 

And there are other benefits to enjoy from doing it that way as well.  

Stop heating feedline and start using more of the antenna, man.  


73
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WB6BYU
Member

Posts: 12976




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« Reply #22 on: May 12, 2013, 12:19:47 PM »

Quote from: WALTERB

If I'm going with RG-213,  and I have this balun, can I use it?



I don't see why not.  You'll want to tie the wires off to a separate insulator to take
the strain, then connect the ends of the wires to the bolts so there is no mechanical
strain on the connections.  And you'll want the support rope or mast to take the weight
of the balun (and the coax hanging down from it).  In fact, providing strain relief on the
coax connector would be a good idea in any case.

Balanced coax feed would have at least as much loss as the single coax with an external
balun.
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WALTERB
Member

Posts: 528




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« Reply #23 on: May 12, 2013, 01:44:49 PM »

Quote from: WALTERB

If I'm going with RG-213,  and I have this balun, can I use it?



I don't see why not.  You'll want to tie the wires off to a separate insulator to take
the strain, then connect the ends of the wires to the bolts so there is no mechanical
strain on the connections.  And you'll want the support rope or mast to take the weight
of the balun (and the coax hanging down from it).  In fact, providing strain relief on the
coax connector would be a good idea in any case.

Balanced coax feed would have at least as much loss as the single coax with an external
balun.

Thanks so much your help.

The horizontal skywire loop is operational!

This morning I hooked up 280' of 12 gauge wire and let it float between 3 dog bone insulators in a diamond shape about 45+ feet in the back yard. I used the 4:1 balun we discussed and it seems to tune fine with my Kenwood TS-590s.  I can tune 80, 40,20, 15, and 10 meters with an SWR 1:1.  Whoo hoo!

Way too early to tell how well its working.  On 20 watts, 15 meters using JT65 I've made contacts in New York, Ukraine, Spain and Brazil only using it for about 30 minutes.
I'll have to wait until the sun goes down to see how it performs on 20 meters (my main band) tonight.
It does seem to "hear" better than the OCF Dipole, but that was expected as was it not being able to tune on 160 meters, which it doesn't.  No problem. I believe I have never made a contact on 160 anyway. :-)

I'm going to have to tweak the support cords a bit.  They are too tight right now for a good storm, and I want to make sure the wire floats correctly.

thanks.
« Last Edit: May 12, 2013, 01:50:51 PM by WALTERB » Logged
WB6BYU
Member

Posts: 12976




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« Reply #24 on: May 12, 2013, 03:04:18 PM »

The real question is, what is the SWR without the tuner?
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WALTERB
Member

Posts: 528




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« Reply #25 on: May 12, 2013, 05:58:19 PM »

The real question is, what is the SWR without the tuner?

here are the results

10m 1:3
20m 1:1  (whoo hoo!)
15m 1:2
40m 1:4
80m 1:6

I would have thought 80m would have been closer to 1:1?

thanks
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K8POS
Member

Posts: 332


WWW

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« Reply #26 on: May 12, 2013, 06:00:35 PM »

Walter,
Congrats, glad to hear it is working better for you.  
If you are able, I would sweep the various bands with an swr meter to see where you can work without the tuner.
I was surprised that even being 100% coax fed how good my swr was on some of the bands.
That is a good Balun you got, first time around I bought a cheap one and the internal connections came apart after just a few months use.
I now run a Balun designs as well.
Give it time to see if it is better than your OCF dipole.  Conditions can make a great antenna bad, and a bad antenna good.

Bob
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KE3WD
Member

Posts: 5694




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« Reply #27 on: May 12, 2013, 06:55:05 PM »

The real question is, what is the SWR without the tuner?

here are the results

10m 1:3
20m 1:1  (whoo hoo!)
15m 1:2
40m 1:4
80m 1:6

I would have thought 80m would have been closer to 1:1?

thanks

Man, every one of 'em is Fine Bidness to me! 

73
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WB6BYU
Member

Posts: 12976




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« Reply #28 on: May 12, 2013, 07:41:32 PM »

Quote from: WALTERB

here are the results

10m 1:3
20m 1:1  (whoo hoo!)
15m 1:2
40m 1:4
80m 1:6

I would have thought 80m would have been closer to 1:1?




SWR is defined to be a ratio of a number greater than 1 to 1.  Reversing the numbers leads
to confusion.  So, for example, the 80m number should be written as 6 : 1, otherwise it
will look like 1.6 : 1 to those who are expecting the proper values.

The SWR of 1 : 1 on 20m is pretty close to what I expected (the model predicted 1.2 : 1,
but having to guess at the actual height and the ground conditions, both of which will
have some impact on the actual values, as well as assuming in ideal balun.)

On 80m it makes a big difference where in the band you take the measurement.  To get the
resonance to align on 20m you generally need to tune the loop to around 3.55 MHz, which
will cause a higher SWR up in the phone segment.  I would expect close to 3 : 1 at resonance
on 80m:  typically the feedpoint impedance is around 90 ohms, depending on height
above ground.)
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WALTERB
Member

Posts: 528




Ignore
« Reply #29 on: May 12, 2013, 09:55:43 PM »

Quote from: WALTERB

here are the results

10m 1:3
20m 1:1  (whoo hoo!)
15m 1:2
40m 1:4
80m 1:6

I would have thought 80m would have been closer to 1:1?




SWR is defined to be a ratio of a number greater than 1 to 1.  Reversing the numbers leads
to confusion.  So, for example, the 80m number should be written as 6 : 1, otherwise it
will look like 1.6 : 1 to those who are expecting the proper values.

The SWR of 1 : 1 on 20m is pretty close to what I expected (the model predicted 1.2 : 1,
but having to guess at the actual height and the ground conditions, both of which will
have some impact on the actual values, as well as assuming in ideal balun.)

On 80m it makes a big difference where in the band you take the measurement.  To get the
resonance to align on 20m you generally need to tune the loop to around 3.55 MHz, which
will cause a higher SWR up in the phone segment.  I would expect close to 3 : 1 at resonance
on 80m:  typically the feedpoint impedance is around 90 ohms, depending on height
above ground.)

thanks.

I'm being Heard on six continents at the same time using JT65 at 20 watts.  So I'm a happy camper.. :-)
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