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Author Topic: Vertical-Dipole Feed Line(s)?  (Read 5269 times)
WB6BYU
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Posts: 13113




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« Reply #15 on: July 19, 2013, 09:39:38 AM »

Quote from: K7RNO

That, and the fact that an HVD shows to have a very favorable (i.e., low) radiation angle (see pp 2-11 and 9-10 of 22nd ed. Antenna Book), was what attracted me to that solution.



Basically any vertically polarized antenna will have a similar pattern.  If you are really talking
about a half wave vertical dipole, then feeding it at the bottom against ground (end-fed
half wave
) will have exactly the same pattern, since the current distribution along the wire
is the same regardless of where you feed it.

However, if you put up a 40m dipole and want to use it on 20m, then feeding it at the bottom
does NOT give the same pattern as in the center, because the current distribution is different.
However, if the wire slopes at around 45 degrees then it should work better on 20m, though
there will be a bit of directivity to the pattern.

But whether the low angle radiation is actually usable depends on the ground characteristics -
see the next answer.



Quote

BTW, being close to the lake, I would expect a high water table under the surface and therefore a very good ground. However, I am not sensing a lot of enthusiasm here and wonder why that is.



Fresh water isn't a good conductor, though it isn't as lossy as some ground types.  If the
lake is fresh water it won't be nearly as good as the Great Salt Lake for low angle radiation
with vertical polarization.  And if it is salt water, you probably won't have a high water table
in an area with tall trees because they couldn't survive with the salt on their roots.

Even with salt water, the antenna needs to be pretty close to the edge of the water to get
optimum low angle radiation.  A couple hundred feet isn't close enough:  I use the guideline
that the distance from the edge of the water can't be more than one to two times the
height of the point of maximum current on the antenna above the water.  You can also think
of this as you would need to see the water looking down from the point of maximum current
at a 30 to 45 degree angle.  Often that means that, if your antenna isn't getting wet from
the spray, it isn't close enough.



Quote

If you (you in general) now say "go ahead and try that HVD", then I am back to square one with my ingoing question on how to design the feed line: Ladder line > ? balun ? > coax > [ATU/TRX] ?



Then it becomes a matter of choice.  The easiest thing, in my experience, is to build
a half wave dipole fed with coax. You can then string it up vertically or sloping, with
the coax running off over a tree branch at an angle then down to the rig.  I tuned up
my backpacking dipole kit ~35 years ago and have used it in many different configurations
since then (vertical, horizontal, sloping, inverted vee, etc.) without a tuner.  The
disadvantage of this approach is that you can't use a single wire on both 20m and 40m
without adding traps.  A balun at the feedpoint makes performance more predictable,
though I don't use one on my kit to save weight.

You will get a bit more gain on 20m if you use a 40m doublet fed with twinlead or ladder
line to a tuner, providing both bands on a single wire.  In that case, run the feedline
to a balanced tuner sitting beside you.  Again, such an antenna can be installed in several
different ways.


Remember, THERE IS NO ONE "RIGHT" ANSWER!  It is a matter of personal preference - so
you have to make the ultimate decision of what works best in your situation.


You certainly can try the 20m whip.  You can also put a 40m quarter wave wire and tune
it with a tuner on 20m, where it will be an end-fed half wave.  (If your tuner doesn't like
high impedances for some reason, lengthen the wire to 40' and use the tuner on both bands.)
You'll want radials with that, of course, just as with the 20m whip.
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K7RNO
Member

Posts: 279




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« Reply #16 on: July 19, 2013, 09:57:10 AM »

  You can also put a 40m quarter wave wire and tune
it with a tuner on 20m, where it will be an end-fed half wave.  (If your tuner doesn't like
high impedances for some reason, lengthen the wire to 40' and use the tuner on both bands.)
You'll want radials with that, of course, just as with the 20m whip.

That looks intriguing. I would put four radials and they would be sloping. How long should they be for this, say, 40' wire?
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73,
aRNO
NAQCC #6870, SKCC #11131
WB6BYU
Member

Posts: 13113




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« Reply #17 on: July 19, 2013, 09:59:33 AM »

Some should be 1/4 wave on 20m (say 17') and others 1/4 wave on 40m (about 33').

That assumes they are elevated above the ground - perhaps running along the top of
a hedge or strung between branches.
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K7RNO
Member

Posts: 279




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« Reply #18 on: July 19, 2013, 10:17:08 AM »

Okay, all that sounds feasible, Dale. I could build something like that.

I would also want to have a ZS6BKW dipole and consider hanging it as an inverted V.
If I interpret the available info on it correctly, I would have the prescribed length of 450 Ohm ladder line PLUS 70' of coax. That would actually help me finding a place in the shade  Grin

Am I missing something that I should also do with these two antennae?

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73,
aRNO
NAQCC #6870, SKCC #11131
WB6BYU
Member

Posts: 13113




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« Reply #19 on: July 19, 2013, 10:28:04 AM »

Quote from: K7RNO

Am I missing something that I should also do with these two antennae?



Have fun.
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K7RNO
Member

Posts: 279




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« Reply #20 on: July 19, 2013, 10:28:59 AM »

Sounds good!

Phew!
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73,
aRNO
NAQCC #6870, SKCC #11131
K5LXP
Member

Posts: 4465


WWW

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« Reply #21 on: July 19, 2013, 10:42:49 AM »

To utilize that branch 90' up in  the tree, I can only use a vertical antenna. At 90' up there I'd be in the thicket of all the other branches of the tree and cannot go horizontal or might not even get into an inverted V with an acceptable spread angle.

Since I've never been to the location you're proposing I can only offer generalities, but here in the mountains I can shoot a line some distance up a ~100ft ponderosa, then into another tree some distance away and raise a dipole right up using the coax as a guide line to pull it back and forth as I hoist to avoid any interfering branches.  Sometimes you can't get it all the way to the top, or it's a bit at an angle or crooked.  It really doesn't matter, you still have a efficient antenna well clear of the ground.  It's going to work great.

Granted, an end fed obviates the need for a 2nd support but it has the pattern issue WB6BYU pointed out if you intend to use it on multiple bands.  If you think getting a dipole up through the branches is a hassle, just wait until you try a vertical groundplane and try to get all the radials tied off.   A vertical dipole is somewhere in middle since you have the perpendicular coax feed issue to deal with.

Something to consider is if the multiband issue is hanging you up, consider the idea of just using 2 antennas, say 2 end feds.  You may be able to launch two end feds faster and easier than one multiband dipole.

Quote
Am I missing something that I should also do with these two antennae?

Yes.  They will be much harder to deploy because they're almost twice as long as a half wave dipole would be.  They also require a tuner but if the one built into the rig will handle it then that may not be a big deal.  I consider the pattern these antennas exhibit to be a disadvantage when operating portable.

We could come up with all sorts of contingencies and configurations but I think a lot of your over analysis stems from your lack of practical experience in the field.  I am convinced that once you do this a few times you'll figure out that simpler is better and not to sweat the details too much.


Mark K5LXP
Albuquerque, NM


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K7RNO
Member

Posts: 279




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« Reply #22 on: July 19, 2013, 11:00:08 AM »

We could come up with all sorts of contingencies and configurations but I think a lot of your over analysis stems from your lack of practical experience in the field.  I am convinced that once you do this a few times you'll figure out that simpler is better and not to sweat the details too much.

I have a hunch you are hitting a nail here. Right on the head. My quest for perfection is often in the way of a quick sub-optimal but still very functioning solution. Oh well, I'll be learning more from this for sure.

Working on an order for some cables and connectors now...
« Last Edit: July 19, 2013, 11:02:51 AM by K7RNO » Logged

73,
aRNO
NAQCC #6870, SKCC #11131
K5LXP
Member

Posts: 4465


WWW

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« Reply #23 on: July 19, 2013, 12:49:52 PM »

I would add that your "field kit" can contain all the above - single band dipoles, balanced line doublets, end feds and whatever else ends up working for you.  I carry several wire antenna configurations in my QRP portable kit, and they all fit in a ziploc sandwich bag with the whole station weighing in at 12 ounces.  Wire is cheap and having an assortment of antenna "tools" in the toolbox comes in handy during situations you didn't anticipate.


Mark K5LXP
Albuquerque, NM
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KI5FJ
Member

Posts: 39




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« Reply #24 on: July 19, 2013, 05:32:59 PM »

I suggest you consider both a vertical and a horizontal loop.
A 85 ft vertical radiating wire and approximately 4 sloping elevated radials will be a 5/8 wl on 40-Meters.
Feed with open line to just outside the shack. Transition with a current balun to less than 25 ft of low loss coax.
A manual ATU should make the match. Great on 80 and outstanding on 40-M.
A 80-M loop up at least 30 Ft will have directivity gain 30-M and above. Again open line feed.
Email me at QRZ address for more info. 73 Joe O, NNNN
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K7RNO
Member

Posts: 279




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« Reply #25 on: July 19, 2013, 06:16:53 PM »

Thank you, Joe, and I need to be staying within a portable solution.
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73,
aRNO
NAQCC #6870, SKCC #11131
ZL3OF
Member

Posts: 20




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« Reply #26 on: July 19, 2013, 10:10:54 PM »

I would also want to have a ZS6BKW dipole and consider hanging it as an inverted V.
If I interpret the available info on it correctly, I would have the prescribed length of 450 Ohm ladder line PLUS 70' of coax.

Don't get too focused on the 70 ft of co-ax that's often specified. My home brew ZS6BKW, set up as an inverted vee, only has 4 metres (about 13 ft) of coax and 1:1 choke balun and works just fine on the bands it's designed for.
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WB6BYU
Member

Posts: 13113




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« Reply #27 on: July 20, 2013, 07:27:36 AM »

And don't over-estimate the heights you are likely to reach with the antenna.  Yes, I can
toss a weight over a branch at 60+ feet, but for casual portable operations I usually find
that 20 to 30 feet is more typical for most setups.  (If I get it higher than that, I raise my
dipole by one leg so it is more vertical.)  Unless you have ideal trees, difficult increases with
the square of height above some point (especially if the park frowns on slingshots and spud
guns.)

I save the tall, large antennas for Field Day (and, yes, I've put up a full wave 80m loop with
ropes up 60' in some corners.  But unless conditions are ideal, something like an inverted vee
at 20' is more usual for a couple hours of operation.
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K7RNO
Member

Posts: 279




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« Reply #28 on: July 20, 2013, 09:23:28 AM »

I am aware of the issues of which you speak, Dale. At this point, I have spent more time searching out ideal trees and locations than actually being on the air waves. Just yesterday, after ordering the cables, I made another run and spotted two more trees that will allow high dipoles (^). They are away from all the picnic tables, where the hordes of people (and kids!) gather (good for deploying the slingshot and the tie-offs, bad for hooking up to the car battery).

I do have a question regarding the dipole design. Somewhere (Antenna Book?) I read about a recommendation to put a 10kOhm resistor between the two 1/4 waves in order to have a benchmark when measuring continuity of the feed line in case of issues. That way, one can tell for sure if the 10K are still present (A-OK) or whether it is indefinite or somewhere in between. Is this trick still a good one, and if so, what wattage do I need to have for the resistor, with a maximum of 10W output from the TrX? In my (portable) situation where I will have to setup and break down the antenna every time I want to go on air, it will be subjected to more stress than one put up stationary, so I might need such an indicator down the line (pun not intended).

Thanks and 73,
Arno
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73,
aRNO
NAQCC #6870, SKCC #11131
K5LXP
Member

Posts: 4465


WWW

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« Reply #29 on: July 20, 2013, 11:04:58 AM »

I read about a recommendation to put a 10kOhm resistor between the two 1/4 waves in order to have a benchmark when measuring continuity of the feed line in case of issues.

I've done that with a few different antennas, "marking" each with a 10K, 12K, 15K, etc resistor to allow remote identification.  The other benefit it may provide is to dissipate any static potential between the two elements.


Quote
Is this trick still a good one, and if so, what wattage do I need to have for the resistor

Quarter watt is plenty for a 10W station.  This only works for 50 ohm antennas.  If you try this with a high feed Z antenna the voltage present will cause the resistor to dissipate more.


Mark K5LXP
Albuquerque, NM

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