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Author Topic: Considering an inverted V in a confined space  (Read 566 times)

KK4GMU

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Considering an inverted V in a confined space
« on: January 05, 2022, 01:22:52 PM »

I have a 10' by 25' back yard surrounded by a 4' high chain link fence.

I am considering an inverted V about 10' high in the center with the ends at about 4 or 5' high, with each leg about 10' long. I would have about a 120 to 130 degree apex angle.

The antenna would be parallel to the house, about 8' away and 1 foot from the fence.

1. Is the antenna performance likely to be better further from the 12' tall house than from the 4' tall fence?

2.  What is the closet any element of the antenna should be to the chain link fence without destroying or severely compromising the signal?

3. Which bands will perform best?  6 and 10 meter?

4. How much of a positive impact would an antenna tuner such as the MFJ-939I have in working other bands?

5. What type of antenna is likely to work better than an inverted V given my constraints?
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K5LXP

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Re: Considering an inverted V in a confined space
« Reply #1 on: January 05, 2022, 01:42:47 PM »

The impact to performance is relative.  Even without the fence and the house, the antenna is compromised in height and length.  Throw in more environmental variables and you're on a slippery slope trying to achieve least worst.  The performance goals also plays a part, whether you're shooting for DX or high contest scores vs just being heard by anyone/anywhere.

All of your options are not great and I would offer it makes little difference which one you pick.  Just hang the thing and you can play around with moving the ends and center around but I doubt it will fundamentally change the result.  As you go higher in frequency indeed the efficiency will go up, but if those bands aren't open then it doesn't matter how well dressed you are.  There are a few bands with propagation like 20 and 17 where an antenna this size will have a useful degree of efficiency so you will definitely get some result with this setup.  It's assumed you are limited to wire antennas, as the obvious answer to what is "better" is a different antenna on the top of the house or a tower with a beam.

Mark K5LXP
Albuquerque, NM
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K0UA

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Re: Considering an inverted V in a confined space
« Reply #2 on: January 05, 2022, 01:42:56 PM »

Quote
5. What type of antenna is likely to work better than an inverted V given my constraints?

A multiband vertical.  Most users of the so called "DX commander" report good results.
I built a version of that out in the woods and it works very well.  You will need radials, either laid on the grass (they disappear after a couple of years) or elevated radials. A commercial trap vertical will work well too, but I am not a huge fan of traps.

Yes your horizontal wire antennas can work on the upper bands. They will not work all that well at such a low height. Using them even at 20 meters with a tuner will not be great, and anything lower will really suck. Another solution is a mobile screwdriver antenna. You will still need radials, and will be a far cry from full size antennas, but It can work multiple bands with some efficiency especially if you use one of the larger ones.
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73  James K0UA

N8CBX

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Re: Considering an inverted V in a confined space
« Reply #3 on: January 05, 2022, 02:39:58 PM »

I agree with the inverted V dipole, great antenna, and try to get it up as high as possible.
I use a vertical antenna for only TX on 80M and receive on a 40M dipole. Dipole is much quieter.
Jan N8CBX
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W1VT

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Re: Considering an inverted V in a confined space
« Reply #4 on: January 05, 2022, 03:57:57 PM »

It is really hard to make contacts with a low dipole, but it can be done.
I worked KV4AA with a helical 20M dipole just six feet long when he was the best OP in the world.  1st floor dorm in Philly. 4 watts.
FG Guadalupe on 12M CW 5 watts.  Indoor dipole strung across room, perhaps 10 ft off the ground.

15M dipole 10 ft high Sand Island next to Pearl Harbor Oahu. 4 watts. Dozens of CW contacts in the ARRL FD operating event.

Zak W1VT
« Last Edit: January 05, 2022, 04:00:37 PM by W1VT »
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KG6PHS

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Re: Considering an inverted V in a confined space
« Reply #5 on: January 05, 2022, 06:52:14 PM »

It would be much better if you could raise the antenna to 20 feet or more, and the ends to 5 or 6 feet off the ground or more. I have a 40m inverted vee dipole and an 80m half sloper which are mounted to my VHF antenna pole at 35 feet. If you had a flagpole or tree, you could mount the antenna to one of them maybe. You can still make contacts at the lower height, but they will be more local. You have to be careful with RF radiation at that height too.
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KH6AQ

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Re: Considering an inverted V in a confined space
« Reply #6 on: January 05, 2022, 06:53:13 PM »

I ran an EZNEC model of your inverted-vee with and without the chainlink fence. At 20.8 MHz, the resonant frequency of the antenna, mounting it directly over the fence looks good. Little current is induced in the fence top rail with almost no current in the vertical posts. The gain shows a 0.2 dB change. I would mount it over the fence to minimize RFI to and from electronic devices in the house.

Without an antenna tuner it is a single band antenna and can be tuned to the 15 meter band. The internal antenna tuner in your transceivet (if it has one) will not have the tuning range to tune this on the other bands.

It should work well on 17 meters and above when fed with ladder line to an antenna tuner. It might also work well enough on 20 meters. It can also be fed with a short length of RG-8 to an antenna tuner. A 1:1 feedline choke should be placed at the antenna feedpoint.

The MFJ-939I ATU has tuning range of 6-1600 ohms and stands a chance of tuning this thru coax on 20-10 meters. With ladder line I would place a 1:1 choke at the MFJ-939I output.

MFJ-939I    https://mfjenterprises.com/products/mfj-939i
« Last Edit: January 05, 2022, 06:58:58 PM by KH6AQ »
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KK4GMU

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Re: Considering an inverted V in a confined space
« Reply #7 on: January 06, 2022, 06:50:12 AM »

It would be much better if you could raise the antenna to 20 feet or more, and the ends to 5 or 6 feet off the ground or more. I have a 40m inverted vee dipole and an 80m half sloper which are mounted to my VHF antenna pole at 35 feet. If you had a flagpole or tree, you could mount the antenna to one of them maybe. You can still make contacts at the lower height, but they will be more local. You have to be careful with RF radiation at that height too.
Sadly, no available trees, and I am probably pushing the height envelope at 10 to 12'.

I am thinking about a PVC sleeve-type arrangement where the fixed, lowest height would be 8' and I could raise the smaller PVC to 15' or so - maybe a third PVC to raise another 6' when in use.  I'm still working this through.

By the way, for the center pole, would a non-conductive PVC be better than aluminum, which is the material of the Harbor Freight sleeve-type flagpole for around $60? I thought it best to stay away from conductive material for the center pole.
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WB6BYU

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Re: Considering an inverted V in a confined space
« Reply #8 on: January 06, 2022, 08:54:21 AM »

Quote from: KK4GMU

...I am thinking about a PVC sleeve-type arrangement where the fixed, lowest height would be 8' and I could raise the smaller PVC to 15' or so - maybe a third PVC to raise another 6' when in use.  I'm still working this through.



I have used PVC for antenna supports, but I generally
don’t think it is a good solution.  It is relatively heavy
and bendy.   Black ABS pipe is lighter and stiffer - stop
by your local store and try holding each type of pipe
horizontal from one end, and you will see what I mean.

ABS has a minimum size of 1 1/4 or 1 1/2 inch.  It will
bow, however, if you try to tighten the guy ropes too
tight.

Fiberglass is a better solution.  There are any number
of fiberglass fishing poles sold for this purpose, with
popular heights of 20’ to 33’.  (SpiderPole sells heavy
duty versions up to 80’.)  The top section is often
too wimpy to support an antenna, so don’t plan for
the full advertised height.

They are, however, useful for wire verticals, in which
case the full height can be used.



Antenna choice will depend on what bands are of
particular interest to you, and whether you can
bend the ends of the wires around the corners
of your property.  With a bit of creativity, you might
fit a 33’ dipole for 20m into that space, or even
a coil-loaded dipole for 40m (which could also
work 10m).  But, unless you feed tha antenna
with twinlead or ladderline instead of coax, the
performance won’t be very good on other bands.
(A 15m dipole probably wouldn’t be too bad on
17m or 12m, but will degrade much beyond that.)


At some point, you just have to put up something
and experiment.

If your budget will stretch that far, consider the
telescoping masts from Max-Gain Systems
(www,mgs4u.com).  These ate much stronger,
and have thumb latches to lock the sections
rather than friction fit.  I have one that I take
out and strap to the back fence, that goes up
26’, and you have the options of section
lengths and diameters to go up to 50’.

MFJ sells similar masts for a similar price,
and if you have a local ham radio store you
might be able to get it there and save shipping.

One of the common options is a painter’s pole,
but make sure you can raise the top section
with the bottom section vertical.  The taller ones
use 8’ sections, which may require a ladder.

My favorite is in the window washing section of
Home Depot - the Unger 24’ telescopic pole,
UPC 61475-97298.  It is aluminum with a
triangular cross section, and has thumb locks
on the sections instead of twist locks.  There
are numerous other telescoping handles of
varying sizes - I also have a 15’ one for
changing light bulbs.  Probably the best
deal was a red and green 13’ pole designed
for stringing Christmas lights, that was on
super sale on 27 December.

Some of these may not be ideal for a
permanent installation, but probably will
work better than a home-made telescoping
PVC mast.


Now, there are some other considerations
to increasing the height.  It means that the
angle in the center of the antenna gets
smaller, unless you can also elevate the
ends.  With 20’ of width, the feedpoint can
only be 10’ higher than the end anchors
for a 90 degree angle.  Yes, inverted vees
can work with sharper angles, but you are
approaching the area of diminishing returns.



Quote

...I thought it best to stay away from conductive material for the center pole...



A lot of hams with steel towers hang inverted vees
from them.

There can be some interaction right at the feedpoint
that can shift the tuning a bit.  You can always add
a foot or two of PVC between the top of the mast and
the feedpoint if you want.  (It is fine for short lengths.)
I sometimes use a wood spacer at the top to move
the feedpoint away from a metal mast a little bit,
but I don’t think it really makes much difference.

Some of the telescoping poles don’t make good
electrical contact between the sections, so they
don’t act like a long piece of metal.

KK4GMU

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Re: Considering an inverted V in a confined space
« Reply #9 on: January 06, 2022, 09:32:23 AM »

I have used PVC for antenna supports, but I generally
don’t think it is a good solution.  It is relatively heavy
and bendy.   Black ABS pipe is lighter and stiffer - stop
by your local store and try holding each type of pipe
horizontal from one end, and you will see what I mean.

Fiberglass is a better solution.  There are any number
of fiberglass fishing poles sold for this purpose, with
popular heights of 20’ to 33’.  (SpiderPole sells heavy
duty versions up to 80’.)  The top section is often
too wimpy to support an antenna, so don’t plan for
the full advertised height.

My favorite is in the window washing section of
Home Depot - the Unger 24’ telescopic pole,
UPC 61475-97298.
Thanks for the suggestion for alternative pole materials.  Black would be good.  I was going to paint the white PVC a dark green or black to reduce its visual prominence, but the already black pipe would be perfect.

Height:  Yes, I will assure my height will make the angle at the apex no less than 90 degrees.  More degrees is better.

I'm currently studying for my General.  So these "antenna exercises" may be good in helping answer a couple of test questions. I'm at least a couple months out from actually using the frequencies this antenna will be designed for.

For my conditions do you think the SWR/tuner in the Yeasu 991A in combination with the MFJ-939 auto tuner would be adequate for helping design this antenna?  Or is there something besides or in addition to the MRJ-939 that would be more helpful?
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WB6BYU

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Re: Considering an inverted V in a confined space
« Reply #10 on: January 06, 2022, 06:10:15 PM »

Quote from: KK4GMU

For my conditions do you think the SWR/tuner in the Yeasu 991A in combination with the MFJ-939 auto tuner would be adequate for helping design this antenna?  Or is there something besides or in addition to the MRJ-939 that would be more helpful?




That's hard to know, without knowing what your interests are.

If I model the antenna (as an approximation, at least, since I might not
have all the details correct), the SWR on 50 ohm coax is under 10 : 1
between about 18 and 22.5 MHz.  It's a reasonable guess that you could
use it on 17m and 15m without too much loss in the coax, as long as
the coax wasn't too long (or too small).  You can move that range by
adjusting the length of the wires, but clearly there will be at most 2 or
3 bands (possibly 15m / 12m / 10m?) where such an antenna will work
well if using the tuner in the shack through coax.

Note, however, that the matching range of the MFJ-939 is specified as
6 - 1600 ohms.  So while the top end of the range is an SWR of 32 : 1,
the bottom end is less than 10 : 1.  (And that doesn't consider reactive
loads.)  While the SWR will be somewhat lower in the shack than at
the antenna due to cable losses, a 10 : 1 limit still seems reasonable.

So then we need to consider which bands you are actually going to
want to operate.  As the sunspots come back, activity is picking up
somewhat on 17m and 15m, and even 10m to some extent.  But it
still is comparatively sparse, and you could be waiting for a while
for an opening to someplace.  (Somewhat different using FT8.)
Not that those bands can't be really hopping when we get good
ionospheric conditions, but we aren't there quite yet.

So you may find it rather quiet and lonely if those are the only bands
you have available, especially if you work during the day and want
to operate during the evenings.

One option is to feed the antenna with twinlead or ladder line,
which reduces the feedline loss.  With that, you might stretch
performance down to 20m, although whether or not the tuner
will match it will depend on the length of the feedline you use.
With some creativity, you should at least be able to cover 20m
through 10m.  To do that, you would want an effective external
balun to connect between your tuner and the balanced feedline.

Ideally you would have a balun at the feedpoint of the antenna
anyway - with the balanced line you just move it to the tuner
output instead.  Such lines require more care in routing, as you
can't tape them to a metal mast or bury them in the lawn.


Now, another option for wider band coverage is something that
you can change on the to shift the resonant frequency.  For
example, you could use a loading coil in each wire, with tap points
you can select for each band.  That isn't necessarily a good permanent
solution, but will be more efficient than trying to just use a tuner to
force it on different frequencies.  You'll need to experiment, but
it would be possible to get it down to 40m that way, although the
bandwidth will be narrow, even using the tuner.  In that case,
to change bands you would lower the antenna until you can reach
the coils, move the clips or plugs, then raise it up again.  That
can go pretty quickly if you have a telescoping mast, or if you use
a halyard and pulley to hang the antenna on the mast, so you can
leave the mast in place and just untie the rope to lower the antenna.

It would require some experimentation to find the right tap points,
but that gives you another idea of what is possible.

You can use a similar approach with a wire vertical hanging from
a fishing pole, but do all your adjustments at ground level (or some
other convenient height) so you don't need to lower the antenna.
It would also be less noticeable, because you wouldn't have the
lumps (coils) in the wires.

If you wanted to get more adventuresome, you could put up a
loop antenna hanging from the tip of the pole, down to some
sort of tuning system near the base.  That would look rather like
your proposed dipole wires, but with the ends of the wires bent
around and stretched back to the mast.  Exact shape isn't
critical.   This still requires some design work to find a good
combination of wire length and manual adjustment, but you
would only need an approximate correction at the antenna,
enough to get it within the matching range of the tuner in the
shack.


By the time you sort through and understand all the options,
you may be in good shape for that part of the general test
at any rate!

KK4GMU

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Re: Considering an inverted V in a confined space
« Reply #11 on: January 06, 2022, 07:01:53 PM »


By the time you sort through and understand all the options,
you may be in good shape for that part of the general test
at any rate!
Absolutely.  I'm also seeking out an Elmer in my neck of the woods so I can spread my incessant questioning harassment around a bit.

For $50 or so more than the MFJ-939 auto tuner is a model that has both balanced (ladder line) and coax inputs and includes a 10:1 balun:  https://mfjenterprises.com/products/mfj-993b?_pos=8&_sid=60670f61f&_ss=r 

That may give me more options after I get smarter about what I want to do.  At the moment it appears I'm confined to the lower bands below 20 above 14MHz.  I'd like to stretch it to decent performance in 20 meters.

What I'd like to eventually try is NVIS. Unfortunately, the Near Vertical Incident Skywave works primarily on 60 meters or longer.  I'm not sure how those old jeeps with the 15' long antenna stretched forward reached those frequencies to bounce back to their target 100 to 300 miles away at those bandwidths - I guess they acted like a cross between a vertical and a horizontal endfed that has unique properties.
« Last Edit: January 06, 2022, 07:06:14 PM by KK4GMU »
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KH6AQ

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Re: Considering an inverted V in a confined space
« Reply #12 on: January 06, 2022, 07:42:52 PM »

I haven't read all the posts and am responding to your last one. You'd like good performance on 20-10 meters and are interested in the MFJ-993B.

Extending your inverted-vee to 26' total will work. One way is with PVC or fiberglass end supports 4' above the fence and then run wires 3' long down the end supports.

The MFJ-993B might tune the antenna on 20, 17, 15, 12 and 10 meters.

At this part of solar cycle 25 the bands most useful to you will be 20, 17 and 15 meters. SSB at 100W will work. CW would be much more productive and with FT-8 you probably can work the world.
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WB6BYU

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Re: Considering an inverted V in a confined space
« Reply #13 on: January 06, 2022, 07:47:13 PM »

Quote from: KK4GMU

...What I'd like to eventually try is NVIS...



You can even work 40m on NVIS, at least during the day, when
we have sunspots.  But you may need a larger antenna,
like a loop around your house under the eaves.

The military vehicles use a tuner at the antenna feedpoint
that is specifically designed to match antennas that short.
You can build your own, or use a wide-range tuner, and
get it to work (to some extent), but the voltages and
currents are higher in the tuner.  Here is an example of
using an antenna about half the size you are proposing
on 40m, 80m and even 160m.  But notice that he built
separate tuners for 80m and 160m to handle the impedances.

It can be done, it just requires more work.

KF4ZGZ

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Re: Considering an inverted V in a confined space
« Reply #14 on: January 07, 2022, 03:15:44 AM »

For your situation I would use a metal pole, as high as possible, attached to the house to one end of the yard.
Then I would build a top loaded half-sloper. A coil loaded sloping antenna with the coil at the top.
Google will give you a few variations. You can check them out and decide which is best for you.
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Matt
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