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Author Topic: EFHW or inverted V or ...? (space-limited QTH)  (Read 1568 times)

HS0ZIB

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EFHW or inverted V or ...? (space-limited QTH)
« on: August 20, 2019, 07:21:21 PM »

My new QTH is rather restrictive for a decent antenna (not a rule, just lack of space). The house sits on a plot which is barely bigger than the house footprint.

I initially intend to only operate on 20 meters.  I have built an EFHW antenna with 1:49 toroid, got this all matched on my operating band and hoisted the wire antenna taped to a fiberglass mast to a maximum height at the tip of about 14 meters. Performance is 'OK', but I'd like to do better.

I wonder if an inverted V antenna would be perform better, supported from the tip of the fiberglass mast, since more of the antenna would be higher than the EFHW (whose feedpoint is about 4 meters above the ground).

What about a 20 meter band vertical Moxon?  It might be possible to construct one using lightweight bamboo poles that could allow me to manually rotate it.

Finally, perhaps a hexbeam antenna, but the problem with that is actually raising it through the palm tree leaves that grow all around the house (pushing up a vertical mast through these large leaves is much easier).

I'm open to suggestions :)
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KH6AQ

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #1 on: August 21, 2019, 10:28:56 AM »

If I understand your installation you essentially have a half wavelength vertical dipole. The simulated gain at a take-off-angle of 5 degrees (medium ground) is -1.6 dBi. An inverted-vee with apex at 14 meters has a simulated broadside gain of -1.0 dBi at 5 degrees take-off-angle. A Moxon at 14 meters could have around 4 dB of gain over either antenna.
« Last Edit: August 21, 2019, 10:42:33 AM by KH6AQ »
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HS0ZIB

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #2 on: August 21, 2019, 03:55:27 PM »

Thanks for the gain analysis.  Concerning the Moxon, I have an idea to use a vertical Moxon that is supported on a single bamboo pole (it's easy to buy very long, straight bamboo poles over here!).

A single pole vertical Moxon would normally mean that a crossbar would need to be mounted at the top, center (to support the coaxial feed) and bottom of the pole, in order to support the antenna wires and to maintain the rectangular shape.

The lower 2 cross beams should mechanically be strong, since the bamboo center pole would be rigid at these lower heights.  But the top crossbeam would be secured to the thinness part of the center pole, and I guess that guy wires would be needed.

But what about this idea?

Build a Moxon where the bottom and center crossbeams are as before.  But remove the top crossbeam and instead run the antenna wires directly from the center crossbar to the tip of the bamboo pole, similar to an inverted V.  This would require an increase in the overall length of the antenna, since the wires now follow the hypotenuse of the triangular layout at the top, rather than following the two shorter sides of the triangle.

The advantage would be a reduction in wind load.  The antenna wires would also now act as guy wires to support the tip of the vertical bamboo pole.  Additionally, since those guy wires form part of the antenna itself, the whole antenna can easily be rotated without mechanical interference.  Mechanically, it would be very easy to construct this type of 'inverted V' Moxon.

How do you think this layout would affect the electrical performance of the antenna?

So the layout would look something like this:

                   .
                 . | .
                .  |  .
               .   |   .
              .    |    .
             .______.
             .     |     .
             .     |     .
             .     |     .
             .     |     .
             .______.
                   |
                   |
                   |
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AC2RY

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #3 on: August 22, 2019, 07:52:16 PM »

My new QTH is rather restrictive for a decent antenna (not a rule, just lack of space). The house sits on a plot which is barely bigger than the house footprint.

I initially intend to only operate on 20 meters.  I have built an EFHW antenna with 1:49 toroid, got this all matched on my operating band and hoisted the wire antenna taped to a fiberglass mast to a maximum height at the tip of about 14 meters. Performance is 'OK', but I'd like to do better.

I wonder if an inverted V antenna would be perform better, supported from the tip of the fiberglass mast, since more of the antenna would be higher than the EFHW (whose feedpoint is about 4 meters above the ground).

What about a 20 meter band vertical Moxon?  It might be possible to construct one using lightweight bamboo poles that could allow me to manually rotate it.

Finally, perhaps a hexbeam antenna, but the problem with that is actually raising it through the palm tree leaves that grow all around the house (pushing up a vertical mast through these large leaves is much easier).

I'm open to suggestions :)

Inverted V still needs space. You may try Hexbeam - that would fit over the house, but you will need a mean to rotate it.
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HS0ZIB

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #4 on: August 23, 2019, 04:08:39 AM »

@KH6AQ, thanks very much again.

I wanted to ask about feeding a standard Moxon.  Usually it's driven at the center of the dipole element.  But I found an example where it's driven at the lower end of the dipole wire.

I currently have an EFHW antenna for 20 meters, which is driven via a 1:49 ratio toroid match and coaxial stub capacitor.  The match is very good :)

Am I correct in thinking that the input impedance of an end-fed Moxon will be the same as for a single EFHW antenna, about 3,000 ohms?  If so, then I can simply use my EFHW matching toroid unit to match to my end-fed Moxon.
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KH6AQ

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #5 on: August 23, 2019, 12:41:01 PM »

That's an interesting question.  I can see why you might want to end feed the antenna given your planned vertically oriented installation.

Fed as an EFHW it should work but the feedline, or the length of feedline between the EFHW wire and a coaxial cable current choke, will radiate thereby deteriorating the forward gain and F/B ratio. The end fed half wavelength wire must have something to work against - it needs the other half -- and that ends up being the coaxial cable shield.

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HS0ZIB

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #6 on: August 23, 2019, 04:48:46 PM »

OK, how does this look?



The top bamboo slat supports are connected to the center pole with a metal pin, allowing them to fall under gravity and held at the correct height by the reflector and dipole wires that also pass through a metal eye that's connected to the center pole at the top.

The reflector wire is installed as a normal Moxon design.

The dipole wire is an EWHW wire that is fed at the lower end via a 1:49 matching toroid and capacitance that is tuned to match the EFHW input impedance, which is about 3,000 ohms.  (Question, will the presence of the reflector wire change that input impedance?).

Finally, the choke balun is located close to the matching toroid to block RF currents on the coaxial feed-line.  The coaxial feed is earthed to a ground stake in the normal manner, as is the earth side of the matching toroid assembly.

So... with this design, the mechanical load is minimised, meaning that no guy wires are needed and it's easy to rotate the antenna.

I already built and use a matching toroid assembly for 20 meters that I tuned to match at 3,000 ohms.  Using this with a 20 meter band EFHW antenna gives me a 1:1 SWR and good performance, but I want to add some directivity to my antenna.

Comments?

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KH6AQ

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #7 on: August 24, 2019, 09:08:14 AM »

It works
A simulation shows your end-fed design working well. I added the "usual" 0.07 wavelength EFHW (vertical) counterpoise wire and the F/B ratio deteriorates by only 1 dB.

Choke
A high impedance choke is needed and for that one of G3TXQ's designs can be used. His 11 turns of RG-58 on an FT240-52 ferrite core choke looks good and that core is available on Ebay. An alternative might be an air-cored choke using 13 turns of RG-58 on a 4.25" form. The air-cored choke narrow band and altering the design could render it ineffective.

http://www.karinya.net/g3txq/chokes/
« Last Edit: August 24, 2019, 09:22:26 AM by KH6AQ »
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ND6M

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #8 on: August 30, 2019, 09:58:41 AM »

Not to discourage your Moxon ideas, have you considered the 3 or 5 band "DX COMMANDER" type antennas?

https://www.m0mcx.co.uk/store/products/dx-commander-premium-build-hf-multi-band-vertical-system/

Easy enough to make your own
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HS0ZIB

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #9 on: August 30, 2019, 05:22:19 PM »

Not to discourage your Moxon ideas, have you considered the 3 or 5 band "DX COMMANDER" type antennas?

https://www.m0mcx.co.uk/store/products/dx-commander-premium-build-hf-multi-band-vertical-system/

Easy enough to make your own

Not possible - see my original post. "The house sits on a plot which is barely bigger than the house footprint."

There is no possibility to lay down radial wires.
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K6BRN

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #10 on: September 01, 2019, 11:20:43 AM »

A decent EFHW antenna can be bent into quite a few shapes (horizontal V, Inverted V, Linear, etc - be creative) and still work very well.  But height above ground and nearby interfereing objects is important.  So if you want better performance at minimum cost, get that antenna up another 5-20 feet, especially at its midpoint, and it will likely perform better.

If that's not enough, buy or build a rotatable dipole for 10-15-20-40M.  They are light, have broad front and back patterns so rotators are optional - and best of all, they can be put WAY up in the air on a simple metal mast.  One of my buddies has his Comet H422 on a 20 foot long un-gyed aluminum mast mounted to the side of his home and is quite happy with it.  You can even salvage old Yagi parts from the driven element and use that as the dipole.  Both old Cushcraft (NOT the MFJ versions) A3S/A4S (especially with 40M add-on) and Mosley Yagi driven elements are great for this (yes, I've done this with good success).

So... not happy with the EFHW you built?  1) Get it up higher and away from interfering structures, 2) Buy or build a rotatable dipole, 3)  Put up a Yagi (2 or 3 element - see the Mosley catalog - something for every situation.

Comet H-422 rotatable dipole (10-15-20-40M):

https://www.gigaparts.com/comet-antennas-h-422.html

Mosley Options: (Note that Mosley has options to "Grow" these dipoles to 2 and 3-element Yagis and has kits to add more bands once the step up to a Yagi has been made.)  These are very well built antennas.

http://www.mosley-electronics.com/dipoles.html

So... when is the anternna raising party?

Brian - K6BRN
 
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K7AAT

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #11 on: September 03, 2019, 09:59:52 AM »


Wouldn't a nice loop around the house perimeter or property perimeter be something to consider.....

Fed with a nice remote tuner, it should give decent performance on a number of other bands, too.
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AC2RY

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #12 on: September 03, 2019, 01:19:18 PM »


Wouldn't a nice loop around the house perimeter or property perimeter be something to consider.....

Fed with a nice remote tuner, it should give decent performance on a number of other bands, too.


Noise level will be extremely high with that arrangement. From my experience antenna closer than 30 feet from the house structure will make any DX contact almost impossible. Today the problem is not weak signal, but high noise on HF bands. Thus the antenna should be primarily designed to reduce noise on receive.
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K6BRN

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #13 on: September 04, 2019, 12:37:59 PM »

All of my antennas, at three different QTHs, are within 30 feet of my homes.  My noise problems are minimal. 

At QTH#1 in SoCal, homes are spaced at 10 foot intervals and back yards are 20 to 60 feet deep.  In general, I try to keep wire antennas as far as practical from the home and as perpendicular as possible.  I also ground the coax before shack entrance and use a CMC choke, too, placed where it is most effective (varies by antenna installation and type).  Had no problem achieving multiband WAS and DXCC + DXCC digital.

My most effective (personal) EFHW install is a horizontal V at 45 degrees, run from the 2nd story corner eaves of my home to a pine tree (the apex of the V) in the back yard.  Pretty much omnidirectional, decent noise level, works from 10M to 80M including WARC bands, just fine.

So, experimentation is GOOD.

Brian - K6BRN
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HS0ZIB

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RE: EFHW or inverted V or ...? (space-limited QTH)
« Reply #14 on: September 04, 2019, 06:49:46 PM »

Thanks again for all your suggestions.

Update:  This is what I've done.  Despite saying that there is no room for radials wires, I decided to install a 5/8 wave vertical for 20 meters, because I had good results using a similar antenna when operating as XZ2A last year.

I had a 10 meter long, straight bamboo pole.  I ran the 12.6 meter wire along this and then hoisted the pole through the leaves of a nearby low palm tree, adding another 3 meters of 3 inch PVC pipe at the lower end to obtain the full antenna length.  The strong leaves help to support the bamboo pole.  I added a matching coil, earth rod and then ran a few radial wires around the exterior of my wooden house.  SWR match at the desired frequency is about 1:1.3 and down to 1:1 with my ATU.

I'm testing out reception right now - you can look on pskreporter with my HS0ZIB callsign to see the results which look encouraging.

I'm collecting my new XW licence hopefully tomorrow, and can then do some QSOs.

Since I already have the EFHW that I made working OK, I have an idea to use that EFHW as the ancillary antenna with my MFJ-1025 noise canceller, to see if I can null out local QRN.  Need also to add a few more radial wires if possible :)
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