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Author Topic: 40m vertical homebrew antenna  (Read 3873 times)
NL-6915
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Posts: 21




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« on: April 04, 2005, 12:20:07 PM »

Hi everyone,

I'm thinking of putting up a vertical antenna next to my house.  But I have some questions....Maybe you can help.  It will be 1/4 wavelength, so for 40 meters, about 10.6 meters high.  I am planning to mount the bottom of the vertical (which will also be the feedpoint for the coax) about 2 feet above ground level, and insulate with a plastic pipe from ground.
The vertical itself will be 2" pipe. Then, I will use my house to attach the vertical to, again insulated with plastic where the fasteners will be.  Now I have read about radials, and talked to some hams about it.  What I plan is to run 3 radials, about 1/4 wavelength long and under almost 90 degree angles.  Like one pointing west, one south and one east.  A fourth radial isn't possible because I am up against the wall of my house.  I'm planning to attach 8 or 10 gauge wire to a wire-ring, which lays underneath the base of the vertical.  The radials would be attached to the ring, and buried a couple of inches into the  ground.

Couple of questions....
How is the performance of a vertical affected being mounted only a foot or so from a brick house?  There are no electrical wires there.

Do I need to bury the radials?  I would prefer to, anyway, so I don't get them caught in the mower.

How would a 10m high 1/4 wave vertical work for the 80, 20, 15 and 10 meter bands?  Any chance for 160?

I have read about loading coils....Are there any remotely operated coils like that, so I don't have to run out every time?

As far as lightning goes...I am sticking a 35 foot high pole next to my house.  Normally I would have  guessed that this could attract lightning, but since this vertical is not grounded, am I correct in assuming that it will actually NOT attract lightning?

What's the correct way of calculating the length I need for a 1/4 wave antenna using pipe of different diameters?

Any general suggestions or comments about all of this?  Your input would be appreciated!

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WB6BYU
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Posts: 13573




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« Reply #1 on: April 04, 2005, 01:27:31 PM »

> How is the performance of a vertical affected being
> mounted only a foot or so from a brick house? There
> are no electrical wires there.

The brick will probably absorb some of the signal,
especially when it is wet.  On 40m most of the radiation
will be from the lower part of the antenna, which is
most likely the part beside the house.  The further the
antenna can be from the brick, the less the losses will
be. (The same applies to receiving, even though the
power levels are much lower.)  So 1m spacing will be
better than 20cm.  But I don't think the losses will be
bad enough to make the antenna unusable.



> Do I need to bury the radials? I would prefer to,
> anyway, so I don't get them caught in the mower.

Actually, radials are often more efficient if they are
NOT buried, as well as being easier to install.  Instead
of burying them, I'd make some U-shaped pieces of stiff
wire and use them to "pin" the radials to the ground.
If you mow the grass very short before putting the
radials down, this will hold them to the ground while
the grass grows up around them and you shouldn't have
problems with the mower.

You are better off using more radials than 3, even if
they are much shorter than 1/4 wavelength.  They do not
have to be heavy wire:  0.5mm to 1mm "magnet wire" is
quite adequate.  Run as many as you can in as many
directions as possible: they can bend around the house
or other obstructions as needed.  I usually do this by
making a loop of wire connected to the base of the
antenna, then soldering the ends of all the radial
wires to this loop.  

> How would a 10m high 1/4 wave vertical work for the
> 80, 20, 15 and 10 meter bands? Any chance for 160?

Yes, it could work on all bands, even 160m (though it
would work best with a top loading coil for that band.)
On 80 and 160m it will be short, and can be matched
with a tapped coil at the base.  On 20m it will be a
half wavelength, so will need an "L" netork or parallel-
tuned circuit at the base to match the impedance (around
1000 ohms).  On 15m the SWR will be fairly low but the
angle of radiation will be higher because it is over
5/8 wavelength.  On 10m the antenna will be 1 wavelength
tall, with maximum angle of radiation at about 45
degrees above the horizon and a high input impedance.

> I have read about loading coils....Are there any
> remotely operated coils like that, so I don't have to
> run out every time?

There are several options for this.  You can make
motorized coils, but that won't work for 10 or 20m because
there you need a different matching network.  Perhaps
the simplest approach is to use a set of relays to
select the desired matching network for each band: the
relays can then be switched from indoors.  There are
a number of commercial auto-tuners for the amateur radio
market which are more expensive and would work with a
transmitter, but aren't practical if you are only
receiving.  (They rely on transmitted power to adjust
the match.)

> As far as lightning goes...I am sticking a 35 foot
> high pole next to my house.... am I correct in
> assuming that it will actually NOT attract lightning?

No.  Some possible matching networks will provide DC
grounding to the antenna.  (And this is a good thing,
as it avoids problems with the antenna building up a
static electric charge.)  You will need to use standard
lightening protection on the cables from the antenna
to the radio.  And plan that you will have to replace
the tuning box if it takes a lightening strike.  (If
you use relays, wire them so that the antenna is grounded
when not in use.)

> What's the correct way of calculating the length I
> need for a 1/4 wave antenna using pipe of different
> diameters?

This will be different depending on the diameter: a
straight 5cm pipe will resonate at a shorter length
than one made of tapered sections.  You can use some
antenna analysis software to calculate this for whatever
diameter element you are going to use, but I'd just
make a good guess and add a bit of capacitance or
inductance in your matching network to make up the
difference.  (For reception, getting it close is all
you need.  Something around 10m is good enough.)
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WB2WIK
Member

Posts: 20666




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« Reply #2 on: April 04, 2005, 02:20:15 PM »

 >I am planning to mount the bottom of the vertical (which will also be the feedpoint for the coax) about 2 feet above ground level, and insulate with a plastic pipe from ground.
The vertical itself will be 2" pipe. Then, I will use my house to attach the vertical to, again insulated with plastic where the fasteners will be. Now I have read about radials, and talked to some hams about it. What I plan is to run 3 radials, about 1/4 wavelength long and under almost 90 degree angles. Like one pointing west, one south and one east. A fourth radial isn't possible because I am up against the wall of my house. I'm planning to attach 8 or 10 gauge wire to a wire-ring, which lays underneath the base of the vertical. The radials would be attached to the ring, and buried a couple of inches into the ground.<

To supplement what Dale said, I'd be concerned about the statement above.

You said you'll install the vertical with its feedpoint 2 feet above ground, then use radials on the ground.  How are you going to do that?

Radials, to be most effective, need to be connected to exactly the same place as the antenna is fed, but to the other feedline conductor (typically the "shield" if you're using coax).  You can't just use radials two feet below this point, attached there, and expect good results -- the two feet of mast between the feedpoint and the radial field has substantial inductive reactance.  The fact that it probably has *zero* DC resistance isn't important.

I've never been able to make this work (as you describe).  If you are going to use ground-mounted (or buried) radials, you need to drop that feedpoint down to ground level.

Better still would be to raise that feedpoint substantially higher, and use elevated radials that won't get in the way because they'll be over your head!  An elevated vertical almost always works better than a ground-mounted one, unless the ground-mounted vertical can be installed in a very clear field with a great many radials.  The elevated vertical doesn't require as extensive a radial system, either: I've found that 4 radials, each 1/4-wavelength long, sloping down from an elevated feedpoint generally work very well, although to force the same vertical into multiband service does call for multiple sets of radials.

Your 35' tall (or so) vertical would do better on 80m if you added "top hat" loading to it, instead of a base loading coil -- however, the proper loading coil should work.  The problem with base loading a current-fed antenna is the loading coil has loss no matter what you make it from -- although big, fat copper tubing would have a lot less loss than ordinary wire.  This is one of the reasons the Butternut (brand) verticals work better than many others on the lower HF bands -- they do use thick aluminum tubing for the loading inductors.

Good luck!

WB2WIK/6
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N7DM
Member

Posts: 671




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« Reply #3 on: April 05, 2005, 08:00:40 AM »

Considering your installation situation, you might benefit from another 'trick'.  The length of a quarter wave, on 40, is... 35 feet. But you said you were going to use 2 inch pipe. Good idea. That 'fattens' your radiator, changing the 'end effect' and 'Q'... to your advantage. I have made similiar antennas using galvanized downspout pipe, and have found that a single pipe resonates on 7 Megs at only 29 feet in length, and if I strapped TWO of them together, I was resonant [on the noise bridge] at 26.5 feet!  So consider doing that. I once had a 80 meter vertical.. you know.. should have been around 68 feet tall? I put a three foot spreader at the top and bottom, and put TWO wires there...joined top and bottom. That 'fattening up' brought it to resonance at FIFTY EIGHT feet!...  Shorter 'physical length' often aids installion..and appearance.
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K5DVW
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Posts: 2193




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« Reply #4 on: April 05, 2005, 08:30:27 AM »

Good advice, all... but I wanted to stress something. If you can, if at all possible, get the antenna out away from the house. Three reasons...

1. The house will tend to distort the pattern
2. you'll pick up EMI and cause RFI to electronics in the house.
3. If it DOES get hit by lightning, it's not going to burn down your house!!
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N7DM
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Posts: 671




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« Reply #5 on: April 05, 2005, 08:43:38 AM »

Yes...all..  You know, on the lightening thing, lightening happens due to charges piling up, concentrated, on things... POINTY things. Like the sharp edges of the upper end(s) of your pipe. Mind you, I've never had to try this, but it does stand to reason that if you could get some kind of metalallic 'ball'... the larger the better, and 'solder' that to the top of your antenna, you would greatly decrease the charge concentration...and reduce your vulnerability to 'fire in the sky'...  It is  FACT that balls are installed on the ends of things like that, commercially.
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NL-6915
Member

Posts: 21




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« Reply #6 on: April 06, 2005, 09:35:33 AM »

Ok, thanks everyone for the good advice.  I will move the feed point close to the ground, not 2 feet up.  Other than that, I am still a little concerned about lightning.  What I am getting out of this discussion is that I should definitely tie my vertical to ground when not in use, correct?

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N7DM
Member

Posts: 671




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« Reply #7 on: April 06, 2005, 09:44:02 AM »

From the 'usual' standpoint, I'd say, "yes".  But when you consider that lightening occurs due to charges in the clouds AND charges FROM THE GROUND...want to get together...  I often wonder if grounding an antenna doesn't actually encourage Fire from the Sky! Airplanes get hit and they are sure NOT grounded. The Seattle Space Needle is grounded, and gets zapped regularly. Maybe somebody with Mid-West experience can be definite about it, I sure can't. SRI
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WB6BYU
Member

Posts: 13573




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« Reply #8 on: April 06, 2005, 12:47:59 PM »

The distance over which the lightening is going to arc
is significantly longer than the distance from the base
of the vertical to the ground, so what would it care about
a few extra cm in its path?

At a minimum you will want some sort of lightening
protector in your coax cable that is well grounded.  This
will prevent a destructive surge from travelling up the
coax cable.  This is not a trivial installation - do a
search on lightening on the Elmers forum here to get a
lot of ideas.  (Many of those are for grounded towers,
and your situation will be a bit different.  But the
stories from people whose towers get struck several times
each year will give you a useful perspective.)  One thing
you probably will want is some sort of "flashover" fitting
that will arc and shunt the current to ground in case
of a strike, but will withstand the normal operating
voltages on the antenna.

There is also a device like a metal hedgehog that can
be mounted on top of an antenna or tower to reduce the
chance of a lightening strike.  It has many sharp points,
and the theory is that these will arc over at a much
lower voltage than that required for a major strike,
keeping the local area at a lower potential difference.
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NL-6915
Member

Posts: 21




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« Reply #9 on: April 06, 2005, 12:52:20 PM »

Yes, I agree..I would love to hear from a ham that lives in an area that has lots of thunderstorms and also has a tall vertical antenna.  I'd love to hear what his or her grounding procedures are....

Just by reading about it on the net, one thing seems to pop up over and over:  Have only one common ground point at the base of your mast, tower or in my case, vertical.  

So here's what I plan to do:  Mount the vertical on a plastic insulator a few inches from the ground, and also insulated from the house.  Drive a long copper ground rod into the ground next to it, and attach some heavy gauge wire to it.  When not in use, I can attach this heavy gauge wire to the vertical's feedpoint.  Other than that, have a bunch of radials running from the base of the vertical for RF ground.  Connect the radials and the ground rod with heavy wire, too.  Then, run a large ground wire from the rod and into the shack to act as a ground terminal there.  

So, when I use the antenna, the ground rod is disconnected from the vertical, and just lays on the ground.  During a thunderstorm, I connect the heavy ground wire from the ground rod to the vertical with e.g. a big wing nut or a clamp, disconnect all coax going into the shack and stick them into a glass peanut butter jar, which are tough to find these days.
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K8AC
Member

Posts: 1478




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« Reply #10 on: April 07, 2005, 04:42:31 AM »

The common ground point ought to be on the outside, where the cables enter the building.  Ideally, that means ALL cables, including antenna coax, power, telephone, etc.  Proper grounding techniques have been dealt with many, many times on the forums here and elsewhere.  Check out the archives for more info, and go to the Polyphaser site for a good set of recommendations.  You really don't want to know how other guys are doing it - you want to know the right way!  Most installations are not up to snuff.  I'm afraid I always disagree with any solution that involves disconnecting cables.  Will you ALWAYS be there to disconnect the cables when leaving home?  

I'd like to try and dissuade you from mounting the vertical against the house.  Chances are, it will end up within a few feet of one or more runs of AC wiring in the walls and substantial energy will be coupled to the wiring.  Even 100 watts can wreak havoc with the electronic controls in modern dishwashers, toasters, etc.  If at all possible, get the antenna away from the house.  

The ground wire should be tied to your electrical service ground rod OUTSIDE of the house.  What you described results in a ground wire INSIDE of your station that is separated from your AC service ground only by your equipment.  During a nearby strike, the difference in potential between the two grounds can rise to a  incredible level, destroying the equipment.  Please check out the Polyphaser site for a good explanation on all of this.  You don't have to buy their protectors - their good ideas are free!
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AA4PB
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Posts: 13032




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« Reply #11 on: April 07, 2005, 05:46:54 AM »

I submit that radials laying on the ground are electrically not much different than radials burried a few inches below the surface so laying them on the ground is not going to improve their efficiency. Burried radials are not resonant therefore there is no need to cut them to a specific length. Longer is better, although going above 1/4 wavelength doesn't provide much improvement. More is always better, even if they are shorter than 1/4 wavelength. They don't have to be in straight runs and they don't all have to be the same length. Because your coax shield is always connected to the radials, they can provide a good deal of lightning protection as well.

I'd move the antenna away from the house. It would be very unusual to have one wall of the house that has no electrical wiring in it (that would mean there are no electrical outlets anywhere on that wall). Even if that is true, electrical wires will not be very far away and the RF from the antenna will couple into them. The opposite is also true, the wires will couple any noise on them into the antenna.

The coax shield needs to be grounded outside the house near the entry point. As others have suggested, take a look at the Polyphaser site for more info.

Don't believe for a minute that leaving an antenna "ungrounded" will keep it from getting hit by lightning. The little bit of insulation you can provide means nothing at the voltage levels present in lightning. Most lightning damage is not caused by a direct hit anyway. Most damage is caused by voltages induced in conductors by the current flow caused by lightning hitting trees and other objects in the area.
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