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Author Topic: random wire antennas  (Read 2873 times)
KC9BUG
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« on: September 09, 2002, 09:33:59 PM »

I just got my general class upgrade and am putting together a HF station.  I am learning alot along the way.  However I have no exprience with antennas.  I what to build a random wire antenna.  How do I connect the feedline to the antenna?  What kind of feedline should I use?  How do I ground my antenna and my station?  Any advice would be great!  Remember I am a greeny so please try not to leave out those details that everyone but me knows.

Thanks,
KC9BUG
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AA4PB
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« Reply #1 on: September 09, 2002, 09:55:11 PM »

A random wire antenna usually means an end fed single wire antenna which is connected directly to the rig or tuner. A random wire antenna needs a good RF ground connected to the rig or tuner. The tuner built in to most rigs does not have a wide enough range to feed an end fed wire directly. Often the easiest way to use such an antenna is to place a tuner (an SGC for example) outside, mounted close to the ground with a short connection to a burried radial system and the antenna connected directly to the tuner. Then you can run RG8 or other 50 ohm coax into the house and connect it to the rig.

Depending on your situation, you might also want to look into a dipole or trapped dipole (for multiple bands) that can be feed directly with 50 ohm coax.
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N4ZOU
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« Reply #2 on: September 09, 2002, 10:08:39 PM »

First of all you will need an antenna tuner! There will be no feed line required as the random wire end will connect to the single wire connector on the back of the tuner. The ground system will connect to the ground post on the tuner. Yes, it's that simple! Something else to look out for is not to have a wire length that is close to 1/2 wave on any band you plan on working as the end of the wire at the antenna tuner will have a very high voltage show up there when you start to transmit. That could burn up your antenna tuner if it can't handle high voltage. Two length's are used in random wire antenna's for multiband operation, 67 feet long for 80 meters and up and 135 feet long to be able to operate with a good signal on the 160 meter band. Simply put, hang it, tune it, and have fun!
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K0IPG
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« Reply #3 on: September 09, 2002, 11:00:50 PM »

Instead of a random wire, I would suggest you start off with a simple, 1/2-wave dipole. There's really not much easier or better performing than a good ol' dipole.

Although, if you do want to go with a random wire, you will have to use an antenna tuner, of course.

Perhaps we can help further if you could tell us which band(s) you think you'd like to work, and what kind of radio and/or tuner you currently have?
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AC5UP
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« Reply #4 on: September 09, 2002, 11:28:16 PM »

The random wire concept looks good at first glance thanks to its simplicity, but I've had poor luck with them due to the lack of an effective ground. You'll see better results from something that's not ground dependent, like an off-center fed dipole (coaxial Windom) or Extended Double Zepp, than an end-fed wire with 500 feet or more wire in the ground radials.

As for a 100' or wire with a shiny new 8' ground rod and 9:1 Balun at the feed point with no radials, FOR-GET-IT. Biggest dummy load I've ever built, but it was a dandy RX antenna on AM broadcast.

I've worked the world with an Extended Double Zepp on 20 Meters (82' tip-to-tip) and continue to have excellent luck with an Off-Center Fed dipole on 75, 40, 20, 17, 12 and 10 Meters (137' tip-to-tip). If you have the room and can get either antenna 30' or more off the ground, that's the way to go. If you're limited on space, an Extended Double Zepp on 15 Meters (57' tip-to-tip) will also work on 40 Meters, just be sure to go slow and watch both bands as you're pruning it to resonance.

Good DX and Congrats on the new privileges!

- AC5UP
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #5 on: September 10, 2002, 03:28:18 AM »

Try here for wire antenna ideas, and back ground info on antennas:http://www.cebik.com/radio.html

It might be more important to get up one antenna that performs good on one band; and then experiment with other antennas for multiple bands.  Dipoles or inverted Vees are easy to make and get to work.

Congrats on the upgrade.

Have FUN
Bob

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WA8NNX
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« Reply #6 on: September 10, 2002, 11:15:07 AM »

I have had great success with random wires over the years on all bands and modes.   The one often overlooked but critical part of the system is a counterpoise tuned to the band you are operating.   Just take a quarter wavelength wire i.e. about 43 feet for 40 meters, and attach it to the ground on your antenna tuner.   Just run it around the baseboard of the shack.  You can attach multiple wires of various lengths for different bands.  It is this counterpoise that provides an effective rf ground.   If you don't have this rf ground your results will be poor.  Good luck.  Mike
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WB6BYU
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« Reply #7 on: September 10, 2002, 03:28:00 PM »

My response may not agree with all the others, but that
is part of the hobby - different people have different
experiences.

A random wire antenna may be a good choice if you have
only one support to tie off the far end, and it has
the advantage that no feedline is needed between the
antenna and the tuner (since generally the end of the
antenna wire connects directly to the tuner.)  It may
also be a good choice for a clandestine or otherwise
inobtrusive antenna, where you don't want to keep
climbing back up to adjust the length to resonance.
In these cases, you just put up whatever length of wire
you can, and make it work.

On the other hand, if you have several supports handy,
so you can string a dipole between them, that may be
preferable.  I like the convenience of being able to
switch bands and having the antenna be resonant so I
don't need to adjust the tuner.  Center-fed antennas
are less critical for grounding and possible RF in the
shack.

First, you will have best results if the random wire is
at least 1/4 wavelenth long on the lowest band you are
planning to operate.  There are two reasons for this:
first, the efficiency will be higher (ground loss is
more significant with shorter antennas) and also the
wire will be easier to match (the feedpoint impedance
will close to - or over - 50 ohms.  This allows for
the use of a matching network that only has to step
the impedance UP.)  This doesn't mean that shorter
antennas won't work, but that you will need to pay
more attention to them to get them to work well.

Now, you will need a tuner of some sort.  You can pay
a lot of money for the automatic ones, or somewhat less
for the manual ones.  However, most commercial tuners
try to be able to match any possible impedance, and,
in the process of trying to do this with practical
components, they can get lossy on extreme impedances.
My personal prefernce for end-fed wires is the "L"
network:  this requires one coil between the center
conductor of the coax and the antenna, and a variable
capacitor from the antenna wire to ground.  You can
wind a coil on a toilet paper tube, and pick up a
variable capacitor at a hamfest (or from the junkbox
of a local ham.)  You can mount these in a nice box
with coax connectors, or jury-rig the circuit on a
piece of wood with clipleads as you mood suits you.

Since "ground" is rather uncertain in an indoor shack,
I recommend the 1/4 wave radials attached to the
"ground" connection of the tuner.  A good starting
point is 33' for 40m and 17' for 20m:  add more if you
have problems on specific bands.  I've also used a wire
to a ground rod or water pipe, but this is best when
the length of the ground wire is much shorter than
a quarter wavelength.  And, usually, it is too long.

Adjust the tuner by varying the coil and capacitor
until the SWR is minimum.  You will probably need to
use a clip to select which turn of the coil to use,
unless you have a "roller inductor"-type of variable
coil.  Once you find the right number of turns for
each band, you can mark it, or otherwise make it easier
to attach to next time you change bands.  One advantage
of the "L" network over the common commercial tuners
is that there is less chance that the setting you find
that gives a low SWR is really dumping most of the
power into the tuner instead of into the antenna.

I've used a number of wire antennas, and my favorites
have generally been around 135 feet (a half wavelength
on 80m, full wavelength on 40m, etc.)  Yes, I know
some would warn you that your tuner will blow up due
to the high impedance, but that has never been a
problem for me.  The "L" network handles this just
fine.

If you change bands often, at some point you will get
tired of changing the coil tap and retuning the
variable capacitor for minimum SWR each time.  So here
is what I did:  I got a multi-pole rotary switch with
8 or so positions.  I made some measurements with a
temporary tuner to find the coil and capacitance values
required to match the antenna on each band.  Then I
made up some fixed coils (either individual ones for
each band, or one coil with different taps) and wired
them to the switch.  For capacitors I used the old
mica "postage stamp" capacitors from tube equipment.
When I was done, I could just turn the switch to the
desired band and it would select the right coil and
capacitance to match the antenna.  (Sometimes I had to
use a mica trimmer capacitor for fine adjustment.)
Yes, it took me some time to set the whole thing up
initially, but after that changing bands involved just
turning the switch, without enabling the transmitter.
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KC9BUG
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« Reply #8 on: September 10, 2002, 05:00:54 PM »

The problem is that I can only have one antenna up.  I think that a dipole could work.  Can I make a dipole multiband?  
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WB2WIK
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« Reply #9 on: September 10, 2002, 05:36:17 PM »

Any good conductor makes a nearly 100% efficient antenna, in that all power applied must be radiated, dissipated or reflected -- and when using good conductor material, "dissipation" is virtually zero.  If you can achieve an impedance match between the transmission line used and the conductor, then reflection will also be virtually zero.  So, power has nowhere to go but OUT, and be radiated.

The only difference between great antennas and mediocre ones is deployment.  If I can match a paperclip, I can make it a "100% efficient" antenna.  But it won't make many contacts.  On the other hand, I can end-feed 1/2-wavelength of wire and make the antenna _system_ nearly "0% efficient," because I'll never be able to match to it, and all the applied power will be reflected, and eventually absorbed in the line.

In your case, if you're looking for multiband operation with a single wire antenna, I'd recommend using 450 Ohm ladder line (or 300 Ohm twinlead) and erecting a loop if possible; if not, then any length doublet will work quite well, with 100' overall length (50' each side of the feedpoint) being a good place to start.  Such a doublet will not be resonant, but that doesn't matter much when you're using low-loss transmission line (like ladder line) and a good manual antenna tuner.

You can make a "single dipole" work using coaxial cable for feedline, but only if that dipole has traps or multiple elements that resonate it on each band.  Such a multiband dipole can work well, but is heavier and more expensive to construct, and more difficult to support, than a simple wire doublet fed with ladder line.

WB2WIK/6
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KG4OXP
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« Reply #10 on: September 10, 2002, 05:57:27 PM »

I rec'd my general not that long ago, like you. I'll share what I did.

I decided that I wanted to try 20 meters first, mostly because it seemed that propagation [to various areas] was quite good, 24 hours per day.

I started with a dipole, tuned to 20 meters, as I had no antenna tuner. It was actually an inverted V, with 1 center support and the ends ties off to 2 trees...I fed this with RG-8mini coax. The center was up about 35 feet. It worked well.

I have also tried a delta loop...1 center support, cut for 1 full wavelength at 20 meters and the apex up at approximately 27 feet. This worked very well too...coax fed (12 feet roughly of 75 ohm coax, RG-6) to RG-8 50 ohm coax. It was a close match and tuned up fine. I find this to be a great portable antenna, but I was not in an area where I could get the base of the antenna high enough to be safe for longer term use.

Over time I bought a good Palstar tuner, and made a dipole, but made it as long as I could and just used the tuner...this was fed with 450 ohm ladderline to just outside the shack, to a 4:1 balun and then an 8 foot run of coax (RG-213) into the shack/tuner. This worked very well for me as I had multiband use and it worked quite well.

I hope this helps...

Pete
W4JPP
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