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Author Topic: 20m "random" wire length?  (Read 4766 times)
NU9J
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Posts: 109




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« on: May 12, 2013, 06:02:47 AM »

I'm in a 2nd floor apartment without a balcony. Finally stopped pouting and ordered an MFJ apartment antenna, but it's backordered for 2-4 weeks.

What I'd like is some advice about cutting a "random wire" antenna as short as possible (25' maximum) that would work only on 20m with a B&W VS300A tuner in my situation. I don't have an RF ground, and all I can do to "mount" the wire is run it horizontally along the brick outside. I have read many websites about multiband random wire, and they all conclude you need > 30 feet, but I was hoping I could force it to work on a single band if it's shorter.

Thanks a lot!
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~Philip
K5LXP
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Posts: 4450


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« Reply #1 on: May 12, 2013, 06:43:06 AM »

I'm in a 2nd floor apartment without a balcony.
Any way you can run a wire from a window to some other support like a tree or a pole?  Does this place have an attic you can get access to?  Is there an area nearby you could put an antenna up, and run coax over to it?  Those were the options I used when I lived in apartments.

Quote
ordered an MFJ apartment antenna,
I think B&W does/did sell something like that.  Can work OK if you can figure out a radial or two for it.

Quote
What I'd like is some advice about cutting a "random wire" antenna as short as possible (25' maximum) that would work only on 20m with a B&W VS300A tuner in my situation. I don't have an RF ground, and all I can do to "mount" the wire is run it horizontally along the brick outside.

If your only option is end fed and you can't control the length and proximity effects being attached the structure, it's nearly impossible to offer an optimum length.  Even with a radial or counterpoise inside or out it's a good bet you're going to have a lot of common mode and a good chance a lot of RFI from electronics in the structure.

About all you can do is experiment.  Wire is cheap/free and while you're waiting for the window antenna you can try all manner of wire configurations to see if you can come up with something usable.  I don't think any idea is a bad one until you've proven it that way.


Mark K5LXP
Albuquerque, NM
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W5WSS
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Posts: 1652




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« Reply #2 on: May 12, 2013, 08:38:41 AM »

I built a multie band(10m to 40m )vertical using the Hustler long shaft 54" with the various applicable resonators fit it and positioned the antenna centered directly midway of the bedroom window.

The antenna and radials are indoors more on the radials to follow.

Down at the vertical base I placed a 1:1 current balun.
and using a male to male UHF connector I next attached an auto tuner (optional)

The radial wire system is made of #14 insulated stranded copper and consists of horizontally oriented and purposed to be horizontal radiation cancelled pairs per band that are tuned and equally positioned traveling or routed along the base board in opposite direction relative to each other exacting centered positioning relative to the vertical base. They are horizontally oriented pairs and return the power back to the vertical to be radiated. If you can not run them straight all the way then if possible turn them down the opposite walls at equal distance from the source to help keep them centered relative to the position of the vertical antenna.I suppose one can attempt to emulate a half moon shaped flat plane radial systemfanning the wires underneath carpet I am not comfortable doing so but truthfully I do not think that is as effective an alternative for this special case.

To connect them I used a 3"x 5" wide aluminum rectangular shaped plate and attached each pair together to already installed bolts thru the plate and repeated this for each of the individual radial pairs via crimped and soldered ring terminalsstar washers for excellent electrical bite then attached the plate to the balun terminal marked shield directly.

The advantage of the quarter wave Hustler and resonators is that it can be adjusted for the individual bands and shrinks the vertical length allowing fitting into the space.

I was able to fit and tune every band from 10m to 30m within my vertical space from floor to ceiling without the need for the tuner.

On 40m the lowest band I could manage in this area I need to modify the whip length so I used #14 copper insulated wire as an add on extension connected with an electrical wire to wire bolt from the tip top of the whip and bent the wire following the ceiling horizontally to complete the required length needed...The system provided service on the ham bands.

The bundling of the radial wires was done using teflon tie wraps then routed as a single harness half and the other half done identically this is faster and the tips of each were allowed to be visible protruding upwards above the carpet and allowing easy access for tuning then stayed in such a position with a high voltage wire nut terminating the ends safely with 20 watts pep.

Are there better more effective radial systems ...yes.
Does this radial system work reasonably yes.

73

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W5WSS
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Posts: 1652




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« Reply #3 on: May 12, 2013, 09:30:41 AM »

As an addition for feed method clarity I attached the Hustler vertical shaft in this case the 54" Mo-1 feed point simply using a wide bonding sheathe and SS hose clamp onto the shaft.

I was able to fashion it this way because the balun was purposely mounted perpendicular to the shaft and very close within inches for good connectivity performance.

The bonding sheathe being pliable simply wraps around the bottom end of the antenna shaft and the hose clamp, makes excellent connection. The sheathe being about 2" length connects to the balun terminal marked center conductor in the usual way slipping over the terminal bolt through and tightly securing with the nuts and washers.

73
« Last Edit: May 12, 2013, 09:38:00 AM by W5WSS » Logged
WB6BYU
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Posts: 13033




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« Reply #4 on: May 12, 2013, 10:30:42 AM »

Quote from: KI4THS

What I'd like is some advice about cutting a "random wire" antenna as short as possible (25' maximum) that would work only on 20m with a B&W VS300A tuner in my situation. I don't have an RF ground, and all I can do to "mount" the wire is run it horizontally along the brick outside.




You can make just about any wire length work, for some value of "work".  Many 80m
mobile antennas are only 8' long, which is equivalent to 24 inches on 20m.  And they "work",
though not necessarily very efficiently.

Some guidelines:

1) the longer the wire the higher the efficiency, up to about 30' or so.  Beyond that there
will be less variation as the length changes.  So if 25' is what you can manage, then do it.

2) Some of your RF will be dissipated in the lossy brickwork.  Hard to say how much.  The
bricks will also change the resonant frequency of the wire.  But if that is the best you
can do, give it a try.  Spacing the wire away from the bricks will help if it is practical.

3) You DO need some sort of RF ground to make it work.  One solution is to connect one
or two 1/4 wave counterpoise wires (about 17') to the ground lug on the tuner and run them
around the perimeter of your room.  (I've also had very good results connecting to aluminum
window frames, if your building has them.)  The same applies to any of the portable antennas
designed to stick out a window, regardless of the marketing claims.

4)  Even a less-than-optimum antenna should make some contacts.  But it will work better
if you can get the wire further from the bricks.
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W5DXP
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« Reply #5 on: May 12, 2013, 11:51:13 AM »

...all I can do to "mount" the wire is run it horizontally along the brick outside.

Why can't you run a 16.5' horizontal wire in both directions and have a normal dipole? Or if you can run a 16.5' wire horizontal and another 16.5' wire vertical, you can have a resonant bent dipole. Or figure out how to run a 25' horizontal wire along with an 8' vertical counterpoise and have an off-center-fed resonant dipole.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K3ANG
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Posts: 177




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« Reply #6 on: May 12, 2013, 03:00:57 PM »

Lengths that work, and don't work, for random wire antennas.
http://www.hamuniverse.com/randomwireantennalengths.html
de K3ANG
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NU9J
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Posts: 109




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« Reply #7 on: May 12, 2013, 05:37:34 PM »

...all I can do to "mount" the wire is run it horizontally along the brick outside.

Why can't you run a 16.5' horizontal wire in both directions and have a normal dipole? Or if you can run a 16.5' wire horizontal and another 16.5' wire vertical, you can have a resonant bent dipole. Or figure out how to run a 25' horizontal wire along with an 8' vertical counterpoise and have an off-center-fed resonant dipole.

Horizontal: 16.5'+16.5'= 33' > 25'. No-go.
L-shaped: there are apartments above and below me. Going into their space would violate the rental agreement.

About all you can do is experiment.  Wire is cheap/free and while you're waiting for the window antenna you can try all manner of wire configurations to see if you can come up with something usable.  I don't think any idea is a bad one until you've proven it that way.

Thanks. I tried a 16.5' wire on the tuner as well as another connected to the tuner ground. It tunes up nicely showing no reflected power, though I have yet to throw out a CQ.
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~Philip
K0ZN
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Posts: 1536




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« Reply #8 on: May 12, 2013, 07:27:57 PM »


WB6BYU is absolutely correct. You  MUST  have an RF ground. Otherwise, your rig and power supply wiring is the ground!!  .....and that is NOT good.

It would not be my first choice, but you could "ground" it to a cold water pipe, building steel, or even with a blocking capacitor, the AC power ground (worst
choice, but *possible*). ....and as noted you can use a 1/4 wave counterpoise wire.  The point is that random wire MUST have something to work "against".
You just don't push power into a wire; that wire is 1/2 of the antenna SYSTEM....the ground is the other part.

Any way you can make a full wave square loop....that would only be 16 ft. on a side, or a 20 M dipole is 16.5 ft. per side. Some sort of balanced antenna
would eliminate the need for an RF ground.

STRONGLY suggest you do a little reading/research in the ARRL Antenna Book to get some ideas and increased knowledge in this area. When you are
in a challenging QTH it is even more important to have an understanding of antenna theory, etc.

73,  K0ZN

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KB4QAA
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Posts: 2275




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« Reply #9 on: May 13, 2013, 12:50:07 AM »

Lengths that work, and don't work, for random wire antennas.
http://www.hamuniverse.com/randomwireantennalengths.html
de K3ANG
Pure urban myths!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Any piece of wire will radiate.  There are no magical lengths.

The purpose of a 'random wire' antenna is putting up whatever is convenient or possible, not putting a tuned antenna.

As long as you your tuner can match the load, that is all that matters.
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W5DXP
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« Reply #10 on: May 13, 2013, 05:28:22 AM »

As long as you your tuner can match the load, that is all that matters.

When the tuner "matches the load", the antenna system is resonant by IEEE definition. The random length of wire winds up being one component in a resonant antenna system just as the caps and coils in the tuner are components in the resonant antenna system. Such can be proven by using a grid dip meter on the random length of antenna wire after the tuner is properly adjusted. Assuming the receiver has a 50 ohm input impedance, the GDO will indicate a resonant dip very close to the transmitter frequency after the tuner has achieved a match.

http://www.w5dxp.com/OWT1.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB0KSL
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Posts: 94




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« Reply #11 on: May 13, 2013, 07:44:49 AM »

KB4QAA,

I think the point of the web page cited by K3ANG is that there are certain lengths that a "random" wire will be easier to match.  Many of the tuners/couplers in use have difficulty matching a "random" wire that is very close to a half wavelength at the desired frequency.  Some lengths manage to avoid that situation on all bands, and therefore the reason for such web pages.  The antenna works "better", simply because it can be matched with commonly available tuners/couplers.

73 de wb0ksl
John

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WB6BYU
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Posts: 13033




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« Reply #12 on: May 13, 2013, 09:09:25 AM »

Quote from: WB0KSL

...The antenna works "better", simply because it can be matched with commonly available tuners/couplers.



While some of us prefer to use wire lengths that are multiples of 1/2 wavelength
because they are easy to match with simple tuners, either homebrew or commercial.
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WX7G
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Posts: 5920




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« Reply #13 on: May 13, 2013, 09:52:38 AM »

I see two good approaches to your antenna dilemma:

1) A dipole composed of two 12.5' wires out the back of the tuner attached to the balanced terminals

2) A single wire, 12 to 25' connected to the tuner random wire post. Connect the tuner ground post to the AC output ground


Run whatever RF power does not cause RFI. #1 will likely cause less RFI as it will not make the apartment AC wiring as "hot" with RF.

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KB4QAA
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Posts: 2275




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« Reply #14 on: May 13, 2013, 07:47:20 PM »

KB4QAA,

I think the point of the web page cited by K3ANG is that there are certain lengths that a "random" wire will be easier to match.  Many of the tuners/couplers in use have difficulty matching a "random" wire that is very close to a half wavelength at the desired frequency.  Some lengths manage to avoid that situation on all bands, and therefore the reason for such web pages.  The antenna works "better", simply because it can be matched with commonly available tuners/couplers.

73 de wb0ksl
John

Sir, I absolutely agree with that that the thought behind the recommendation is to make matching easier.   But the myth that is propagated is that 'random' antennas can/should be cut for specific lengths for better performance. This is untrue.  If we choose to cut an antenna for a particular length (for easier matching, i.e. lower impedance) then it is no longer a random wire antenna.  

Further, the while matching may be easier, the antenna performance does not change.
« Last Edit: May 13, 2013, 07:49:50 PM by KB4QAA » Logged
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