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Author Topic: multi band antenna  (Read 555 times)
KE5PNY
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« on: July 12, 2009, 05:42:12 PM »

on a balanced wire antenna to cover all the bands what would be the length of each side also am using an antenna tuner.  bands from 160 to 10 meters
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WB2WIK
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« Reply #1 on: July 12, 2009, 05:43:43 PM »

The only way to do this without parallel dipole wires or traps is to make a 160m dipole and feed it with parallel transmission line like ladder line or open-wire line (not coax!).

Each "side" of the dipole would be 130 feet long, to hit 160m.

WB2WIK/6
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N7ZM
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« Reply #2 on: July 12, 2009, 07:11:26 PM »

Bill,
     I agree with Steve You need about 130 feet on each side of the dipole to hit 160. But if space is an option, then why not go make a 80 meter dipole and use
traps to get to 160. Will save space, and will load up nice with 450 ohm LL. 80 meters is about 65 feet on each side and then you start trimming.
       Now you know why not many hams use 160, length is too long for their size lots and 160 is mainly a winter band and is very very noisy.
       73 Ron N7ZM
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N0OKS
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« Reply #3 on: July 12, 2009, 09:09:33 PM »

I had an 80 mtr dipole fed with twin lead and worked 48 states from Wisconsin with 100 watts. I used a home brew L tuner with a balun. Length is over rated. The longer the better, but you can load anything with a decent tuner.

NØOKS
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K5LXP
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« Reply #4 on: July 13, 2009, 06:25:23 AM »

"cover all the bands"

What does this really mean?  It could mean matchable with a tuner, have a specifc pattern, "best" length to fit your available space, or what's your goal?

There is no one length that's "optimum" for any of these criteria.  What is it exactly you're trying to achieve?  A doublet cut to work well on 160M will suck on the higher bands, is that what you really want?  Or is just having a wire length that gives you 1:1 SWR the most important?  Or are you just trying to fit a wire within an available space?  


Mark K5LXP
Albuquerque, NM
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WB5JEO
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« Reply #5 on: July 13, 2009, 08:03:59 AM »

The original question was the cut length of a multiband wire antenna fed with balanced line through a tuner. At least that's the way I read it. I think the real answer is that (1) such an antenna will work and will work well, and (2) it will take some fiddling to make it work as you want. If you cut for 160, fine. You have to start somewhere, and more wire is good. No matter how you cut it, you will have issues to work through. There will likely be one or more bands with a problem to overcome that you will address with length of antenna and length of feedline. There's no way to come up with a calculated length that assures you of being able to put it up and work it well on all bands. Even exactly duplicating someone else's set-up will have enough factors introduced by your version and environment that you'll have to mess with it, unless you're just lucky.

Once you get it working, meaning you can tune it okay without the tuner arcing or other nasty things happening in the shack, it's going to be a different antenna on each band. Different patterns and take-off angles. That's only to be expected, since it really is quite a different antenna when you think of its design with reference to each band. It will be a "good" antenna on every band, but "good" in different ways and not so good as other designs in other ways.

What you're developing is a very basic antenna, one evolution up from an end-fed wire (and which avoids some difficulties of end-fed). And I think it's not entirely untrue to say that, if multiband operation with one wire antenna without a literal farm of real estate to work with is the "got to" parameter, this one is the least compromise and very likely the least fiddling. Or maybe better so say that the fiddling will be toward something more productive than accommodating other compromises, like adjusting traps. (The multiband "fan" dipole looks attractive but can be tricky to bring to resonance because of the interactions among the wires, and you're doing it pretty much just so you can coax feed it, which isn't a good reason, unless coax is another "got to" parameter.)
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N1LO
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« Reply #6 on: July 13, 2009, 11:06:12 AM »

Hi Bill,

I agree the 160m doublet (a dipole, not necessarily resonant, fed with parallel line) will do what you want.

That's how I got started. As with all antennas, there is a compromise: once your doublet is more than 3-4 waves long, the lobes of gain on the highest frequencies will become very narrow and much less omnidirectional.

I gave up 160m and for the same amount of wire, but more complexity, went with an 80m horizontal delta loop, fed with parallel line at a corner, which is an excellent, 80-10 multibander.

I haven't modeled this, but you could try a 160m doublet with a separate 10m dipole in parallel to help even out the 10m pattern. This can be rigged easily as an inverted V using a single suspension point.

You'll also need a 'current' type balun at your tuner. Built-in baluns can work, but when multibanding you may have to adjust your feedline length +/- 5-10' to get it to tune all bands.

Having different ratio baluns helps with this problem. Elecraft and MFJ both sell a switchable 4:1/1:1 unit or build your own: http://www.qsl.net/n1lo/build.htm

Good Luck,

--...MARK_N1LO...--
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WA7NCL
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« Reply #7 on: July 13, 2009, 01:09:17 PM »

If you want a good description of the trade offs, go to cebick.com and read about the 88foot dipole.  He covers the optimum length to get a good feed impedance and a bidirectional pattern.  You will not be able to get that on 160 thru 10 but you will understand the trade offs and be able to refine what to compromise.

You need to register with antennex to get access to the site.  It is well worth the time, because there is a wealth of information there and old LB covered it thoroughly.
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N8NSN
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« Reply #8 on: July 13, 2009, 01:38:30 PM »

Just finished up a project here, recently.  

Now working 160 through 10 meters with a 135' doublet.

In the interest of not taking up too much band width with E-Ham;  Have a look at my QRZ page for details on how to work 160 with a 135' doublet "efficiently".

I sent in an article on this project complete with photos and all to E-Ham and I am hoping that the article gets selected for posting soon.

73,

Jim N8NSN
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AD7WN
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« Reply #9 on: July 13, 2009, 08:21:30 PM »

The antenna doesn't really have to be resonant on any band you are working.  If you are using real open wire line (not the twinlead type with some windows cut out of the web) the line losses will be negligable, even on frequencies where the antenna is somewhat shorter than a quarter wavelength.  The only critical factor is that your transmatch have enough tuning range to transform the feedline input impedance (which will vary widely with frequency)to 50 +j0 ohms, so that the transceiver will like the load that it sees.

73 de John/AD7WN
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W4MLO
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« Reply #10 on: July 23, 2009, 08:57:20 PM »

Another vote for the 135' 450 ohm ladderline fed doublet. At Lindy's recommendation, I built one specifically for this year's field day.

From Georgia, we worked Alaska, Hawaii, 4 California's, Oregon, several Washingtons, several Montana's several Dakota's, Idaho, 2 Newfoundlands, several other Canada's, Mexico, Peurto Rico, 2 Virgin Islands, a total of 146 contacts in only 7 hours mostly on 15M and 20M, 30 something on 80M. All on phone.

I realize some of these were by the other end doing the heavy lifting but the darn thing works, period.

We were using a IC-7000 @ 80 Watts w/ a stock mike and a MFJ 993B Autotuner. The antenna was only up 40'.  It cost less than $35 including the 145' of ladderline i picked up at a hamfest.

It was the first antenna I ever built but definately not my last.

Whatever you end up with build it yourself and enjoy the fruits of your labor.

73 Milo W4MLO
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W5DXP
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« Reply #11 on: July 24, 2009, 06:43:52 AM »

The 135 foot ladder-line fed doublet is a good all-band HF antenna. Here's how I do it without a tuner.

http://www.w5dxp.com/notuner.htm

To use this antenna on 160m, I would short the ladder-line wires together and feed it as a T-antenna against a radial ground plane.
--
73, Cecil, w5dxp.com
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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.
N5LRZ
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« Reply #12 on: July 24, 2009, 07:23:40 AM »

WB2WIK is correct...

You will need a center fed half full size half wave dipole cut for the lowest frequency you intent to transmit on.  Feed the antenna with window/ladder line because this transmission line is far less effected by high standing wave ration loss compared to coax.

As to your tuner which you mentioned that you have.  IF it is one of those inexpensive mobile tuners two words: FORGET IT.  You will need a rotor that has a varable roller inductor and adequately large caps to cover the wide range of matching offsets.  Look to spend at the very least 250 on up for a half way decent tuner and 500 bucks on up for a good inductor rotor tuner (legal limit probably).
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G3TXQ
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« Reply #13 on: July 24, 2009, 08:17:30 AM »

If space is tight, and you want to cover 160m, you can go as short as 200ft with the doublet - much less and efficiency begins to suffer.

Depending on its height you may find that strapping it as a vertical (like Cecil suggested) is the better option for 160m.

Steve G3TXQ
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W5DXP
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« Reply #14 on: July 24, 2009, 09:04:24 AM »

> G3TXQ wrote: If space is tight, and you want to cover 160m, you can go as short as 200ft with the doublet - much less and efficiency begins to suffer. <

A ~202 foot dipole fed with ~66 feet of ladder-line is a double-sized G5RV good for 160m, 80m, 40m, 30m, 17m, and 12m operation with a tuner. I didn't realize the double-sized G5RV covered all the WARC bands until I ran EZNEC just now. Here's what EZNEC says about the impedance looking into the ladder-line.

1.935 MHz, 11 ohms, 50 ohm SWR = 4.6:1
3.88 MHz, 14.3 ohms, 3.5:1
7.203 MHz, 123 ohms, 2.5:1
10.125 MHz, 22.4-j54.3 ohms, 5.1:1
18.14 MHz, 43.7-j50 ohms, 2.8:1
24.95 MHz, 28+j18.6 ohms, 2.1:1

Of course, on frequencies above 7 MHz, the radiation pattern is multi-lobed.
--
73, Cecil, w5dxp.com
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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.
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