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160 METER DIPOLE USE ON MULTIPLE BANDS?

Aug 9th 2011, 13:07

N8CHR

Joined: Apr 4th 1998, 00:00
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I have a question about a 160 meter dipole. I want to use 50 ohm coax for feed or since I have a lot of it, maybe 75 ohm coax.
My question; if I make a dipole for 160 meter @ full length, is 253 feet or there abouts. Can I use it on the other bands with a tuner? I have a Kenwood TS-450 with auto tuner will it tune? I have 80 ' plus trees all round me and I want to try and get something up in them.
Thanks Tom N8CHR
Aug 9th 2011, 14:21

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
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You can feed the dipole on other bands with a tuner. On some bands, it will work fairly well. On other bands, it may work, but the losses on the feed line may be higher than you would like, while on some bands, depending on the matching range of your antenna tuner, you may not be able to find an adjustment of the antenna tuner that will provide a 50-ohm load to your transmitter.

When a feed line is terminated in its characteristic impedance -- 50 ohms in this case, the voltage and current along the line have a 50-ohm relationship at all points. All of the power delivered to the load is accepted by the load. If the load is an antenna, the power is either radiated or, depending on the antenna, may be lost as heat in the antenna if the antenna is lossy.

However, if the feed line is not terminated in its characteristic impedance, some of the power is delivered to the load, with the remainder of the power returned back down the feed line. The interaction between the forward power and reflected power creates what is termed a standing wave on the feed line, with some points having a voltage/current ratio greater than 50 ohms and some points having a voltage/current ratio less than 50 ohms. The ratio beween the voltage or current maximum point along the line and the voltage or current minimum point along the line is called the standing wave ratio, or SWR.

When the SWR increases, because power is sloshing back and forth along the feed line before it is radiated, that power is subject to the feed line losses multiple times, so the losses of a feed line increase as the SWR on that line increases. If a line is very low loss, such as open-wire line, the increased losses due to a high SWR will still be very low. If a line is lossy -- thin RG-174 coaxial cable, for example -- the increase in SWR can increase the loss substantially. Naturally, the longer the feed line, the more loss you will see. A "high" loss of 10 dB/100 feet will be only 1 dB if the feed line were 10 feet long.

Larger coaxial cables tend to be lower loss than thin ones, so if you are going to do this, I recommend that you use RG-8 size coax (RG-8, RG-213, Belden 9913, etc.). The rule of thumb I apply on HF for RG-8 is that if the SWR on the line is 5:1 or better, I can match it with an antenna tuner and generally have acceptable losses. For short runs of feed line, the SWR can be somewhat higher.

There are also losses in your antenna tuner, although these can be difficult to predict. The ARRL Antenna Book disk has a TLW.exe program that can be used to predict feed line and tuner losses.

You also need to consider the power ratings of the feed line and antenna tuner. At 100 watts, typical on HF, your feed line should be able to handle the load presented by the antenna on any band, and a 300-watt tuner would probably suffice. However, on some bands and for some lengths of coaxial cable, your tuner could see an impedance that is outside its ability to match, or at least to match at full power.

With a standing wave on the feed line, the voltage/current ratio varies along the line, from points of high impedance to points of low impedance. So the actual impedance presented to your tuner will vary with the impedance of the load, the impedance of the feed line and the length of the feed line. You can do one of two things -- you can carefully analyze this on all bands, using TWL.exe or other tools, or you can put it up and see how many bands work for you. On a few bands, you may lose an S unit or two, but most of the time, it works out pretty well.

The ARRL Antenna Book (any edition) and the ARRL Technology page on Antennas has a lot of good tutorial articles. You can also do a Google search on SWR or Antenna Tutorial for additional articles.

73,
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator
73.
Aug 9th 2011, 18:26

W1VT

Joined: Apr 4th 1998, 00:00
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Modeling the 253 ft dipole at 80 feet with 100 ft of LMR 400 transmission line results in line losses of 6, 6, and 4 dB on 3.8, 7.2, and 14.2 MHz. This is pretty significant on SSB--not so much for casual CW and PSK31.

Zack Lau W1VT
ARRL Senior Lab Engineer
Aug 9th 2011, 18:53

N8CHR

Joined: Apr 4th 1998, 00:00
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Thanks for the info. I guess if I'm going to try to get a wire up 60 feet into a tree for 160 , I might as well try to figure out how to get 3 or 4 wires up there and put up a multi-band dipole.
Aug 10th 2011, 10:34

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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If you can run ladder line for all of that run, or a good portion anyway, you can cut down the losses substantially. While 4 to 6 dB is a fair amount of loss, it could still be a better-than-no-antenna choice for some.

A few of the articles on ARRL's "How Antennas Work" page may be helpful.

ARRL's Mike Gruber, now W1MG, wrote an article about multi-band antennas. The Technology page on feed lines has a number of other articles that you may find useful, as well.

73,
Ed, W1RFI
ARRL Lab
Technical forums moderator
Aug 10th 2011, 11:29

W0BTU

Joined: Apr 4th 1998, 00:00
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I suggest that you forget about a dipole on 160, unless you can get it up VERY high.

Put up a vertical (or inverted-L) instead, with a good radial ground system under it. That's what the loud stations on 160 use. And that's what they use on the AM broadcast band, which has similar characteristics to 160. (No AM broadcast station uses a dipole.)

http://www.w0btu.com/160_meters.html

73, Mike
www.w0btu.com
Aug 10th 2011, 16:09

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
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Quote by W0BTU
I suggest that you forget about a dipole on 160, unless you can get it up VERY high.


I agree. It's funny how many of us were so focused on the impedance, SWR and loss issues that we missed the obvious point that a low horizontal dipole on 160 meters is pretty much a regional-coverage antenna at best.

To me, this points out the value of these forums, because when a member asks a question of HQ staff, he or she will receive an answer that is written from one perspective. But here, not only have members often received answers from multiple people, each of those answers has built on the previous answers, and by having members answering other members' questions, those asking questions are being better served.]

Not only that, but not only does the member asking the question benefit, other members with an interest in the topic get to read the answer, too.

I like this approach a lot. I look forward to additional knowledgeable hams joining these discussions.

73,
Ed Hare, W1RFI
ARRL Lab
Technical forum moderator
Aug 16th 2011, 20:08

KB0HAE

Joined: Apr 4th 1998, 00:00
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Hi Tom. I say try the 160m dipole fed with coax. You will probably need an external tuner to use it on some other bands. The internal tuner in the TS-450S will probably not tune or match an SWR over 3:1. To use ladder line to feed an antenna, you will need a balun at or near the radio. Many external tuners have a built in 4:1 balun. Ladder line must also be kept a certain distance from metal objects. I seem to remember 6-12 inches.

Other ideas for wire antennas could include an off center fed dipole, or a horizontal or vertical loop. I use an 80m dipole with an MFJ 949D tuner. It works surprisingly well for the center only being 17 feet high. All antennas are a compromise for multiband use, don't let others talk you out of trying what you want to do. Personally, if I could get a wire up at 60 feet, I would like to try an OCF dipole.

ttyl
Aug 17th 2011, 04:13

N0NB

Joined: Apr 4th 1998, 00:00
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Quote by W1RFIAdmin
Quote by W0BTU
I suggest that you forget about a dipole on 160, unless you can get it up VERY high.


I agree. It's funny how many of us were so focused on the impedance, SWR and loss issues that we missed the obvious point that a low horizontal dipole on 160 meters is pretty much a regional-coverage antenna at best.


You write that as though that's a bad thing, Ed. ;-)

Actually, my primary use/value of 160m is as a regional band. Yes, I'm aware that DX is possible (even worked my first out of W/VE DX during the last ARRL 160m 'test) on the band. It's all a matter of priorities.

My doublet is a center-fed 3/8 wave on 160m up about 25 feet. No, I'm not going to work or even hear much from outside north America on that wire even on the best of nights, but that's not my interest. I use 160m primarily as a local ragchew band as 80m has gone too "long" on winter nights for local coverage the past several years. I feed it with 450 ohm window lead via a Palstar AT1500DT tuner and it works well. As it is a 3/4 wave center-fed doublet on 75/80m it works very well on that band plus 40 and 30m as well.

Off topic a bit, but over the years it seems there has been a bias towards the DX performance of antennas (nohing wrong with that) while local/regional coverage antennas seem to have red headed step child status. I found out quickly when starting out in the mid '80s that a vertical did not let me work Kansas Slow Speed CW net on 80m very well. However, I put up a so-called "cloud warmer" in 1985 that was maybe 15 feet up and it was like a new world opened up to me. Stations nearby literally boomed in as a result. This from an antenna that the manuals sternly intoned that it was not high enough for DX. Well, that was the point! I didn't need to work DX, I wanted to engage in the local HF nets. Since then the NVIS term has been applied to such antennas and their value is more accepted especially in these times of emergency preparedness.

My point is, decide on your operating priority and build your antenna accordingly.

73, de Nate >>

http://www.n0nb.us

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