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Author Topic: 40 meter loop  (Read 1546 times)
KE5YQG
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Posts: 11




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« on: June 27, 2009, 01:40:30 PM »

I am trying to make a 40 meter full wave loop antenna for when I get my general lic. I have the antenna cut to 140 ft. with 450 ohm ladder line going to the shack and I put a 4:1 balun at shack then a short piece of 50 ohm coax to transmitter. My swr is way high using antenna analyzer shows I am tuned for 21 mhz. should I have the balun at the antenna? what is going on. I thought these antennas were supose to be easy to set up. I was up all night and spent all day in this 100 degree weather trying to get it to work. I added 6 feet of wire and 10 meters was almost 1:1 swr but 40 meters was still way off I took the wire off again and figured I would ask and see if having the balun where I got it is causing the problem before moving it in this heat only to find out it never made a difference. thanks for anybody's help on this.
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KI8DJ
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« Reply #1 on: June 27, 2009, 01:51:50 PM »

These type of antennas are usually used with a tuner so swr isn't critical,with the ladder line losses are low anyway. If your antenna is low to the ground coupling lowers the resonant frequency so your antenna was probably too long to begin with. You can cut and trim but my advise is if you have a good tuner just use it the way it is. You will never get a good match on all the bands without the tuner. No one will hear a difference If you just use it the way it is. Last of all your balun is where it should be. Have fun on hf 73 Gary ki8dj
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KE5YQG
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« Reply #2 on: June 27, 2009, 02:15:06 PM »

I have a tuner, it helps on the recieve, but will not tune any of the bands with a low swr not even close. Maybe I need a new tuner. it is a Tokyo HY-power HC-200 It tunes a all bander dipole just fine, The loop is at 40 feet I am using the all bander dipole wich is 135 feet long with 5 more feet of wire added to it. Maybe it won't tune loop antennas I don't know.
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K7UNZ
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« Reply #3 on: June 27, 2009, 03:15:39 PM »

As I read it, your balun/coax run is between the tuner and the openwire feeder, and that just may be your problem.  The antenna impedance will change every time you change frequency, and definately jump around all over the place with changes in band.  A 4:1 balun won't help much if the inpedance is 8, 10, 12, or more times the 50 ohm coax.

Try just bringing the openwire line directly to the tuner, without the balun/coax.

That will, naturally, only work if you have a tuner capable of dealing with open wire line (which is what you need for multi-band use).

73,

Jim/k7unz
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EI4GYB
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« Reply #4 on: June 27, 2009, 03:19:33 PM »

I dont see any reason why your loop should be causing so much trouble. A closed loop antenna would come in at about 100 ohms and could be fed like you say with 450 ladder via a balun to 50 ohm coax.

A few things to look at first, is the loop closed? it should be joined at both ends
Is the loop insulated correctly from the supports, at each support it needs to be isolated from any metal objects.
Is the antenna and ladder line kept away from any metal objects.
Where is the antenna fed from? one of the corners or .25 wavelength from the apex if hung vertical is usually good.

Another thing you could try is to change the matching arrangement.... a quarter wavelength (in free space) aprox 32ish made from good quality 75ohm coax, this is then directly connected to the standard 50ohm coax which goes to the radio ( no ladder line used or needed with this method). This is my perferred method of feeding a loop.

I used a 40m delta loop here and the matching as mentioned above. The SWR is 1.2 to flat accross the whole band. Loops are very broadband but often need a good deal of adjustement to get right but well worth the effort.

Have a look at the ARRL antenna handbook for some good advice and detail

This web site is very helpful

http://ka1fsb.home.att.net/loopcalc.html

Hope this helps
73
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W5DXP
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« Reply #5 on: June 27, 2009, 04:06:38 PM »

The standard way of feeding a resonant 1WL loop is 1/4WL of 75 ohm coax attached to the antenna with 50 ohm coax the rest of the way to the shack. When you deviate from that design all bets are off. You seem to be suffering from a number of amateur radio myths, e.g. a 4:1 balun always works well with 450 ohm ladder-line. Reality is much different. If you want your present system to work well, you need a balanced antenna tuner.

450 ohm ladder-line driving a 100 ohm antenna transforms the impedance all up and down the transmission line. It varies up to 2000 ohms with reactance with a subsequent sky-high 50 ohm SWR.

You can understand what is happening by downloading the free demo version of EZNEC from www.eznec.com and modeling your antenna. Otherwise, you probably need help from a good Elmer.
--
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.
KE5YQG
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Posts: 11




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« Reply #6 on: June 27, 2009, 05:51:38 PM »

Yes my loop is closed and Isolated with insulators and away from any metal. the ladder line is not near metal either. the loop is horizontal. I have tried all the shapes, square, diamond, and now triangle. the loop is closed. I guess i am going to try the matching stub. I saw a web site by Callum McCormick in the UK and it sounded as though his antenna worked great and he used a 4:1 balun at his feed point on the antenna then ran coax. That is why I thought of the balun. only I used mine at the shack. I have been trimming and 40 meters is not getting any better. I guess it's time to start from scratch and use the matching stub. I will do it tonight after the sun goes down and the kids are in bed. it will be alot cooler. just need to watch my step
  Oh and yes I have been to that web site with the calculator, and many others. I have built many antennas, even before getting my ham ticket. Yagis, quads, dipoles and mobile antennas. I have allways managed to have what I expected when finished. I figured this antenna would be an easy one, Hopefully it will be when using the stub we will see thanks for everyones help. I am having fun and will be pleased when I figure it out I am sure.
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K0ZN
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« Reply #7 on: June 27, 2009, 06:20:22 PM »

Hi,

That antenna *should* be "easy" to tune (with a good quality tuner) on all HF frequencies above 7 Mhz.

First, it sounds like you are caught up in the low SWR thing. SWR is NOT a "problem"; it is just an electrical condition on the transmission line. If you have low loss transmission line, like ladderline, high SWR is a NON-event because the line loss is low. That is NOT the case with almost all common coax which is why there are so many myths about SWR. However, you do need the appropriate matching network (e.g. an "Antenna Tuner") to create a match between the radio's fixed 50 ohm output and the antenna because the impedance and SWR will vary WILDLY between different bands. That said, it is not at all unusual to run into one band that is hard to get a match on...that is just the nature of the beast with so many HF bands.

FYI: in a mismatched condition, such as you have in your system a "balun" does almost nothing to reduce SWR. Just because you put a balun in the line doesn't "fix things".

With your system, if you can mount the balun right at the Tuner output, it would probably be best. Coax, even short lenghts can be pretty lossy at high SWR. Do NOT use small size coax in this application. Use the "real" stuff that is full sized if possible.

The "best" answer is as stated in a previous post: Use a Balanced Tuner to the ladderline. That said, LOTS of people do have good luck with a balun in the line between an UNbalanced tuner and the Balanced ladderline. It probably makes little or no difference whether the balun is 1:1 or 4:1 because, again, the impedance that the balun "sees" will jump all over the place from band to band. You can make a case for both and there are heated academic arguments over this, but the real world fact is that both "work".

Respectfully, BY FAR....the best investment you can make in your station and antenna system at this point it to buy a copy of the ARRL Antenna Book and put in a little book time picking up some basic knowledge of antennas and impedance matching. This is too long a subject for a quick blurb on an internet website. It will save you a LOT of frustration (out in the hot sun!!) and give you a better signal. Antennas are not complex, but they do have some specific parameters which must be met to work well....or even work at all in some cases! You can pick up an older copy of the ARRL Antenna Book on Ebay very cheap and the basic info is still totally accurate. You will be really glad you made the effort.

Good luck on your antenna and General ticket.

73,  K0ZN
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W5WSS
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« Reply #8 on: June 27, 2009, 09:09:38 PM »

My all band horizontal oriented horizontal polarized loop is installed indoors on my ceiling (Not in the attic)It is 88' square shaped.I feed my loop with 300 ohm Belden low loss transmission line that is soldered directly to the loop, is very short in total length from the ceiling to the wide range tuner is only 6ft. I do not use a balun for this application because the wide range tuner can handle the all band impedence variations. an 88' loop will work reasonably as an all band antenna and your height of 40' is nice. High enough to present a low enough toa for dx work. I would omit the coax and connect your antenna and feedline ladder line(balanced) to a balanced tuner(one that provides for a ladder line connection)choose a band and adjust your wide range tuner you will hear the signals increase dramatically when the tuner is in fact adjusted properly. You will notice some problematic locations within the all band hf spectrum where perhaps a perfect 1:1 may not be possible but 1:5,6,7,8,9 etc may be as low as the tuner can do...use a wide range tuner for best results. a T match tuner is good. I work decent sky wave state side and dx as well with the indoor version and only 20 watts pep the height is low at only 18 ft up. I live in an apartment with a no outdoor antenna rule so The loop works great for my adverse handicap. 73
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W7ETA
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« Reply #9 on: June 27, 2009, 09:52:25 PM »

What does your antenna analyzer indicate on 40 meters?

My MFJ unit provides more than just SWR information.

Bob
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KM6CZ
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Posts: 23




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« Reply #10 on: June 27, 2009, 11:25:47 PM »

Something is no right with your antenna... it could be the lenght of 450 ohm feedline (should not be a resonant length at 40m) or more likely a bad balun. A good 4:1 current balun is what is needed for this installation.

Loops are the ultimate in simplicity and ease in tuning. Moreover, they work great! From what you're describing, something is seriously wrong and its not likely to be the antenna wire. (bad coax, short or bad balun.)
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G3TXQ
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« Reply #11 on: June 28, 2009, 04:38:54 AM »

There seem to be a lot of misunderstandings here!!

If your loop was resonant on 40m - which it wont be if it's 140ft long - the feedpoint impedance would be around 100 ohms. Fed with 450 ohm line that would give you an impedance anywhere between 100 ohms and 2000 ohms depending on the line length. In other words you **should expect** a VSWR that is 2:1 at best, and might be as high as 40:1 - that would be perfectly normal for the antenna system you describe.

If you want to use this loop on multiple bands, fed via 450 ohm ladderline, you will have to use an external tuner.

Steve G3TXQ
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KI8DJ
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Posts: 190




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« Reply #12 on: June 28, 2009, 07:15:10 AM »

I think maybe the issue is your tuner,my cheap mfj 948 will tune a forty meter loop much like you describe even on 160 meters, higher bands tune really easy. See if you can borrow a different one. The tokyo highpower is a very high quality unit but it still may have a limited matching range.
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KI8DJ
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Posts: 190




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« Reply #13 on: June 28, 2009, 07:15:17 AM »

I think maybe the issue is your tuner,my cheap mfj 948 will tune a forty meter loop much like you describe even on 160 meters, higher bands tune really easy. See if you can borrow a different one. The tokyo highpower is a very high quality unit but it still may have a limited matching range.
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KI8DJ
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Posts: 190




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« Reply #14 on: June 28, 2009, 07:21:24 AM »

If I am not mistaken your tuner is an automatic tuner. There is the problem,no auto tuner has the tuning range of a manual other than some of single wire types like an sgc or icom ah-4. I have had some supposedly wide range ldg units and they had nowhere near the matching range of my mfj.
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