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Author Topic: Proper un-un for random length antenna  (Read 3919 times)
N2YEG
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Posts: 1




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« on: September 05, 2012, 06:02:57 PM »

Moved into an antenna restricted community (xyl's choice, not mine) and would like to attempt a random length/longwire antenna.  I have about 50-75 feet to work with, but I'm confused about the impedance/proper un-un.  Reading the 'net, I see a random/longwire rated both 450 ohms, requiring 9:1 un-un or 250 ohms, requiring a 4:1 for random length antennas.  Any help gratefully appreciated.  Thanks.  
« Last Edit: September 05, 2012, 06:05:54 PM by N2YEG » Logged
K0ZN
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Posts: 1563




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« Reply #1 on: September 05, 2012, 07:02:56 PM »

Hi.

If you are end feeding a random 50 to 75ft. wire, the (feedpoint) impedance at the end of that wire will vary RADICALLY by band. It could could be 5 ohms on 160 or 80 and several thousand on some of the other bands. The feedpoint impedance at the end of that wire is related to the length of the wire as a part of a wavelength and the frequency.  Bottomline:  there is no UNUN or any other FIXED value device that will allow multiband operation.  You have to have a variable matching network ("Antenna Tuner"). You *could* put some kind of UNUN or Balun (connected to the end fed wire) at the end of a piece of coax and run coax back to the shack and subsequently to a Tuner, but that has pro's and con's too. The SWR on the coax and in the balun or UNUN will be sky high on most bands in that situation. Another very good solution is a remote auto tuner at the antenna feedpoint and then run coax to the rig. In this case the SWR on the coax will be quite low.

Note that a Balun or UNUN is a FIXED RATIO transformer, however, a lot of people "use" it in a highly mismatched situation feeding various antennas on multiple bands.....and, depending on a lot of variables, it may work OK or may not. This is kind of "forcing" the balun to do a job that it is not really intended for, but, in reality, often does work. The price paid in that setup is often a significant power loss in the balun core since the balun is not operating anywhere near its design parameters. The other side of this set up can also be very high SWR on the coax and high losses there too depending on the length of the coax. It is not at all an optimal situation, but it is used fairly commonly with various levels of success. This is probably what you saw 'on the net'.

Caution: there is a HUGE amount of misinformation and myth out there on the "net".  Go to a known ACCURATE source of information on antennas such as the ARRL Antenna Book.   You will save yourself a lot of frustration and problems by using credible published sources.  

Most respectfully:  If you are in an antenna restricted QTH, you absolutely need to get a good handle on the basics about antennas. A restricted QTH almost always requires a unique antenna, designed to fit the circumstances and HOA situation and physical availabilities. This means you will most likely have to homebrew an antenna and know what kind of matching network (tuner), ground or counterpoise or balanced system you CAN install that is likely to work WELL. That all takes knowledge. Antennas are not particularly complex or difficult to understand, but they also are not forgiving of assumptions, guesses and misinformation. To work efficiently and well, certain parameters must be met; it is just the laws of physics. Again, and I mean this in order to help you: the absolute best investment you could make in your ham station at this point is to pick up a copy of the ARRL Antenna Book and put in several hours of study on basics and various wire antennas and how they could be fed/matched and worked at your QTH. You can pick up an older copy off of Ebay quite cheaply and the basic info in the older books is just as accurate as the new ones. Restricted QTH's can be a challenge, but armed with some good basic knowledge of antennas it is usually possible to come up with a system that is EFFICIENT and will yield a good signal, which is of course, more fun.  Good Luck.

73,  K0ZN

« Last Edit: September 05, 2012, 07:33:18 PM by K0ZN » Logged
W5WSS
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Posts: 1783




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« Reply #2 on: September 05, 2012, 07:04:20 PM »

I would use a 1:1 unun current balun on the output side of the network  since the antenna needs a network tuner place both "at the antenna". Back to that in a minute.
 In this case I mean to say locate them (The tuner and balun) where the end of the wire just before it enters the shack if not possible just after it enters the shack in either case this technique allows one to place the equipment farther away from the antenna entry where more radiation is present. Because One can use a longer run of  coaxial cable via the matched side of the tuner to facilitate better placement of the equipment and you as far away as possible. You should seriously consider connecting the proper rated high power 1:1 current balun to a counterpoise wire as follows: Connect the antenna lead to the terminal marked + and the -lead to an elevated counterpoise wire that runs parallel and directly underneath the main antenna wire that goes upward. There is no need to make it any longer than the main antenna wire however one should keep it about 3ft off of the ground for better results.This is as suggested by W8ji and he may have further thoughts on the subject.I have done so from a window with satisfactory results. Keep the counterpoise wire ground isolated meaning do not connect the counterpoise wire back to an earthing ground. Good project 73
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W5WSS
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« Reply #3 on: September 05, 2012, 07:15:47 PM »

I use a wander lead apx 4ft long to a nearby ground rod when I disconnect the antenna. This is an 8ft ground rod with an attached clamped wire on the rod end and at the antenna base end an attached crimped and soldered ring terminal to a bolt junction. This diverts the antenna circuit to an outside earthing safety ground when one is away or bad weather. Unplug evrything from the wall outlet.
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WB6BYU
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« Reply #4 on: September 05, 2012, 07:26:27 PM »

That's because there is no such thing as a "proper" un-un for a random length wire.

There may be cases where one can be used, but it certainly isn't a requirement.
I've never used one with an end-fed wire of any sort, and I've used quite a number
of such antennas over the years.

The impedance of an end-fed wire varies widely depending on the length, frequency,
wire size, shape, and ground connection.  For example, a 20' wire fed against a
good ground system would look like about 9-j400 ohms on 7 MHz, 36 ohms at
1/4 wave resonance (around 12 MHz), 700+j1000 ohms on 15m, about 2500 ohms at
half wave resonance (23.7 MHz) and 250 - j800 ohms at 29 MHz.

The SWR relative to 450 ohms is never better than 5 : 1 across the whole HF range.
At 200 ohms it is slightly better - it gets as low as 4.8 : 1 at 13 MHz.

The reason why an un-un is sometimes used is because the range over which the
SWR is better than 10 : 1 is wider:

50 ohms:  4 MHz below 10 : 1
200 ohms:  9 MHz  "  "
450 ohms:  17 MHz  "  "

However, these occur over different frequency ranges for the same wire length:  the
higher impedances tend to give a better match at higher frequencies.  (And that ignores
the changes in the radiation pattern that affect how useful such an antenna is on those
frequencies.)

So if you have to feed such an antenna through a long length of coax cable, an
un-un may help to reduce losses due to SWR in the feedline.  But you really need
to run the analysis for your specific wire length, coax type and length, and bands
of interest, as there are a lot of tradeoffs.  Oh, and many un-uns don't give the
expected transformation ratio over a wide range of impedances, so that makes it
even more difficult to predict.


In my experience, the best way to feed a random-length end-fed wire antenna is
with a tuner at the feedpoint, fed against a good ground system.  (The latter is
important.)  Sometimes I've brought the end of the wire into the shack to a tuner
beside the rig, sometimes using some sort of switched remote tuner to match the
antenna - at least to some extent - at the feedpoint and done final tuning at the
rig if needed.  Generally I avoid operating coax at a high SWR, but that can work
also, as long as you make sure the losses are acceptable.  Actually my current
end-fed wire comes in though the wall via a twisted pair, then splices into a short
coax jumper to the tuner, but it is a carefully chosen length, and includes some
series capacitors at the feedpoint to resonate it around 3.9 MHz so I can operate
75m without a tuner.  (You can do that when the electrical length is an odd
multiple of 1/4 wavelength:  my first 75m antenna was a quarter wave wire
plugged straight into the back of my rig.)

I've also used manually-switched tuners at the feedpoint, adjusted for each band
and selected with a big switch, as well as a similar setup utilizing a stepping relay
from a slot machine.  These approaches require some experimenting to set them up,
but keep the feedline losses low.  Of course, you can also use a remote autotuner
at the feedpoint:  I have one of those out in the barn to try out when I get it
repaired, but haven't had a need for it yet.


So there really isn't a "proper" un-un that works with any random wire length, and
often the best approach is not to use one at all.  Each installation requires
evaluation of the feedpoint impedances, potential coax losses, impedance that
the tuner will have to match, and your personal tradeoffs for performance on one
band vs. another.
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G3TXQ
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« Reply #5 on: September 06, 2012, 06:03:23 AM »

I made some measurements on the effect of using various UnUns on an end fed wire; you may find some of it of interest:
http://www.karinya.net/g3txq/unun/

I'll repeat the conclusions here:

*An UnUn can significantly reduce the range of SWRs on the coax feedline - and therefore reduce feedline losses - on a majority of bands
* All four of the UnUn designs tried produced a broadly similar SWR reduction, but with this particular length of antenna the 9:1 UnUn had marginally the best overall performance
* UnUns can be lossy, particularly at low frequencies where the antenna is electrically short, and small cores can easily be overheated at quite modest transmit powers
* Lossy ferrite mixes are a poor choice for this application
* Typical UnUns wound on iron dust cores are far from being ideal impedance transformers at low frequencies when handling the reactive loads represented by electrically short antennas. Any theoretical analysis which assumes they produce 4:1 or 9:1 impedance transformations is suspect


73,
Steve G3TXQ
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WX7G
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Posts: 6327




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« Reply #6 on: September 06, 2012, 10:52:29 AM »

Since you will be feeding the random wire from an antenna tuner against ground no unun is required. Run the wire to the tuner RF output post and connect an RF ground to the tuner ground post.

If you experience a "hot" chassis (RF burns) on the tuner/rig, the tuner antenna wire and ground wire can be run together through a ferrite core or a 1:1 balun then connected to the antenna and ground.
« Last Edit: September 06, 2012, 11:01:01 AM by WX7G » Logged
K3AN
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Posts: 787




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« Reply #7 on: September 06, 2012, 04:22:10 PM »

Install the random wire as an inverted L, and feed it at ground level with a remote autotuner (MFJ, SGC, etc.). You will need some radials but they can be buried and therefore out of sight.

The 18 gauge black insulated "stealth" wire sold by The Wireman and others is pretty darn invisible to non-hams even though you will be able to plainly see it. Put the wire up very early on a weekend morning, or any workday if your nearest neighbors all work during the day.

Get to know your neighbors and be real neighborly toward them before you put the wire up, and as long as you don't QRM any of their home electronics they might be willing to ignore the wire if they see it.
 
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W8JI
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« Reply #8 on: September 06, 2012, 05:41:26 PM »

Moved into an antenna restricted community (xyl's choice, not mine) and would like to attempt a random length/longwire antenna.  I have about 50-75 feet to work with, but I'm confused about the impedance/proper un-un.  Reading the 'net, I see a random/longwire rated both 450 ohms, requiring 9:1 un-un or 250 ohms, requiring a 4:1 for random length antennas.  Any help gratefully appreciated.  Thanks.  

In order to not have 25 answers, I have 5 questions:

What can you use for a counterpoise or ground system?

How far will the feedpoint be from the shack?

Do you have a tuner, or can you buy one?

How much power do you run?

What bands do you want to operate?

 
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