View Full Version : homemade dipole?
KD8IIS
06-11-2008, 10:20 AM
If my SWR is too high on a homemade dipole do I shorten or lengthen to tune?
KI4POT
06-11-2008, 10:50 AM
It depends on whether it is too long or short to begin with. High SWR at one frequency doesn't tell you much. What does the SWR do if you move down a bit in frequency? If the SWR drops, then your dipole is too long. If it goes up even more, it's too short.
Chris
K4LSX
06-11-2008, 10:59 AM
How high is too high?...that is quite subjective...
but in any case, your question can't be answered based upon the data you provide...there are several things that could cause your problem, in addition to the antenna length - such as bad transmission line, bad connections, etc.
But - assuming that there are no problems with the aforementioned, you must determine if the SWR is worse at higher or lower frequency...or conversely, is it better at higher or lower frequency - in the same band of course. Once you determine where it appears better, then you can determine whether to lengthen or shorten the dipole. A better approach is to get your hands on an analyzer and determine where it is absolute best then go from there.
If the better SWR is at the high end of the band that the dipole was made for, then you need to make the dipole longer. If the best SWR is at the low end of the band, it needs to be shorter.
SWR too high is somewhat relative - if it is not horrible, you can just use a tuner and make the radio see a good match and forget it.
I use a non-resonant vertical on most bands - the VSWR is obscene without a tuner...but with the tuner the radio is happy and so am I - work just about any station I want to.
You might also benefit from reading a good book - perhaps the ARRL antenna manual. You can have a lot of fun building and experimenting with antennas - build it, determine why it doesn't work, solve the problem, get bored with it and build something new as a challenge. I haven't read the ARRL Antenna book in years, but it was once upon a time quite decent. Don't waste your time with highly technical books such as John Kraus' Antennas unless you have a good background in math and are truly interested in field theory and other such esoteric material - good stuff, for sure - but a very long path to fixing your dipole problem.
Good luck.
KB4UHK
06-11-2008, 03:21 PM
Doug Demaw had a good book called "W1FB's Antenna Book" or "Book of Antennas" or something like that. I don't have it anymore.
I would agree with everything John above me just said. Find out where the dipole is most resonent. Got to be somewhere.
If you are testing it out at QRP levels, you can use some extensions with alligator clips as a makeshift test of how much longer it needs to be. Hopefully your dipole is not 80 feet up in a tree.
Just cut it, no harm no foul ;)
GQ, that really isn't such good advice. If the dipole is already too short, cutting it will just make the condition worse.
It is always good to use the formula 468/freq (mhz) and add a few inches ( feet in the case of the lower bands ) to start with. Then you can almost be sure the antenna is long with to start. After measuring the actual result, THEN and only then should you consider cutting the wire shorter!
I can remember back to my days as a beginning ham. I worked on a 40/15 meter dipole a whole afternoon before I got it correct. Now days, I can put up a new antenna inside a half hour! And I'm a heck of a lot older, and less spry than I was back then !
73, Jim
It depends!
First of all, how high is too high? If the SWR is less than about 2:1 or so, then you probably don't have to worry about it a great deal. If you want, just throw away the SWR meter and get on the air. It's good enough.
If it's higher than that, you first need to determine whether it's too long or too short. Let's say you are making a dipole for 10 meters. You start out with the formula 468/f. So if you want the antenna for 28.4 MHz, then the total length will be 468/28.4 = 16 1/2. In other words, you want 8 feet 3 inches either side of the center insulator.
If your SWR is too high on the frequency where you want it, you first need to determine whether it's too short or too long. The easiest way to do this is to find the SWR on various frequencies and see where it is lowest. If you are above 2:1, then chances are, this frequency is not in the amateur band. But you can still figure out if it's too long or too short.
Check the SWR at various frequencies where you are allowed to transmit. So if you are a technician, I would check it on various frequencies between 28.001 (CW, and you'll have to ID in CW) and 28.497 (if you ID in SSB, don't get any closer than this to the top of the band).
At which frequency is the SWR lowest? If it's 3:1 on 28.001, but 4:1 on 28.497, this means that it's 1:1 at some frequency below 28 MHz. This means that the antenna is too long and will need to be cut. If it's 4:1 on 28.001, but 3:1 on 28.497, this means that it's 1:1 at some frequency above 28.5 MHz. This means that the antenna is too short, and will need to be lengthened.
Just add or cut a few inches at a time, and then repeat the process. There's no need to get too obsessive. If you get it below 2:1, it will work just fine.
I just add "clip leads" to the ends of the dipole and check the low VSWR frequency again. If it is near the frequency desired, then I know it needs to be longer. If not it needs to be shorter. No soldering or cutting is needed when using clip leads for your test. They can hang down from the ends and do not need to be in the same plane for this test. The clip lead length will vary for different bands. I use about a foot long lead for 40 meter antennas.
An antenna analyzer will tell you if you need to be shorter or longer, however I assume that you only have a VSWR meter for this project. Many antennas have been successfully built with only a VSWR meter for test gear.
Happy building.
73,
Terry, K7FE
EI8DRB
06-11-2008, 04:42 PM
There's no need to get too obsessive. If you get it below 2:1, it will work just fine.
Who you calling obsessive? It's got to be 1:1 every time! ;)
W4INF
06-11-2008, 05:10 PM
Who you calling obsessive? It's got to be 1:1 every time! ;)
1.5:1 here and Im happy. ;)
Your correct Jim, I was just trying to be goofy, :D
GQ, that really isn't such good advice. If the dipole is already too short, cutting it will just make the condition worse.
73, Jim
K4LSX
06-11-2008, 08:27 PM
and radiator stretchers are gettin 'wicked hahd' to find these days!:D
Your correct Jim, I was just trying to be goofy, :D
Sorry if I sounded "anal", OM, but sometimes "goofy" advice is taken seriously by some "newbies", and even us O.F.s !
;) :o
73, Jim
K8YZK
06-11-2008, 09:55 PM
If my SWR is too high on a homemade dipole do I shorten or lengthen to tune?
Yes to both, but it's according to what band you are trying to build the dipole for.
My suggestion is to go to AC6V.com and take a look at the info on making a dipole. A lot of good information.
73
Kurt
G0GQK
06-11-2008, 10:07 PM
Amazing ! What an answer ! It depends whether its too long or too short in the beginning ! Simple answer, if the frequency is too high, you lengthen it , if the frequency is too low, you shorten it.
G0GQK
K0RGR
06-11-2008, 10:57 PM
Perhaps, our writer doesn't know how to find the resonant point for dipole?
First, to check your SWR, your antenna needs to be up in the air where it will operate normally, and away from metal objects. Then, you need to check the SWR at various points in the band.
You may find that there's a point in the band where the SWR is lowest - if so, you are lucky enough to have cut the antenna to actually resonate in the band. Most likely, though, you will just see that the SWR is lower at one end of the band than the other, indicating that it is actually cut for a frequency higher or lower than the band. This is where an SWR analyzer really helps, because you can see the resonance no matter where it is.
If the SWR appears to be better at the low end of the band, the antenna is probably too long. If it appears to be better at the high end, it's probably too short. However - be certain that your transmitter is putting out the same amount of power at each of the test points. The circuit that rolls back your output in the case of high SWR can make the SWR readings look better than they are. If the transmitter is shutting down on one end of the band and not the other, that means that the SWR is worse on the end where the transmitter is shutting down.
BTW, SWR measurements should ALWAYS be taken at the minimal amount of power necessary to get a full "forward" reading! This is not because the SWR changes as power changes, BUT the "foldback power" feature of just about every modern rig will cause all sorts of confusing results.
If you start out with a power of only a few watts ( my rig drops down to a minimum of 5 ) there is no or at least an extremely small chance for the foldback feature to kick in and confuse the issue.
I am convinced that so many reports of "SWR changes with power" are not being caused by a real condition ( some could, but not most ) but rather by the foldback feature interfering with the test.
Something to think about !
73, Jim
W4HAY
06-12-2008, 01:34 PM
A split-bolt connector (http://www.farwst.com/fwst/cable/servit.htm) at each end of the antenna makes adjustment a lot easier. None of that cutting, twisting and soldering foolishness. Just cut the antenna a few feet longer than the 468/f formula suggests, then fold the surplus back through the insulators and clamp with the split bolt.
If you change locations or your interests switch from SSB to CW or vice-versa, retuning is much easier.
Most electrical supply houses carry them. Not sure about Lowe's or Home Depot though.
KA9VQF
06-12-2008, 02:01 PM
A split-bolt connector (http://www.farwst.com/fwst/cable/servit.htm) at each end of the antenna makes adjustment a lot easier. None of that cutting, twisting and soldering foolishness. Just cut the antenna a few feet longer than the 468/f formula suggests, then fold the surplus back through the insulators and clamp with the split bolt.
If you change locations or your interests switch from SSB to CW or vice-versa, retuning is much easier.
Most electrical supply houses carry them. Not sure about Lowe's or Home Depot though.
My local Home Depot and Lowe’s have them in the electrical department. So does Ace, True Value, Farm and Fleet, Paul’s discount, Target, even our local Radio Shack has them with a shiny gold plating on them in their ‘monster killer’ mobile audio department, right next to the oxygen free heavy duty speaker wire.
OHHHH , I GOTTA get some of that oxygen free heavy duty speaker wire to build my next antenna ! ! You think I can make the DXCC with the stuff ? ? ?
;) :p :D
KA9VQF
06-12-2008, 02:30 PM
With out a doubt Jim.
You don’t even need a radio if you use that stuff do you?
To my mind copper is expensive enough but putting what I consider a bogus label on it and selling it for three times the market value seem to be working for many retailers.
Don’t get me wrong, I enjoy the subwoofers in my car and occasionally do turn it up a bit more than is needed but many of the younger people I know are way over kill with their “systems”.
In my opinion they suffer from the dreaded “more money than brains” disease. Some actually read the spec’s of these systems and believe it completely. I’ve seen some of the blogs dedicated to ‘Pro audio’ and usually have a pretty good laugh at them.
If you want a good laugh, be sure to pick up a copy of "Stereophile" magazine! 10K dollar amplifiers, tuners, TURNTABLES, etc. etc. They even have FUSES that have gold plated end caps and probably the wires inside too, and cost maybe $20 a pop !
You will hear all kinds of superlatives being lauded by the authors of the articles. If such hyperbole' were used to push the current brands of radios, you would think that DXCC would be possible in one evening's worth of work ! ( oh, you mean it ISN'T ? ? ? )
73, Jim
KL7AJ
06-12-2008, 05:26 PM
If my SWR is too high on a homemade dipole do I shorten or lengthen to tune?
The answer is....
yes
KL7AJ
06-12-2008, 05:31 PM
BTW, SWR measurements should ALWAYS be taken at the minimal amount of power necessary to get a full "forward" reading! This is not because the SWR changes as power changes, BUT the "foldback power" feature of just about every modern rig will cause all sorts of confusing results.
If you start out with a power of only a few watts ( my rig drops down to a minimum of 5 ) there is no or at least an extremely small chance for the foldback feature to kick in and confuse the issue.
I am convinced that so many reports of "SWR changes with power" are not being caused by a real condition ( some could, but not most ) but rather by the foldback feature interfering with the test.
Something to think about !
73, Jim
Or you can use an attenuator. I have one in the form of 300 feet of really crappy rg/58. Easily dissipates a kilowatt without even a load connected.....and still shows a perfect 50 ohm match! :)
If you want a good laugh, be sure to pick up a copy of "Stereophile" magazine! 10K dollar amplifiers, tuners, TURNTABLES, etc. etc. They even have FUSES that have gold plated end caps and probably the wires inside too, and cost maybe $20 a pop !
You will hear all kinds of superlatives being lauded by the authors of the articles. If such hyperbole' were used to push the current brands of radios, you would think that DXCC would be possible in one evening's worth of work ! ( oh, you mean it ISN'T ? ? ? )
73, Jim
Jim,
I call your $20 fuse, and raise you a $485 KNOB.
http://www.boingboing.net/2005/11/07/astronomically-overp.html
I am so in the wrong line of work :(
Rege
I'll just HAVE to send this URL off to a friend of mine! He and I have shared laughs over the YEARS at some of the totally off-the-wall items that "audiofools" have shelled out beaucoux bucks for !
"an expensive amplifer for better sound" 10K + dollars !
" a set of wooden knobs to make it sound EVEN BETTER " Priceless ! ( almost ! ) :eek: :rolleyes: :p :D
WR5AW
06-14-2008, 09:26 AM
Call me a fool. But I'll put my Sennheiser studio monitors (phones) against (insert favorite ham brand) headphones any day. Awesome sounding set of phones. So maybe they sound a bit better listening to music. But what the hell. Gotta splurge once-in-awhile yanno. I'm still templed to pick up that $34 grand McIntosh tube amp some day. Which is a bit, as the engineers say, "foolish" considering by the time I can afford it, the ears will be shot anyways.
KD8IQO
08-11-2008, 02:27 AM
If my SWR is too high on a homemade dipole do I shorten or lengthen to tune?
Hi Brian
Ya know who I am. When you get done moving I will come down with my 'tenna 'lizer and tune your dipole for ya. You have to find the resonate frequency to know if you need to shorten or lengthen the antenna.
73's
Ron
KD8IQO
:):rolleyes::)
KB3LAZ
08-11-2008, 03:01 AM
Who you calling obsessive? It's got to be 1:1 every time! ;)
Straight up!;)
KL7AJ
08-11-2008, 04:18 AM
If my SWR is too high on a homemade dipole do I shorten or lengthen to tune?
The SWR on a dipole is infinite.
W0WLS
08-13-2008, 12:08 PM
Brian,
Everything in your setup will effect SWR. Is it near metal objects? Is it straight or slightly inverted? How close to the ground is it? Even the much debated, how long is the coax? Are you using balanced line or coax? I'm assuming you have a standard wire dipole with coax feedline. As mentioned earlier, just use the formula 468/freq. - each leg will be half that length. Cut it a little long for trimming. Just remember, higher freq need shorter antenna. Lower freq need longer antennas. Since you're a tech, I'm also assuming you are building a 10m, 6m or 2m antenna. A small length trim on these higher freq antennas goes a long ways. 10m trimming should only be done about 1/2" at a time. 2m trimming only 1/8". Antenna tuning takes time and patience. Once you've done a few, it gets easier. Good luck, hope your friend shows up with the analyzer.
73
Jeff
[QUOTE=wr5aw;1252353]Call me a fool. But I'll put my Sennheiser studio monitors (phones) against (insert favorite ham brand) headphones any day. Awesome sounding set of phones.
Along these lines.....If I plug in Boze noise-cancelling phones into my TS520, will they eliminate all the lightning static on 80M? Will they faithfully reproduce the 500-700 hz tones from the CW I listen to ?????:confused: