Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Antenna tuning..SWR and reactance vs. resistance  (Read 3887 times)
KG6WHN
Member

Posts: 48




Ignore
« on: August 06, 2009, 02:45:12 PM »

Everything I've read so far about antenna tuning focuses
on getting a decent SWR, but when I last tuned my vertical
I was looking at the reactance and trying to min that as
well.  In other words I was trying to get the antenna input
impedance as close to 50 + j0. Ohms as possible.  Is this
overkill and do people just focus purely on SWR?
Logged
W6OU
Member

Posts: 186




Ignore
« Reply #1 on: August 06, 2009, 02:56:04 PM »

Striving for an impedance of 50 + j0 ohms is the same as striving for a 1:1 VSWR in a 50-Ohm system. VSWR meters are more commonly available than impedance meters so most hams strive for minimum VSWR.
Logged
KG6WHN
Member

Posts: 48




Ignore
« Reply #2 on: August 06, 2009, 03:02:27 PM »

True, but I was wondering if it's pointless to bother with trying to minimize the reactive bit of the impedance.  When I tune I often notice multiple dips of SWR and sometimes the low SWR doesn't always correspond to a low reactance too.  Then again, last time I tuned my antenna I didn't realize that (although it's easier) it's not quite as accurate to tune them when they're horizontal than when vertical in the mount.
Logged
WB6BYU
Member

Posts: 13168




Ignore
« Reply #3 on: August 06, 2009, 03:24:07 PM »

It depends on what sort of antenna you are tuning.  For a
simple dipole or quarter wave vertical you should only
find one SWR minimum near the expected point, and it will
be very close to resonance.  In this case you only have
one adjustment - the length (ignoring the ground system
for the vertical) which affects both R and X.  But close
to the desired point the X varies much faster than R.  In
that case, tuning for minimum SWR is as good as anything
else.

Matters get more complicated when you have something like
a gamma match with 3 variables (element length, gamma length,
and capacitor setting).  I've seen folks get way off when
they sweep the band with an analyzer and try to move the
peak response around.  Much better to keep the analyzer
on the desired frequency and adjust for the best match
there.

Besides, in most cases the transmitter is not right at
the antenna feedpoint:  there is a transmission line
between them.  Adjusting for minimum SWR means the load
looks best at the other end of the transmission line.
Adjusting for X = 0 doesn't: there is no guarantee that
the transmitter sees a non-reactive load unless the
SWR is exactly 1 : 1 (ignoring the fact that the
characteristic impedance of most feedlines is slightly
reactive due to the losses, of course.)


So basically if you have the ability to adjust both the
R and X, then (presuming 50 ohm coax) aim for 50+j0,
which is the same condition as SWR = 1.0 : 1.  If you
only have one adjustment to make, tune for minimum SWR
to keep the transmitter happy.
Logged
AC5UP
Member

Posts: 3844




Ignore
« Reply #4 on: August 06, 2009, 03:27:21 PM »

Ask six hams about this and you'll get at least seven opinions...

Speaking only for myself, in the case of a homebrew wire the testing phase involves measuring the SWR across the band(s) of interest. I'm not looking for an absolute value at this point, only where the SWR hits a minimum. If it's too low in frequency I'll prune, otherwise the antenna is hauled up to final working height and is checked again.

The only time I'm concerned strictly with the SWR number is if it's too high for the TX to load without folding back on power. I don't see much value in worrying about the difference in field strength between a 1.3:1 SWR and 1.5:1, but anything that could possibly harm the radio does concern me.

If I can't tame the wire down to something the TX can live with I'll double check my numbers and the noise bridge gets dusted off. If I'm real lucky it rains the next day so I have a good excuse to spend some quality time on the web researching what to do next before I start wearing a path in the lawn.

My three tools at hand are an SWR meter, Noise Bridge and Field Strength Meter. Listed in order of most used to least used.

BTW: It has been my experience that when I do run into problems they're usually related more to my choice of location, feedline, homebrew Balun(s) or general dumbassery than the antenna itself. On two occasions I've had stray wisps of wire in exactly the wrong place inside a PL-259 and I can tell you from experience it is possible to install the tuning ring on an AR-10 'Ringo' upside down... Looks good, lines up, just won't tune no matter what.

Wink
Logged

Never change a password on a Friday                
WA7NCL
Member

Posts: 625




Ignore
« Reply #5 on: August 06, 2009, 04:00:51 PM »

Measure at the antenna feed point or use some method to correct for the feed line impedance transformation.

Look at the R part and find where X is zero if the idea is to have a resonant antenna.

If the R part is not what it should be, say 35 ohms for a 1/4 wave or 70 ohm for a dipole, you know there is something to look into.  50 ohms on a 1/4 wave means you probably have a lot of loss (but a good match). You might need more radials etc. Variation in a dipole tells you about height above real ground.  Real low dipoles will be below 70 ohms.

When you know the R part, you can decide how you want to match to 50 ohms.  You can just take what you get at the end of the feed line, you can tweak the radiator length and hope that between the feedline and the reactance caused by changing the length, you get some sort of good match.  Or maybe add some intentional matching system in the form of stubs or lumped elements.

You see if you do not know what the feed point impedance is or the resonant value, you could end up like some guys, cutting and trying for the elusive 1:1 match that really might not be possible by just adjusting the radiator length.  There isn't anything wrong with SWR tuning, just don't expect that you can always get 1:1 by adding or subtracting length.  It may be that the combination of reactive and resistive parts will not allow a "perfect" SWR.

Thats what the impedance meter tells you that is "worth the money".
Logged
K0BG
Member

Posts: 9867


WWW

Ignore
« Reply #6 on: August 06, 2009, 04:09:31 PM »

I'll hand it to you, because you're doing your best to learn what really counts! It sure isn't SWR!

Far too many amateurs haven't a clue to what SWR really is. As stated above, ask 6, and you get 7 opinions. It is also unfortunate that we call it SWR, when a better term might be reflection coefficient.

SWR doesn't have anything to do with an antenna's efficiency, how well it "works", or its suitability for any given use. But you'd sure think so by reading some of the on-line palaver associated with it.

Speaking of palaver... If you want a dose of malicious palaver, just get into a discussion about HF mobile antennas,  with a higher unmatched SWR is better than one with a low SWR.

Alan, KØBG
www.k0bg.com
Logged

KG6WHN
Member

Posts: 48




Ignore
« Reply #7 on: August 06, 2009, 04:20:36 PM »

Well, being an engineer I definitely understand the idea of an ideal load and matching for optimal power transfer.  However, the part that still confuses me is why a seeming insulator (i.e. A TREE, or a BRICK house) can detune an antenna?  Now if the antenna is surrounded by a grounded faraday cage..sure but a wall of concrete or a tree?  I have no idea why that matters other than the directions say so. ;-}  Anyhow, I am having a bit of trouble getting my vertical to hear or radiate as I'd hope it would.  Granted my location for the antenna is lousy, but it's my only option at present.  :-(  Also, being a relative newbie at homebrew antennas..I saw a photo of a guy who made an HF antenna out of empty stacked soft drink cans...surely this isn't what is meant by a beverage antenna right?  ;-}
Logged
N3OX
Member

Posts: 8854


WWW

Ignore
« Reply #8 on: August 06, 2009, 04:33:39 PM »

"However, the part that still confuses me is why a seeming insulator (i.e. A TREE, or a BRICK house) can detune an antenna?"

Well let's look at two issues.

One is that trees and bricks, especially when damp, are weak conductors (bricks and mortar more weakly than trees).  

But they're also *dielectric* materials different from air.  They have a polarizability (especially in the case of highly polar water containing stuff) and they respond differently to electric field than does air.   You change the electric field around the antenna, you change the tuning, even if you don't totally block or absorb any energy. If you surrounded your antenna or laid it right up against a few inches of 100% pure Teflon, an excellent RF insulator, you'd still detune it a LOT.  You'd detune it in a relatively lossless way, but you'd detune it.  (Those losses that you could possibly get would come from increased currents in the antenna conductors themselves).  

You should also remember that a wall isn't just made of bricks.  It might have a bunch of wires and pipes embedded inside, which then run to weird terminations.  Some of those wires or pipes might be resonant where you want to operate.  

And maybe it's got rebar in there... again, bunch of metal bars surrounded by a weak conductor.

You can't nestle antennas right up against stuff, even wonderful insulators, and expect them to behave the same, unless that insulator has a dielectric constant of about 1.00 and is really just an insulator, not an insulator with conductors embedded in it.

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AC5UP
Member

Posts: 3844




Ignore
« Reply #9 on: August 06, 2009, 04:42:40 PM »

http://www.rexresearch.com/squier/squier.htm

Anyone know if the ARRL offers a " Worked All Conifers " certificate?

Just Askin'...........
Logged

Never change a password on a Friday                
KG6WHN
Member

Posts: 48




Ignore
« Reply #10 on: August 06, 2009, 04:43:06 PM »

Well, that all is quite true.  The thing that often confuses me a bit is that dipoles are typically hung in trees and verticals are supposed to stay away from them.  You'd think the dipoles would be effected by the nearby trees too.  Either way, I'm on the fence right now about trying some magnetic loop antennas since my backyard setup is less than optimal for verticals with long runs of radials.  I'm either too close to a tree, the house, my garage, or right in the middle fo the yard and then don't have the room for long-ish radials.  Maybe it's time to move? ;-}
Logged
N3OX
Member

Posts: 8854


WWW

Ignore
« Reply #11 on: August 06, 2009, 05:16:46 PM »

"SWR doesn't have anything to do with an antenna's efficiency, how well it "works", or its suitability for any given use. But you'd sure think so by reading some of the on-line palaver associated with it."

While that's true in one sense, if you constrain the argument to some assumptions, then SWR matters a lot.

For example: you put up a roughly half-wave dipole and the connections are done right and known to be good.  It's in the clear suspended between a couple of good insulators.  At that point, once you reach the lowest SWR by pruning, you can be quite sure it's working right.

The same roughly goes if you follow the instructions on a reputable trap antenna over a reasonable radial field and it's not being unduly influenced by its surroundings.  eedpoint impedance and how that relates to the transmission line in question are not meaningless quantities unless there's reason to think the system in question can show a very low SWR and have very low efficiency at the same time.  That's actually pretty hard to do with a lot of antennas, and very easy to do with a lot more.

For random antennas fed with wide range tuners, this is a good thing to bring up and discuss.  You can match a dead short with a high pass T network to a perfect 50 ohms in a lot of cases, a situation with zero efficiency.

And good SWR from a badly installed antenna can be pretty meaningless.  But let's not stretch it too far.

There's not really any situation in which tuning for low SWR is the wrong thing to do with a commercial trapped vertical fed against a fair number of radials.  Maybe it needs to be moved away from the wall, but if you accept the constraint that it needs to be up there, it's probably still the right thing to do to tune it for lowest SWR at the antenna and accept the consequences.

And tuning for X=0 independent of SWR is probably not what you want to do if the antenna has been designed for low SWR on each band.  Independent measurements of R and X are very useful in your measuring tool, but who's to say that your average commercial trap vertical doesn't have an "X=0" tuning solution for each band where the SWR is worse on each band, the feedline losses are higher, and the antenna is actually mis-adjusted in terms of best operation and power handling.

Commercial trap verticals are designed to present an acceptable SWR on each band they work on, not necessarily to be actually strictly resonant within each band they work on.  Just close to resonant on each band.  The reactance can't be very large in these cases, but it's too much to expect it to be simultaneously zero on all bands unless that's what they're saying in the manual.

Some people will point out that the lowest SWR is seen at X=0 *for a given R*, but that may be kind of a red herring, because you're not trying to resonate the antenna on a single band and get low SWR only on that band.  

And there can be a different R2+jX2 that is a lower SWR than some R1+j0.  You can't have very extreme reactance values and still have good SWR, so you're unlikely to be unable to find a tuning solution by shooting for X = 0.  Another consequence of that is that usually you'll find X=0 somewhere in the band in question.  But it's not absolutely necessary, especially for higher bands where reactance is changing more slowly with frequency.   And it might be even if there's an X=0 point in each band that tuning for low SWR and forgetting about X for a while is actually the best procedure to moving exact resonance into each band.
 
73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
Member

Posts: 8854


WWW

Ignore
« Reply #12 on: August 06, 2009, 05:36:47 PM »

"Well, that all is quite true. The thing that often confuses me a bit is that dipoles are typically hung in trees and verticals are supposed to stay away from them. You'd think the dipoles would be effected by the nearby trees too"

Trees are vertically polarized :-)

But seriously, that matters.

The thickest part which will be the best conductor and highest density dielectric fill is at the bottom and vertical.  The horizontal components are much thinner branches, so much higher resistance and less likely to be able to support currents of troublesome magnitude.  

But look, plenty of hams live in forests and get out just fine with verticals, so don't sweat trees too much.   People even do quite well with wires up the side of trees.  They're not that bad, certainly not even close to bad enough to totally give up on verticals if you've got trees.  

I wouldn't staple gun a wire vertical to a tree unless I didn't have another choice.  That's how I feel about trees.  But if you do put a vertical next to one, you shouldn't be surprised if you have to retune it.

"I'm either too close to a tree, the house, my garage, or right in the middle fo the yard and then don't have the room for long-ish radials"

In my opinion, it is far better to put the vertical in the clear middle of the yard, cutting all radials to half their length, than it is to put the vertical in the corner in an attempt to maximize radial length.  Covering the area matters more than getting to long length.

http://n3ox.net/projects/flag/layout_lg.jpg

I tossed the long ones into the front yard just because I could but they didn't do much.  If anything, they  made my 160m signal 1dB weaker all around or made my antenna 1dB more directive.  I can't conclude much from that without more measurements, so I just left 'em with the blind faith belief that more is better, but they're not doing the bulk of the work even on the lowest band.  The naive interpretation of the field strength measurement I did, though, is that they actually made the antenna worse.

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
Member

Posts: 8854


WWW

Ignore
« Reply #13 on: August 06, 2009, 05:49:43 PM »

By the way, trust me, without spending a lot of money and time on a pretty fancy magloop with "best practices" like big conductor, all soldered, vacuum variable cap (if you want to be able to run 100W input) , a commercial trap vertical with even a poor radial system will beat or equal it.

They're very cool antennas but they're finicky and critical and way more sensitive to location than a vertical is.  I tried to make a new mounting tripod for mine that only had it a foot and a half above the ground on the bottom and I lost like an S-unit worth of signal vs. having it 4 feet high.  This is not something I've been able to capture in a model.  

It's a neat concept and they do "work" surprisingly well when done right but I'd consider them sort of an antenna of last resort.
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
Member

Posts: 20574




Ignore
« Reply #14 on: August 06, 2009, 06:02:19 PM »

BTW, everything other than air and some other gasses have dielectric constants higher than 1, and will *all* impact antenna tuning and performance if they're in the near field.

Wood can vary a lot with the variety and how wet it is.

Bricks are basically clay and have the same dielectric characteristics as the earth from which they came...which is not good at all.

Cement is a conductor.  Its components all come from the earth and when melded, becomes a more lossy dielectric than earth, by far: Especially since to "hold it in place" on a vertical surface requires the use of a wire lath behind the cement, and the wire lath is steel. Cement or stucco walls are as conductive to RF as a solid plane of metal.

WB2WIK/6
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!