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Author Topic: Should I match for min SWR or closest to 50 ohms  (Read 4404 times)
K7NHB
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Posts: 226




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« on: June 25, 2013, 10:30:56 AM »

I'm asking here instead of the antenna forum because I know QRP'ers need to get as much out of their system as possible - and I don't mean getting things out of your system when some Big Gun lands on 7.035. As I watch the Analyzer, I see my SWR is nice a low in the 1.3 range but the R value is around 84 ohms. How much effort should I go getting that to 50 ohms if I have to sacrifice a little higher SWR. Or should I make SWR my God and just worship that (apologies to anyone who thinks I'd really pray to swr gods, Elecraft maybe, not swr).
73, Paul, K7NHB
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KQ6Q
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Posts: 963




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« Reply #1 on: June 25, 2013, 07:59:01 PM »

1.3 is decent SWR. Unless you're running a really LONG segment of RG-58, don't worry about it. Take a look at the attenuation/loss figures for the different types of coax at the frequency you're concerned with - and those are for 100' lengths, Lots of QRP'er use RG-174 because it's really lightweight, and they're not running it very far.
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K8AXW
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« Reply #2 on: June 25, 2013, 08:52:23 PM »

Tune for minimum SWR.  Never heard of anyone going wrong doing that.
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K7NHB
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Posts: 226




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« Reply #3 on: June 25, 2013, 09:03:24 PM »

My coax lengths would be 50 ft at most and usually 25 ft. RG-174 in intriguing, except or putting on connectors - either UHF or BNC. Do you usually order it with connectors already attached (wireman?)?
73, Paul, K7NHB
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G4AON
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Posts: 512




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« Reply #4 on: June 26, 2013, 01:47:58 AM »

A 1.3:1 SWR is pretty good. For ordinary "back yard" antennas, such as dipoles, adjusting the antenna for best SWR is generally all that is required. The only time you probably need to concern yourself with the R value (as seen on an antenna analyzer) is when you are matching a 160 or 80 metre loaded vertical, a hair pin matched yagi, or a HF mobile antenna.

On my 160m ground mounted loaded vertical, the minimum SWR (without matching) is around 2.5:1. The antenna was initially adjusted for resonance without any attempt at obtaining a good match to the feeder. The resonant point is where X is the lowest value. The SWR at this stage is too high unless you have significant ground loss (usually caused by not using enough radials, or just using an earth rod).

I added a springy coil of 2mm diameter enamelled copper wire between the base and the buried radials, this coil is approximately 3 uH. By squeezing the turns together with Ty-raps I was able to obtain a 1:1 SWR, X near zero and R=50 at the previously set resonant frequency.

73 Dave
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AA4PB
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« Reply #5 on: June 26, 2013, 04:30:58 AM »

I'd say that low SWR is most important because that will minimize the feed line loss.

This points up something about antennas that we often don't realize. If an antenna is tuned for resonance the reactance will be zero but that doesn't necessarily mean that the feed impedance will be 50 Ohms. Some antennas allow you to adjust both resonance (the length) and impedance (a feed point match device) in order to obtain both resonance and a 50 Ohm feed.
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WB6BYU
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« Reply #6 on: June 26, 2013, 09:32:32 AM »

Quote from: K7NHB

As I watch the Analyzer, I see my SWR is nice a low in the 1.3 range but the R value is around 84 ohms.



If R is actually 84 ohms, then the SWR can't be as low as 1.3 : 1. 

85 / 50 = 1.68 : 1, the minimum possible SWR.  (Any reactance will increase that value.)

That in itself will give you a sense of how much to trust your analyzer.



Quote

How much effort should I go getting that to 50 ohms if I have to sacrifice a little higher SWR.



SWR is a measure of how close your load impedance is to 50 ohms.  It's a 2-dimensional
world with both R and X components:  adjusting one for a perfect result at the expense of
the other isn't useful (unless the purpose is to add some further matching.)

Rarely is there any advantage in tweaking the SWR below about 1.5 : 1, regardless of the
specific readings for either R or X.  If you are measuring your antenna through a length of
coax, then the readings won't necessarily match the impedance at the antenna anyway.
If the SWR isn't 1 : 1 then the R and X values will vary with the feedline length even though
the SWR stays the same (except for some reduction due to feedline loss.)

So tuning for minimum SWR gets you closest to 50 ohms even if your R meter shows otherwise,
but don't make a fetish of trying to make it too low.  The dipole impedance will change each
time it is set up:  I tuned mine originally at a relatively low height (something like 8' to 10'
in the center, 4' at the ends) with the idea that I need the best performance when the
antenna is in the worst situation.  When I can get the dipole high in the air it will work well
enough that I'm willing to suffer a bit higher SWR.


I use RG-174 for my backpack antenna kit.  Since my QRP rigs at the time I built it both used
RCA plugs, that's what I put on the end.  (The other end is permanently attached to the center
insulator, to which I attach whatever set of wires I plan to use each time I set up.)  I recently
found some 25'(?) cables with BNC connectors on each end, and crimp-on connectors are
available as well.  It's quite easy to melt the insulation when soldering if you aren't careful,
and if you look around you may find some of the surplus military types of the same size with
Teflon insulation.  If you don't trust your ability to solder or crimp on the appropriate connectors
you can get cables made for you, otherwise the simplest approach is probably to solder on an
RCA plug (which is less critical than most coax connectors) and use an adaptor from that to
BNC or PL-259.
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K7NHB
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Posts: 226




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« Reply #7 on: June 26, 2013, 11:28:32 AM »

Thank you for the tip on the RCA connector - much easier than BNC.

In my situation, I hoped to have the tuner at the Antenna feed point and run coax from that back to the rig. It's either that or cheap twin lead to a 4:1 balan at the tuner by the rig.

73, Paul, K7NHB
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WB0HZL
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Posts: 24




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« Reply #8 on: June 27, 2013, 02:04:14 AM »

I usually adjust for minimum SWR. I make my measurements close to the antenna but far enough away that I am not effecting the measurement.  With an 1.3:1 SWR I would not worry, unless it is through a long feed line and measured at the transmitter, it that is the SWR at the antenna - it is good enough. Hope this helps.

73,
Trent WB0HZL, DW5HT
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AA4GA
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Posts: 118


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« Reply #9 on: June 30, 2013, 08:18:14 PM »

My coax lengths would be 50 ft at most and usually 25 ft. RG-174 in intriguing, except or putting on connectors - either UHF or BNC. Do you usually order it with connectors already attached (wireman?)?
Crimp-on BNCs work pretty well for RG-174...I make my own for jumpers in the shack - usually less that 3'.  I am thinking about a little longer piece for using with portable antennas, but will try to keep the length as short as is practical for the antennas I'm using.
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WB6BYU
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Posts: 12986




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« Reply #10 on: July 01, 2013, 12:17:15 PM »

The loss in 25' of RG-174 is about 1dB (25%) at 10m and half that on 40m.
I made up a 25' extension (which is easy using RCA jacks and plugs) to use
when I have the height to make use of it, but it doesn't make a lot of difference
for local work on 40m and 80m, and I only used it a couple of times.  But for
DX on the higher bands the added height may well provide enough gain
improvement to make up for the added loss.

(When I do have enough support height for 40m, I may put up a delta loop
with the point down instead:  that gets maximum current higher in the air
for the same length of feedline, though the SWR is higher.)


What sort of antenna are you using that you need to put the tuner at the
feedpoint?
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KA0HVE
Member

Posts: 117




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« Reply #11 on: July 08, 2013, 07:16:32 AM »

Ideally you should tune the antenna for resonance and leave it there and put a tuner at the antenna feed point to match the antenna and feedline.  That's assuming you're using a 50 ohm feed line to the antenna and that your xcvr expects a 50 ohm load.

What many of us end up doing is trimming the antenna for a minimum SWR to keep the xcvr happy.  The antenna can easily end up being non-resonant.

This is why I'm becoming more and more interested in doublets and similar antennas.  They can be close enough to resonance on multiple bands and the antenna feedpoint is within arm's length of the operating position, i.e., the tuner is in the right spot and within reach.

Everything has its pro's and con's and compromises.
« Last Edit: July 08, 2013, 07:27:52 AM by KA0HVE » Logged
AD5X
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Posts: 1426




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« Reply #12 on: July 08, 2013, 09:01:42 AM »

...I made up a 25' extension (which is easy using RCA jacks and plugs) to use
when I have the height to make use of it...

For portable use, I use a 15' or a 25' section of RG174 coax with RCA plugs.  Usually one or the other section is all I need.  I attach them if necessary with an RCA coupler.  I use a 20-10M switched dipole with 40M & 30M clip-on coils (info in the "Articles" section of my website at www.ad5x.com).  Any minor SWR problems are handled by my KX3 autotuner.

Phil - AD5X
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WB6BYU
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Posts: 12986




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« Reply #13 on: July 08, 2013, 02:05:31 PM »

Quote from: KA0HVE

Ideally you should tune the antenna for resonance and leave it there and put a tuner at the antenna feed point to match the antenna and feedline.  That's assuming you're using a 50 ohm feed line to the antenna and that your xcvr expects a 50 ohm load.



From a radiation/reception perspective there is no advantage in the antenna itself
being resonant if you are also going to add a tuner or some other type of matching.  A
5/8 wave vertical or an EDZ, for example, is not resonant, but still works well as an antenna,
and changing the length to make it resonant prior to matching changes the gain and/or pattern
so you end up with a different antenna.

Here is an explanation:
http://www.vk1od.net/antenna/iobr/

For simple antennas such as a dipole or quarter wave vertical that is suitable for direct coax
feed, there is very little difference between tuning for resonance or for low SWR (as long as
you are taking the measurements right at the antenna feedpoint.)   That's because the reactance
changes faster than the resistance as you vary the length, and the point of minimum SWR will
be very close to the resonant frequency - within the accuracy that you can determine it with
most SWR meters.  One reason for that is that you can't have a low SWR and still have a
significant amount of reactance at the feedpoint.


Quote

What many of us end up doing is trimming the antenna for a minimum SWR to keep the xcvr happy.  The antenna can easily end up being non-resonant.




For an antenna such as a dipole or quarter wave vertical it is quite difficult to invent a situation
where the lengths for resonance and for minimum SWR are far enough apart to make a difference.
As an example, some of the simulations I've run had a shift of about 10 to 20 kHz on 20m, so
either setting gave about the same SWR across the band.  That's because the R component of
the antenna feedpoint impedance changes much more slowly with frequency than the X value.

Where you do run into problems, however, is when you try to measure resonance at the shack
end of the feedline without correcting for the impedance transformation along the feedline.  If
all one is interested in is X = 0 on your analyzer and there is any significant length of coax
between the analyzer and the antenna, then it isn't difficult to shift the antenna resonance
clear out of band in order to get the coveted X = 0 reading.

Consider the case where you have a 20m dipole with a radiation resistance around 72 ohms
and connect the analyzer to it using a 4' jumper of RG-213.  At resonance (70+j0 ohms at
the antenna) the analyzer will read 56 - j19 ohms, so it looks like you need to lengthen the
antenna even though it is already resonant.  So we'll lengthen it by 20ohms of reactance:
with 72 + j20 at the antenna, the analyzer now reads 74-j19 ohms.  If we lengthen it to
add another 20 ohms of reactance (which in practice might move the R component up a
few ohms, but we'll ignore it for the moment)  we end up with 96 - j26 ohms at the analyzer.

You can keep going until you are blue in the face, but you'll have to make it resonant
far out of band before achieving X = 0 on the analyzer through the 4' coax jumper.

The problem is that the impedance varies along the feedline when it isn't perfectly matched.
What will give X = 0 with one length won't do so if you add a feet, and for random lengths
of coax ther is no correlation between X = 0 at the shack end of the coax and at the
antenna feedpoint (where X = 0 means the antenna is resonant.)


On the other hand, adjusting the antenna so that the dip in SWR is in the middle of the
desired operating band pretty much assures you that the antenna is resonant in the
band regardless of the length of the coax or the reactance measured at the far end.

So if you are having the problem you describe, it may be due to adjusting it for X = 0 at
the shack end of the coax rather than at the antenna end.  (VK1OD's calculator will allow
you to correct measurements at the shack end of the feedline to find the actual load
impedance at the antenna:  http://www.vk1od.net/calc/tl/tllc.php.)


Quote

This is why I'm becoming more and more interested in doublets and similar antennas.  They can be close enough to resonance on multiple bands and the antenna feedpoint is within arm's length of the operating position...



Hmmm...   I've rarely used a doublet with the feedpoint that close to the operator:  I'd rather
have it up high in the air to improve my signal, then connect it to the tuner with a length of
feedline. 

The same rules still apply, of course:  it doesn't matter whether the antenna length is resonant or
not; the impedance at the antenna feedpoint is transformed by the feedline connecting it to the
shack; and the tuner (if used - it might not be needed for some combinations of antenna and
feedline lengths) matches the impedance at the end of the feedline to 50 ohms.
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KA0HVE
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Posts: 117




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« Reply #14 on: July 09, 2013, 06:45:10 AM »

WB6BYU,

I hear where you're coming from and you did cite a source.

I didn't quote it but my information is taken from the old ARRL Antenna Book and is quite solid.

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