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Author Topic: Shorter or Longer to Alter SWR?  (Read 4367 times)
W8JI
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« Reply #15 on: August 14, 2012, 03:54:45 PM »

Robert-
- my bad, I didn't read your statement carefully- now understand.
- roger wasting my time until I fix radial problem.
- so I need to extend each of the radials from 65 ft to 128 ft.
- earlier post said I need to "tune" the radials-- how do I do that?

will work on radials and then report back.

Tnx and 73
Paul

I am trying to write something on counterpoises, at least to give some general guidelines, but it might take an hour a day for weeks. :-)

The way you can "tune" counterpoises is to feed the counterpoise like a dipole. Break they two wires in the center, and cut or extend them to length for the lowest SWR on 160 meters.

Then you can short them together and install the vertical.

The problem is a small counterpoise will have considerable voltage to earth. That will drive the coax shield with RF current, so you will need a good current balun or line isolator at the feedpoint. DO NOT ground the counterpoise wires, if you use a tuned small counterpoise.

73 Tom


 

 
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KC4MOP
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Posts: 729




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« Reply #16 on: August 15, 2012, 03:31:18 AM »

Robert-
- my bad, I didn't read your statement carefully- now understand.
- roger wasting my time until I fix radial problem.
- so I need to extend each of the radials from 65 ft to 128 ft.
- earlier post said I need to "tune" the radials-- how do I do that?

will work on radials and then report back.

Tnx and 73
Paul



I am trying to write something on counterpoises, at least to give some general guidelines, but it might take an hour a day for weeks. :-)

The way you can "tune" counterpoises is to feed the counterpoise like a dipole. Break they two wires in the center, and cut or extend them to length for the lowest SWR on 160 meters.

Then you can short them together and install the vertical.

The problem is a small counterpoise will have considerable voltage to earth. That will drive the coax shield with RF current, so you will need a good current balun or line isolator at the feedpoint. DO NOT ground the counterpoise wires, if you use a tuned small counterpoise.

73 Tom


 

 
OK Tom
very valuable info. I think another person, or you, said a similar thing about high RF voltages on the elevated radials. But I never knew NOT to ground them to the ground rod as we do for ground mounted radials. That may be why my attempt at elevated radials was a waste of time 2 yrs ago.
I removed them cuz it was going to be a hazard to my family, deer and me on my riding lawn mower. If a Ham op has about 5 acres, then these out-of-the-ordinary antenna systems can co-exist with normal human living.
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KC2NYU
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« Reply #17 on: August 16, 2012, 10:27:27 AM »

OK guys, I have lengthened my two above ground radials from 65 ft to 128 ft each. The back side of my lot is 131 ft, so both of these, 3 ft above the ground radials, make a 90 bend at the corners of the lot. I also added a 1:1 balun at the end of the antenna/radials connection. I took a new set of readings, on all bands just for an overall reference of where I stand. Keep in mind the original goal was to get on 160M. Overall the change in radial length didn’t improve SWR that much but did lower the impedance readings on many bands.

-   SWR on 160M went up, but impedance dropped by half.
-   On 80M SWR and impedance readings are about the same -- very high
-   60M stayed the same
-   40M SWR went up
-   30M SWR slightly up
-   20M SWR about same but impedance is down
-   17M SWR slightly better, impedance is lower
-   15M  SWR about the same, impedance is way down
-   12M  SWR up slightly but impedance way down
-   10M  SWR up slightly but impedance way down
-    6M   SWR slightly better, impedance slightly better. This band has overall best SWR averaging 5.
   
-   So … overall making the radials longer improved impedance, didn’t affect SWR all that much. Looking over the readings on all bands, the SWR is going down as frequency increase, so I do I deduce that the antenna is too short??

Tnx Paul
kc2nyu
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KB4QAA
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« Reply #18 on: August 16, 2012, 11:31:33 AM »

Hi Paul.

-The minimal change with adding length to the radials is not unexpected because we want them to be 'radial' or straight.  Having 90deg bends is not that helpful, but take what we can get.   I  would reckon that the 90bends mean the effective length of the radials is less that the physical length, meaning even longer length is needed to reach a resonant electrical lengths. 

**Tom's suggestion for tuning the radials is still appropriate**
-Disconnect the radials at the feed point.  Attach one radial to the shield of your analyzer, the other radial to the center conductor and see where they are resonant.  Add or subtract length to make them resonant at 160m!!  Once the radials are resonant, adjusting the antenna will be a breeze.

-Since 160m is your goal, take your readings there rather than looking at the other bands.  If you don't see a definite curve with perhaps 5 readings across the band, then start expanding readings above and below.  (What to measure is the next question).

-You don't say what analyzer you have but since you are measuring impedance I assume you it is one that can read X (Reactance).  You need to pick which characteristic you are using to tune/adjust your system.

-X, Reactance:  When a perfect circuit is resonant X=0.  We should be looking for a low figure.

-SWR:  In a typical antenna SWR and X are closely related, and with simple analyzers, SWR can be used as a substitute for X.  I can't say how typical your system is, though.

-Impedance:  Important primarily to know if our tuner can match the antenna system.  Impedance can tell us other things, but is not normally used as the primary reference for adjusting antennas.

I recommend using X, Reactance.  Adjust your radials first if possible.  Make perhaps 5 readings across the band and see where X lowest.  If X is lowest at the low end of the band, then radials are too long.  If X is lowest at the high end of band, radials are short.  Same applies to your radiating element/system as a whole.

Keep in mind that with an inverted L, the vertical portion is the main radiator and the top horizontal run acts as a capacitive hat.  So adding/shortening length is going to have less effect than formulas predict compared to actually making the vertical portion taller.

The simplest shortcut would be to add a honking big tuner  right at the feed point!  Smiley  Bill
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WB6BYU
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« Reply #19 on: August 16, 2012, 12:09:41 PM »

Measure the SWR at several points across the 160m band.  If the point of minimum SWR
is below the desired frequency, or if the SWR is lowest at the bottom of the band, the
antenna is too long and you need to shorten it.  If the frequency of minimum SWR is
above the desired frequency, or at the top end of the band, then you need to lengthen
the antenna.

If you have an analyzer that measures reactance, you can use the point where X = 0
as an indication of resonance only if you use a short coax jumper between the
analyzer and the antenna feedpoint.  If the antenna is about 25 ohms at resonance
(not a bad guess) then adding coax between the feedpoint and the analyzer shifts
the point where X = 0 down by about 2 kHz for each foot of coax.  So if the antenna
is resonant at 2 MHz with an SWR of 2 : 1, measuring through 100' of coax moves the
X=0 point down to 1.8 MHz where the SWR is 20 : 1.  Given the bandwidth of the
antenna, you're probably OK if your coax is less than 10' or so on 160m, but shorter
is better on the higher bands.  Checking for the point of minimum SWR still gets you
close enough, even though a long piece of coax.


Ignore the rest of the bands until you install tuned radials for those bands.

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W8JI
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« Reply #20 on: August 17, 2012, 07:44:16 AM »

This is slowly growing:

http://www.w8ji.com/counterpoise_systems.htm
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KC2NYU
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« Reply #21 on: August 18, 2012, 07:45:52 AM »

OK- took some readings on the analyzer of just the radials. X is lower at the high end of the 160m band -- indicating the radials are too short. So... my next project is to make them longer. The question is how much longer. Each is 128 ft long now-- what if I were to add another 64 ft and retest? Or is there a more scientific way too determine length to add??

This is proving to be an excellent experience for me, appreciate all the contributions from the group.

73
Paul
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N4CR
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« Reply #22 on: August 18, 2012, 07:53:33 AM »

You have two raised radials 3 feet above another radial field or am I reading this wrong?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
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Posts: 13029




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« Reply #23 on: August 18, 2012, 08:20:50 AM »

Quote from: KC2NYU

...X is lower at the high end of the 160m band -- indicating the radials are too short. So... my next project is to make them longer. The question is how much longer...



10 to 20 feet might be a good starting point.

You can make a reasonable guess by extrapolating the reactance curve.  For
example, say that the reactance is 300 ohms at 1.8 MHz and 100 ohms at 2 MHz.
Linear extrapolation would suggest that the reactance would be zero at 2.1 MHz
because the reactance is changing about 1 ohm / kHz.

Using the standard quarter wave formula of 234/f, at 2.1 MHz that is 111 feet.
If you want the radials resonant at 1.9 MHz, that would be 123 feet.  Now, those
numbers won't be exact due to the effect of the ground, but if you take the
difference (12') and add that to your existing radials and repeat the process,
you should converge pretty quickly.

The radials don't have to be exactly resonant, but I'd recommend trying to get
them somewhere in the band.
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KC2NYU
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« Reply #24 on: August 18, 2012, 09:38:34 AM »

W6BYU- OK got it- hope to get to this over the weekend- thanks for explanation and suggestions

N4CR- I have two raised radials only, no In Ground radials.
Each runs in opposite directions from the base of the vertical leg of the Inverted L. Unfortunately - configuration is not optimum due size of lot- ergo not straight , had to bend around corners of lot to get necessary length.

73 Paul
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N4CR
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« Reply #25 on: August 18, 2012, 08:49:36 PM »

In that case, you tune for resonance by adding or subtracting length while doing empirical testing. Ultimately, it's better to start long and trim because it leaves less splices, but that's water under the bridge already.

If you have them a quarter wave long now and they aren't tuning into the band, they are probably too close to the earth. Raising them more might be a better answer than extending them.

Also, putting them in a gullwing configuration might help a lot if you haven't raised them near the feed point. Open the following .PDF and search for "gull wing".

http://om6bb.bab.sk/files/HAM%20kniznica/Magaziny/NCJ/06%20November-December%202000%20NCJ.pdf
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KC4MOP
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Posts: 729




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« Reply #26 on: August 19, 2012, 05:23:05 AM »

I thought (maybe dangerous) that we or me were trying to get Paul, KC2NYU, to abandon the raised radials and go for good ole ground mounted radials.
His description of the property seems that it will support a lot of nice radials. Try for 40 radials as long and as straight as you can.
The antenna is too short. Lengthen the horizontal wire. I'm guessing at this point of the thread that you still have the inverted "L"Huh? The Vertical part is ok.
The magic of the MFJ 269 is the readout of X. Getting SWR down to 1.5:1 (not a mandate) but close and getting the Xs below 20 usually means that the antenna is good and ready for use.
The raised radials are complicating your adjustments of this new antenna system.

Fred
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KA0GKT
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« Reply #27 on: September 03, 2012, 09:33:41 PM »

Check the SWR at different frequencies. If the SWR goes down as you increase frequency then the antenna is too short. If SWR goes down as you decrease frequency then the antenn is too long.


Add to that "If the VSWR goes up whether you increase or decrease frequency, you have started at or near the lowest VSWR.  Remember, low VSWR isn't necessarily an indication of resonance, just of match between the load and the transmission line.

For instance:  in free space, an equal-sided folded dipole at resonance will have a charecteristic impedance of around 300 Ohms.  When fed by a transmission line with a charecteristic impedance other than 300-Ohms, standing waves will form on the transmission line.  Feed that same dipole with 75-Ohm coaxial cable through a 4:1 balun and a low VSWR will be the result.  The antenna remains resonant; however the VSWR goes up due to the mismatch between the transmission line and the antenna.

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