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Author Topic: What a double SWR peak means for a dipole ?  (Read 403 times)

EA3HUF

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What a double SWR peak means for a dipole ?
« on: March 22, 2023, 07:04:20 AM »

Hello,
I have been refurbishing my Cushcraft MA5B multiband yagi.
This antenna has a dipole for 12 and 17 meters.
The SWR graph obtained with the RigExpert AA-600 shows a double minimum in these two bands. To give an idea of the shape (I do not see the way to insert an image), the SWR minima at these frequencies are like a "W"

I would be very grateful if someone with more experience than me could tell me about the meaning of this double peak, if it is something worth solving and what would be the way to do it.

Best
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K1KIM

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Re: What a double SWR peak means for a dipole ?
« Reply #1 on: March 22, 2023, 08:02:11 AM »

Hard to tell what you are asking, but.....

2 dips below 2:1 VSWR between 18.110 MHz-18.168 MHz and 24.920 and 24.930 means you are resonant on those bands.

You need to have the band width you are measuring properly set for the frequencies you are measuring.

I would measure each band separately

I won't even get into Smith charts values here.
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W4HRL

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Re: What a double SWR peak means for a dipole ?
« Reply #2 on: March 22, 2023, 08:08:09 AM »

Usually a double dip in SWR for a single band is indication of excessive Common Mode Current on the transmission line (outside coax braid).

This has been a discussion item on the groups.io forum for Off Center Fed Dipoles (ofcd@groups.io).

See DJ0IP's (D-J-zero..) website for a good explanation.  < www.dj0ip.com/ocf-double---dip >

W4HRL
Mike in Poquoson VA
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WB6BYU

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Re: What a double SWR peak means for a dipole ?
« Reply #3 on: March 22, 2023, 09:06:43 AM »

If you are seeing two dips across the same band,
there are two likely causes:  either it is due to
other resonances within the antenna, or due
to a resonance in the coax cable (common
mode current).

Some wideband antennas are designed to have
such a curve, particularly for 80/75m (3.5 to 4
MHz in IARU Region II).  But there should be
no need for such behavior across the narrow
WARC bands.

The manual doesn’t give sample curves, so
we don’t know if this is considered normal.

In this antenna, the 17m and 12m bands
have adjacent elements that are close to
resonance:  the 15m reflector will be tuned
close to 17m, and the 10m reflector close
to 12m.  That can cause the response you
are seeing, depending on the exact tuning
of the parasitic element.

If the antenna doesn’t have an effective
balun, then the length of the coax shield
(and what is connected to it) can add
other resonances to the antenna SWR
curve.  A quick check would be to add
3-4m of coax and see if one of the
SWR dips moves in frequency.  I’ve
seen this a couple times when tuning
an 80m dipole:  dips at 3.55 and 3.95
MHz, with one of them affected by
tuning the antenna and the other by
the length of the feedline.  The solution
in that case is to add an effective balun
at the antenna feedpoint.

K1KIM

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Re: What a double SWR peak means for a dipole ?
« Reply #4 on: March 22, 2023, 10:07:18 AM »

Hard to tell what you are asking, but.....

2 dips below 2:1 VSWR between 18.110 MHz-18.168 MHz and 24.920 and 24.930 means you are resonant on those bands.

You need to have the band width you are measuring properly set for the frequencies you are measuring.

I would measure each band separately

I won't even get into Smith charts values here.

I misunderstood the OP. You were referring to 2 dips within the same band I gather from reading the responses.
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WA3SKN

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Re: What a double SWR peak means for a dipole ?
« Reply #5 on: March 22, 2023, 11:44:25 AM »

I don't know.
The MA5B is a multiband antenna, so there will be multiple peaks and dips.  Specs claim it is 50 ohms but do not mention frequency.  They do show bandwidth for 2:1 SWR.
But the AA-600 will measure at multiple impedances.  I must assume yours is set for measuring at 50 ohms.
And you did not mention the values of the peaks and dips.  Are they anywhere near the 2:1 claimed?
so how high are the peaks and dips?  Are they anywhere near the bands in question?  And what frequencies are they at?  Antenna height above ground plays a role here... how high is the antenna?
The peaks and dips are assumed at resonant values and probably centered around .25,.5,.75, 1.0 etc. wavelength values, but for which bands or frequencies?
So, just how bad are the peaks and dips?
Is the test set set to 50 ohm Z?

-Mike.
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EA3HUF

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Re: What a double SWR peak means for a dipole ?
« Reply #6 on: March 22, 2023, 12:45:23 PM »

Hard to tell what you are asking, but.....

2 dips below 2:1 VSWR between 18.110 MHz-18.168 MHz and 24.920 and 24.930 means you are resonant on those bands.

You need to have the band width you are measuring properly set for the frequencies you are measuring.

I would measure each band separately

I won't even get into Smith charts values here.

I misunderstood the OP. You were referring to 2 dips within the same band I gather from reading the responses.

Yes, two minima in each band of the two-band dipole of the five-band MA5B yagi.
In any case, I appreciate the intention. Thanks
Best
Joaquin
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EA3HUF

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Re: What a double SWR peak means for a dipole ?
« Reply #7 on: March 22, 2023, 01:15:31 PM »

I don't know.
The MA5B is a multiband antenna, so there will be multiple peaks and dips.  Specs claim it is 50 ohms but do not mention frequency.  They do show bandwidth for 2:1 SWR.
But the AA-600 will measure at multiple impedances.  I must assume yours is set for measuring at 50 ohms.
And you did not mention the values of the peaks and dips.  Are they anywhere near the 2:1 claimed?
so how high are the peaks and dips?  Are they anywhere near the bands in question?  And what frequencies are they at?  Antenna height above ground plays a role here... how high is the antenna?
The peaks and dips are assumed at resonant values and probably centered around .25,.5,.75, 1.0 etc. wavelength values, but for which bands or frequencies?
So, just how bad are the peaks and dips?
Is the test set set to 50 ohm Z?

-Mike.

Yes, I am measuring at 50 ohms.
I have seen the same behavior at 3, 6 and 9 meters high (counting from the top of a 4-story building).  The frequencies of the minima change a bit with height but the shape is similar.

Maybe the following table for the 17m band will give you an idea of the shape:

FreqSWR
17.44
17.72.6
17.951.51st minimum
18.01.7
18.151.42nd minimum
18.31.8
18.63.9

Only the dipole 12 and 17 bands show these minima.
Bands of the two elements antena (10,15 and 20m) show nice, narrow dips.

Best
Joaquin
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EA3HUF

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Re: What a double SWR peak means for a dipole ?
« Reply #8 on: March 22, 2023, 01:26:45 PM »

Thanks everybody for your answers.
I have learn some new things.
Now I have an idea of ​​what might be the factors involved in that behavior, so I can play around with the antenna a bit to see how it responds.
Thanks
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EA3HUF

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Re: What a double SWR peak means for a dipole ?
« Reply #9 on: March 22, 2023, 01:30:10 PM »

Usually a double dip in SWR for a single band is indication of excessive Common Mode Current on the transmission line (outside coax braid).

This has been a discussion item on the groups.io forum for Off Center Fed Dipoles (ofcd@groups.io).

See DJ0IP's (D-J-zero..) website for a good explanation.  < www.dj0ip.com/ocf-double---dip >

W4HRL
Mike in Poquoson VA

The first DJ0IP graph in that website is exactly what I have.
Thanks!

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AA5MT

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Re: What a double SWR peak means for a dipole ?
« Reply #10 on: March 23, 2023, 11:39:42 AM »

You may be seeing separate resonances.  There are two halves of the driven dipole.  Each half may have separate resonances.  You can prove this with your analyzer.  Position yourself where you can reach each half.  Dial in the resonant frequency 1.  Touch one side of the dipole and you should see a response on the swr analyzer dip for that frequency.  Do the same for frequency 2, and the other side should be seen to respond at that frequency.  Normally, the dip would occur on the same frequency.  By watching the analyzer live, you can tune either or both side to a particular frequency. 

You might get a response about what difference in the field pattern might change.  Not really enough for most people to notice.

This was a common way to broadband a dipole in the old days.  Even though the swr dip may not go to 1:1, it was still quite usuable.  So, a common practice was to tune one half to the cw net and the other half to the ssb/am net frequency.

This was intentionally done with the Hy-gain TH-7.  It uses two driven elements, with a 1:1 swr on both frequencies within the same band.  The highest swr seen within the band is around 1.7:1 at the band edges. (10,12,15 m)

Tom
« Last Edit: March 23, 2023, 11:42:26 AM by AA5MT »
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