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Author Topic: Increased mast length ==> higher SWR at resonant frequency. Why?  (Read 2965 times)
HS0ZIB
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« on: December 18, 2011, 04:40:00 PM »

On the basis that more metal in the air should improve my 20 meter band mobile signal, I increased the mast length of my Hustler resonator system from 24 inches to about 48 inches.  (I use a fold-over roof mount and my car is long enough to accommodate this extra mast length without it hanging over the rear of my vehicle).

I adjusted the stinger to obtain the maximum length of mast+resonator+stinger as possible, and then tuned the resonator to 20 meter band resonance by adjusting the number of coil windings.  (I am using an RM-15 15 meter band resonator, but actually that's not so important since I'm only using the resonator as a sturdy coil former and then winding the relevant number of turns to achieve the desired frequency resonance for the length of my antenna system

However, whilst the SWR at resonance on my previous system (24 inch mast) was about 1.7, it is now 2.4 at resonance.

My question is - why should the SWR increase for the same resonant frequency when I have increased the mast length?

Would it be better to increase the stinger length above the resonator, rather than the mast length below it?  I was under the impression that it was better to get the resonator as high as possible

Simon
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W5DXP
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« Reply #1 on: December 18, 2011, 06:11:52 PM »

However, whilst the SWR at resonance on my previous system (24 inch mast) was about 1.7, it is now 2.4 at resonance.

The resonant feedpoint impedance probably went from ~30 ohms down to ~20 ohms. That's good news for efficiency. What you probably need is a shunt matching device at the base of the antenna. It can be either a shunt coil or a shunt capacitor. I believe The ARRL Antenna Book has information on that subject. I have also seen a 2:1 UNUN used to obtain a better match.
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HS0ZIB
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« Reply #2 on: December 18, 2011, 08:17:34 PM »

Actually, it would help if I put my brain into gear before posting!  I was measuring on the output of my DX Engineering matching balun, not directly at the antenna!

If I measure directly at the antenna itself, I get an SWR of 1.3, (about 1.4 at resonance), and with an input impedance of about 18 ohms.  My balun will match that nicely at the 25 ohm connection.

So, all looks good, but let's see how it performs in practice

Simon
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W5LZ
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« Reply #3 on: December 19, 2011, 06:26:17 AM »

After knowing about that balun, the 'simple' answer is that it's the wrong impedance transformation ratio.  Before knowing about that balun, it would be because of a change in the amount and type of reactance introduced by the increase in the mast length.
That "more metal in the air" thingy has limits.  If the antenna is resonant to start with and you want to increase the height of the loading coil, then add to the bottom mast and reduce the top whip keeping the length constant.  There will still be some changes in measured SWR, but they shouldn't be huge.  Any time you make changes in an antenna there will be some 're-tuning' involved.  (Unless you are very, very lucky!)
 - Paul
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AC4RD
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« Reply #4 on: December 19, 2011, 09:25:57 AM »

Simon, I usually work on getting a new mobile antenna nicely resonant at the frequency I want, and worry about SWR later.  A simple coil or a capacitor will get the SWR flat, but flat SWR isn't as important as resonance, I think.

My own setup uses a 54" DXE mast with various Hustler resonators and whips; I've found in the past that changing from the 54" mast to a 36" one didn't change the resonant frequency nearly as much as I would have expected.  I wish I had written down the results, for reference.

Sounds like you're making good progress on the mobile rig!  Congratulations and I hope to catch you on the air some time!  :-)
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HS0ZIB
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« Reply #5 on: December 19, 2011, 06:37:38 PM »

Well, I tested out my new, longer-length Hustler antenna yesterday afternoon whilst parked at the beach.  As usual, the band is virtually dead during the daytime - the occasional Chinese station.  But as dusk approached, more signals were received, (and QSOs made), with Taiwan, Doha, India and Australia.

Enticingly, I could hear some weak signals with rag-chewing in American English.  Could be Guam or Hawaii, but could also be west-coast USA.

When I listen to some VK stations (strong signals), I could sometimes hear them in QSO with US stations, (which were very weak).

Now all I need are better conditions, and I might be able to get a QSO with NA from my car Smiley

Simon
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KW6LA
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« Reply #6 on: January 06, 2012, 04:34:35 PM »

My own setup uses a 54" DXE mast with various Hustler resonators and whips; I've found in the past that changing from the 54" mast to a 36" one didn't change the resonant frequency nearly as much as I would have expected.

That is correct, mobile antennas need the least inductance / coil length when base loaded. The longer the mast, it becomes more like top loading and more turns are needed. While the antenna gets longer and the coil higher, It
will almost stay at the same  resonant frequency. The good news is longer antennas perform better, installed correctly.
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W5DXP
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« Reply #7 on: January 07, 2012, 11:18:26 AM »

I've found in the past that changing from the 54" mast to a 36" one didn't change the resonant frequency nearly as much as I would have expected.

That is exactly as predicted by the wave reflection model applied to standing wave antennas. The Z0 of the loading coil is maybe seven times the Z0 of the bottom mast and the phase shift at the bottom of the coil is a small negative value having little effect on the electrical length of the antenna and therefore on the resonant frequency.

OTOH, changing the stinger length causes a large change in the positive phase shift at the coil/stinger junction and therefore has a large effect on the resonant frequency.

However, changing from the 54" mast to a 36" mast degraded your radiated power considerably - guaranteed.
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