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Author Topic: Problems tuning full 1/4 wave mobile vert.  (Read 3559 times)
M6GAS
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« on: January 30, 2010, 10:06:38 AM »

Brand new 9ft tank whip from rocket radio last week.measures 2770 mm.

1/4 wave of 27.555mhz is 10.879mm divided by 4 = 2724mm so i expect to cut 46mm off.

as it is now,uncut,the flattest my SWR will go is 1.2:1 at 33.000 mhz !!!!

and steadily rises either side of 33mhz.

1/4 wave of 33mhz = 2271mm so my whip seems to be 501mm to short!!! half a meter....dont think so.

Having built and tuned many antennas,im baffled to understand whats wrong here.

given that every other part of my install is exactly the same as it always been(1.2:1 with my wilson 5000) apart from the new tank whip AND the sirio 3/8 adaptor i also purchased to convert my wilson 5000s so239 roof mount(central on top of the roof drilled/fixed) ,am i missing something about which i do not understand?
i.e do unloaded 1/4 waves employ different characteristics than any other antennas,require,more/less Rf grounding,have wild feedpoint impedance......whatever.

ive since,removed,cleaned,resanded the metal where the ground connection on the underside of the roof,swopped out the older thin coax,for a shorter run of big stuff just as precaution to a 2 year old install,and reduce feedline loss a little in future)and continuity,tested all plugs.
Basically im stuck,dont want to cut the whip yet nor should i have to,and cant see how to solve this.
could something as simple as the adaptor be at fault? how could i test this?

http://www.rocketradio.co.uk/so239--...dio-1087-p.asp

just had i thought.could my mfj 941e atu,s swr meter be misreading at 3mhz (33mhz)over hf? still doesnt sort the problem though.

Whilst i appreciate the cb frequency at which im trying to tune up is undesirable to many Hams,its how i started and still have many friends there.my main concern is being able to use this 3/8 adaptor as most dedicated band whips are in this 3/8 thread format.and i wish to use my recently aquired licence on Ham bands,without tuning my wilson through the lossy atu.
--- cheers for any help.
yaesu FT857d,,wilson5000(10m) or 9ft full 1/4 wave tank whip,2element 10m cubical quad(the BBq).split charger aux battery system.170a truck battery,mfj 941 versatune2
100percent mobile station

laters peeps 73s M6GAS Brent
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K0BG
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« Reply #1 on: January 30, 2010, 02:31:54 PM »

The first question I have is, how are you measuring the SWR? And, where are you measuring it? The best thing to do, is measure the input as close to the base of the antenna as you can, with an antenna analyzer. Remember, depending on the actual impedance, resonance may not be at the same point as the lowest SWR. Fact is, it's almost never at the same spot.
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M6GAS
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« Reply #2 on: January 30, 2010, 07:15:58 PM »

hi.just a basic swr reading,taken next to the radio im afraid.whilst i understand your correct angle of diagnosis,5.5mhz out seems way off the mark for a box fresh,sold as 11meter whip,why 33mhz aint even close to the top of 10meters.people of a lot lesser understanding  have purchased one of these and simply swopped over from 1 base loaded to a 9ft whip no problems,my worry is if when i purchase,say, a 20m whip,will this too give a  low swr reading at around,maybe 18mhz? resonance aside,my radios finals will still feel the pain.i opologise if ive missed the point here due to inexperice but any vert ive ever tuned,has been in this fashion,and has corresponded to a vswr and stated bandwith as stated in the manufactures instrutions at least.

the 3/8 adaptor still remains suspect and i was wondering if you had any ideas about how i could test this in situ.LOL bet your gonna say with an antenna analyiser hihi.
 best 73s Brent m6gas.
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M6GAS
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« Reply #3 on: January 31, 2010, 02:28:38 PM »

out of interest,today i wound a 20m 1/4 vert onto an old 2 meter long bamboo spreader i used to use on my quad kind of like a fire- stick.this had a 3/8 thread bottom so i took it out for a test on the problem adaptor.
so even if i get the wind spacing on the bottom load slightly wrong,the lengh of the wire was cut right so its gotta tune up close to 20m give or take half a meg.(it deffo Rxed loudest at 20m centre.

Guess where SWR was flattest......33.000mhz.same as with the 9ft whip and im sure any other whip will be the same.(wilson back on=0 problems)
so while im non the wiser,somehow THAT 3/8 to so239 adaptor is making any whip put on it, use the the van as a parrasitic elelment or something.
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K0BG
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« Reply #4 on: January 31, 2010, 05:24:48 PM »

When you use an analyzer to measure an antenna's impedance, the coax between them needs to be short; inches, not feet! If you're measuring on the rig end of the coax, and is just happens to be the right length, the reading you get could be anywhere.

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KL7AJ
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« Reply #5 on: March 10, 2010, 04:53:15 PM »

" When you use an analyzer to measure an antenna's impedance, the coax between them needs to be short; inches, not feet!"

Yep
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WX7G
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« Reply #6 on: March 10, 2010, 05:36:23 PM »

You can use an antenna analyzer anywhere along the length of coax to obtain a VSWR measurement.

If you want to measure the complex impedance (X and R) the antenna analyzer can be anywhere along the length of the coax. However, to obtain the complex impedance at the antenna while measuring elsewhere you will need to move the measurement plane to antenna using a Smith Chart of some mathematics.

At this point, with only the VSWR meter in the radio, I would add a 1/2 micro Henry inductor in series with the antenna base. If this moves the lowest VSWR point close to where you want it that is the way to proceed.
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WB6BYU
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« Reply #7 on: March 10, 2010, 08:52:37 PM »

The use of a magnetic mount might account for the increase in frequency because the
capacitance between the mount and the roof is in series with the antenna.  Otherwise there
aren't to many simply ways to raise the resonant frequency.  (The resonant length is
dependent on the diameter of the antenna and the taper, and a simple calculation of 1/4
wavelength in air may get you close, but usually a bit long.)  Fatter whips will be shorter,
while those that are wide at the base and thin at the top have to be longer.

Adding an inductor in series with the antenna to lower the frequency might not be convenient,
so you can try adding a capacity hat instead.  I'd start with about 2' (60cm) of fairly stiff wire
with a croc clip on each end.  Clip one to the top of the antenna and the other about 1' (30cm)
below it.  See if that lowers the resonant frequency enough.


Meanwhile, there are some other comments that still don't quite make sense...

You said that you measured the resonant frequency of the antenna with an SWR meter and a
transmitter?  So you have a transmitter that goes up to 33 MHz?  What is the SWR at the
desired frequency?  You are, I presume, using the tuner simply as an SWR meter and have
the tuner bypassed?  If not, that could cause all sorts of quirky measurements!  (The tuner
is approaching the top end of its useful range, and the internal wiring may add enough
stray reactance to shift the resonant frequency even when the tuner is bypassed.)

Also, don't expect a perfect SWR regardless of what length you cut the whip.  There is no
guarantee that it will hit exactly at 50 ohms at resonance - it might be 60 or 40, depending
on the ground losses.  As long as you can get it down below 2 : 1 at the operating frequency
you should be fine.

The shape of the ground plane also affects the resonant frequency of an antenna, and if it is
mounted in the center of the roof your ground plane doesn't extend as far as it would if it
were on the boot lid. The center of the roof is still generally better, but the metal chassis is
part of the antenna, too, and it will impact your results.

Simply winding a quarter wave of wire onto a stick will NOT make it resonant on 20m, or
necessarily anywhere close.  33 MHz is probably more likely, depending on the length and
diameter of the former.  You'll find some helical CB whips advertised as "5/8 WAVE",
meaning they had to wind 5/8 wave of wire onto the form to get it to resonate rather than 1/4
wavelength.  (The whip is still a short antenna, not a 5/8 wave type, regardless of how much
wire they wound in a small space.)  Generally a helical whip will require 1/2 to 3/4 wavelengths
of wire to be resonant.

While there doesn't appear to be a lot that could go wrong with the adaptor (as long as it shows
electrical continuity through to the center conductor of the coax), there is one test that you can
try to see.  Remove the adaptor from the SO238 fitting.  Wrap a piece of bare, stiff wire around
the threaded base of the antenna and stick one end into the center pin of the roof mount - basically
making your own replacement adaptor.  This won't hold the antenna vertical, so you'll have to guy
it with cords to the corners of the van.  (You aren't going to be driving anywhere with it this way,
I hope!)  See if you still get the same reading that you did using the adaptor.
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KL7AJ
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« Reply #8 on: March 11, 2010, 10:26:13 AM »

"You can use an antenna analyzer anywhere along the length of coax to obtain a VSWR measurement."

But THEN what do you do.  SWR will tell you if something is matched or not....won't tell you which WAY it's off.  (Unless you know the transmission line length, of course.)

Eric
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W0FM
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Posts: 2056




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« Reply #9 on: March 11, 2010, 11:50:13 AM »

Hi Brent,

I was wondering if you were using the ball and spring mount for this whip or just the 3/8 adapter?  Most 9 FT CB antennas are expected to be mounted on a heavy spring with an adjustable ball mount.  If you don't have these added items, it could be the reason your reasonant frequency is measuring high.  Both the spring and the ball add to the RF length of the antenna which would lower your measured frequency somewhat below 33 MHz.  Just a thought.

73,

Terry, WØFM
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VE3WMB
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« Reply #10 on: April 25, 2010, 06:50:26 PM »

FYI to get 1/4 wave resonance with a helically wound antenna you would need to wind
about 1/2 wavelength of wire on the bamboo spreader so it is not surprising that you ended up
resonant at nearly twice the frequency that you were shooting for in this case.

Michael VE3WMB

out of interest,today i wound a 20m 1/4 vert onto an old 2 meter long bamboo spreader i used to use on my quad kind of like a fire- stick.this had a 3/8 thread bottom so i took it out for a test on the problem adaptor.
so even if i get the wind spacing on the bottom load slightly wrong,the lengh of the wire was cut right so its gotta tune up close to 20m give or take half a meg.(it deffo Rxed loudest at 20m centre.

Guess where SWR was flattest......33.000mhz.same as with the 9ft whip and im sure any other whip will be the same.(wilson back on=0 problems)
so while im non the wiser,somehow THAT 3/8 to so239 adaptor is making any whip put on it, use the the van as a parrasitic elelment or something.
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M6GAS
Member

Posts: 6




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« Reply #11 on: August 17, 2010, 01:41:00 PM »

Cheers guys.the problem was the faulty mount 3/8th adaptor.So guys with the benefit of hindsight on my side,i now can see that the answer was amongst your replies.
i have reposted this after a long time because i re-stumbled on the post and in case,anyone else buys one of these sirio thread adaptors in what seems an electrically sound state (i.e, no breaks in continuity) and cannot find the answer.
i can also see that i was making some "school boy errors" trying to wind my own helical whips as test whips.
i now use a hustler 20m super resonator,and my 9ft whip which is sweet and swr flat at 28.500 and stays broad enough through c.b.,with a 3/8 to so 239 adaptor made by Hustler.To make an observation,the 9ft whip does a decentish job on 20m whilst using my atu,and so less and less i having to swop it out when using one whip for 10.11 and 20. static mobile.
Terry W0FM,a good point about these whips being short,because they are expected to be used with a big spring..but a lot of good advice made by all you lads..cheers.
Brent M6gas
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KI4SDY
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« Reply #12 on: August 18, 2010, 07:58:34 AM »

I am glad he found the "problem." I thought all this techno babble about checking the resonance of a 102" whip from our resident "eHam.net experts" was some kind of joke. A CB whip is like a brick. It isn't supposed to do anything but work. They are very broadbanded. If you install a CB whip and it isn't working right, don't look at the whip, look at the rest of your installation! Wink   
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W0FM
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« Reply #13 on: August 18, 2010, 08:28:53 AM »

Congrats Brent!  And thank you for taking time to stop back and let us all know about your findings and your solutions.  Too many times Elmers give pages of advice to a ham here on this site regarding a problem and never hear of the results.  I applaud you for responding even after "a long time".  Hopefully others will take note.

Enjoy the hobby!

73,

Terry, WØFM
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