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Author Topic: Telescopic mast and rising VSWR  (Read 380 times)
G7VBA
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Posts: 2




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« on: August 31, 2005, 06:08:44 AM »

I am hoping someone can help me with a problem re. my vertical antenna that I use for 28mhz.

I have the antenna mounted on am aluiminium telescopic mast at end of my garden the rear. Without an atu, the antenna will tune to almost all of the 10 metre band. However, this is with the mast telescoped down ie at a height of 8 ft. As soon as the mast is erected the SWR shoots up to over 3 on some frequncies - specifically the ones I would usually use (it is around 1.5 when down).

I have tried

- using different coaxial feed - no change
- insulating the base of the antenna from the clamps fixing it to the telescopic mast (by wrapping Jiffy bubble wrap around the base before mounting) - no change

I am at a loss as to what could be causing this. I have tried a different vertical antenna and that seems to be much the same.

For info the antenaa is a Shakespeare 2010 which should easily have the bandwidth, the mast fully erected is 30 ft and has 3 ft concreted in the ground. I have approx 120ft of RG213 running to my spare bedroom on the fist floor of my house. My radio is earthed. There is no earth running from the antenna to ground.

The antenna when mast is erect is well over a 1/4 wave from any trees shed etc.

I would welcome any thoughts.

Many thanks.
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KB4QAA
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Posts: 2279




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« Reply #1 on: August 31, 2005, 08:06:18 AM »

Using good coax and swapping for comparison sounds good so far.  

My next suspicion is the length of the coax causing false SWR readings in the shack due to wavelength.

Try checking the true SWR by using a short length of coax that is just long enough to reach the ground and let you stand near the pole for comparison.  If the SWR is good then you know you have an SWR effect in the feedline.

Half wave at 28.4 Mhz is about 16ft and full wave about 33ft.  120ft of feedline is about 3.6 wavelengths.  Try shortening or lengthening the coax.  You want to avoid even wavelenghts so either revmove about 16ft to get to 3L or add 49ft to get to 5L.  If you are careful you can use a coax stretcher. Smiley

One other possibility is that of the coax radiating and interfering with your meter.  Insert a 1:1 current isolator close to the antenna to check if this improves your readings.   This is always a good idea anyway to prevent annoying the neighbors and reduce RF in the shack.

GL,  Bill
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KB4QAA
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Posts: 2279




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« Reply #2 on: August 31, 2005, 08:20:13 AM »

One other thought.

I can't find any info on the antenna but.  Rather than insulating the antenna base from the mast, you may need to ensure it has a good physical and electrical connnection to the mast.   Many verticals use the mast as the counterpoise to the radiating element (and sometimes the coax shield, so the current isolator may not be a good thing).  Check the manual to see what the mounting requirements are.  

Be sure that you don't have any painted over joints in your mast, that the set screws on the antenna are touching bare metal.  You may be able to improve things by running a ground wire from the antenna base down to a ground rod.  

Bill  
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K4JSR
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Posts: 513




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« Reply #3 on: August 31, 2005, 08:32:54 AM »

Matt,
I am not at all familiar with the antenna you mention.
Is it a 1/4 wave vertical?  If so, you have not mentioned any "ground plane" radials.  As you elevate the antenna further from ground you lose any and
virtually all counterpoise action from old Mother Earth.  You will need to add four radials to the ground side of you antenna to replace the "ground"
that is missing at 30 feet.  These radials should be
at right angles to the vertical element and for an
"X" just under the vertical.  The length of each radial should be about 9 feet in length. Let them droop at an angle of 45 degrees.  Once installed you can prune the radials for best VSWR.
Check some of your RSGB antenna books for more info about ground plane antennas.
Good luck and 73.   Cal  K4JSR  
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G7VBA
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Posts: 2




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« Reply #4 on: August 31, 2005, 09:26:34 AM »

Thanks for the replies.

Bill - I will try to vary the coax length by adding a section ie a longer patch lead. Will leave the current isolator for now. The instuctions I have which are very vague indicate the antenna should NOT be insulated from the supporting mast but it appears to have the same SWR either way. I will try the grounding option (which is mentioned in the instructions but is not clear whether it is for the radio or antenna) - what would you recommend - copper wire jubilee clamped to the antenna just above the SO 239 connection and then run to a copper earthing rod buried in the ground ?

Karl - It is  5/8 wave vertical originally made for CB bands I believe but also suitable for 10 metres. It is much like the Antron / Solarcon 99 but a little longer. My understanding is that such antennas do not need a couterpoise / radials.

Thanks

Matt  
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KC8HKI
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Posts: 20




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« Reply #5 on: August 31, 2005, 09:52:05 AM »

The antenna is using the mast as a counterpoise. You said the mast wasn't grounded - I'd try that first.
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WA6BFH
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Posts: 646


WWW

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« Reply #6 on: August 31, 2005, 09:59:30 AM »


If you place the antenna in an open area (at least ¾ wavelength from conductive objects on its horizon), and place it as high in the air as you can and still get to the base ( such as, put it on a 10 foot mast, and place a wooden table next to the mast to stand  on) you should be able to measure the resonant curve of the antenna over the extent of frequency range that you would like to use it.

When the antenna is truly resonant (when Xc = Xl at the center of the frequency range) the antenna should work nicely over this range, and antenna height (as in ground capacitance) will not de-tune or re-tune the antenna. Coax length will not matter, until the coax gets so long as to just be too lossy,

Also, never use any consideration of changing coax length for a non-resonant antenna, unless you realize why this is happening, and are doing it for a specific reason. Always know the velocity factor of the coaxial cables you use. You will not be able to trust the stated velocity from most manufacturers!
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N6AJR
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Posts: 9891




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« Reply #7 on: August 31, 2005, 11:20:58 AM »

I have used the 17 foot tall antron 99 on 10 meters a while back and it was attached to a push up mast, and it worked fine and it did have radials on itn (3) and worked better with than with out.. mount the antenna to the mast as it states in the directions, and radials always help.  make some guys, attachedat the base of the antenna go out 8' 6" and put in a insulator.. presto, instant guys.
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X-WB1AUW
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Posts: 559




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« Reply #8 on: August 31, 2005, 12:35:47 PM »

Can you tune the antenna?

If so, chart the SWR, then adjust the antenna, chart it again.

73
Bob
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WB6BYU
Member

Posts: 13038




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« Reply #9 on: August 31, 2005, 02:06:11 PM »

Adjust the height of the telescoping aluminium pole for
lowest SWR - I'd guess you will find a better SWR around
24' rather than at the full 30'.

Even better:  attach a set of 1/4 wave radials to the
mast about 4m below the feedpoint.  You can try moving
the radials up and down the mast to lower the SWR if
needed.


Clearly the Marketing department decided that being able
to advertise it as a 5/8 wave vertical took precedence
over the Engineering department (if such a thing existed.)
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VK2XDZ
Member

Posts: 48




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« Reply #10 on: August 31, 2005, 08:44:00 PM »

Why not try an inverted v dipole.You already have the coax,get some wire and wind an aircore choke balun and give that a go.
regards
Pete vk2xdz
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