Newbie needs antenna SWR help.

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07Yamaha

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Hello all.
New to the ham world so sorry for the basic questions. I have an N9TAX dual band slim jim (2m/70cm) I am currently feeding with a Yaesu FT-2980. Antenna is hanging from a 12 foot high ceiling. About 12' of coax between the antenna and the radio. SWR is around 1.4 at 148.000, 1.7 at 146.700 and goes up to almost 2.5 at 145.170 Tried a different radio and a different piece of coax connecting to the meter from the radio. Same readings. Any ideas? Antenna location? Antenna bad? Where do I start? (just got the SWR meter today and it is set up correctly. I triple checked) Never heard anyone complain about the SWR on these antennas. All great reviews. Thanks in advance. 73
 

popnokick

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Is it hanging away from all metal objects? J-pole antennas like the N9TAX will couple to nearby metal and affect performance. I had an N9TAX with similar SWR and recently gave up on it when I tried the Edison Fong J-pole antennas. Much lower SWR and better reports from a half dozen of our club members who also got them. Be certain to get the Amateur band version (there is also a commercial bands version). The DBJ-2 is an all-wire rollup version, the DBJ-1 is to be placed inside PVC (type specified by Dr Fong).
What you need to know is here - Ed's Antennas
 

_sundown_

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Is this the roll up antenna? The Yaesu 2980 does 80 watts max. Check your wattage setting for each freq. He site says 100 watts for the antenna but it could be a bad one he delivered just by chance...it happens.. Try all three freq's at 10 watts first and see what you get to start...
 

keary

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Based upon the VSWR numbers you provided, it appears that your antenna may be too long. You may consider trimming its length some to bring it back down to resonance. I would try 1/4" increments till you get a good match. While that is a very conservative approach, it is easier to prune the antenna slowly than it is having to add onto it. I use 146.520 MHz as my reference frequency as that is mid band and offers a good operating bandwidth. Keep in mind that surrounding objects do affect antenna resonance and performance as well so if you relocate the antenna the resonant frequency will change.

73 de Keary
 

WB9YBM

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Hello all.
"SWR is around 1.4 at 148.000, 1.7 at 146.700 and goes up to almost 2.5 at 145.170"

it sounds like the antenna may be tuned for the top end of the band--to lower the frequency, you need to lengthen it a bit...
 

WB9YBM

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Based upon the VSWR numbers you provided, it appears that your antenna may be too long. You may consider trimming its length some to bring it back down to resonance. I would try 1/4" increments till you get a good match. While that is a very conservative approach, it is easier to prune the antenna slowly than it is having to add onto it. I use 146.520 MHz as my reference frequency as that is mid band and offers a good operating bandwidth. Keep in mind that surrounding objects do affect antenna resonance and performance as well so if you relocate the antenna the resonant frequency will change.

73 de Keary
Doesn't an antenna need to be lengthened to lower the frequency?
 

prcguy

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Before you lengthen or modify anything, find some snap on ferrite beads like those that come on computer cables, etc. Snap three of them on your coax and slide them up and down the coax near the antenna and see if you can find a spot where the match gets better. The type of antenna you have lights up the coax with RF and the ferrite will stop most of it where they snap on and you might be able to tune a resonant 1/4 wavelength in coax that can sway the overall match. One snap on bead will not do anything and it usually takes three at 2m or two wraps of coax through one bead can also work. Most of the beads from computer cables are similar to #43 mix which is good for the VHF/UHF range.
 

W5lz

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prcguy
It's pretty simple to lengthen a slightly short antenna. Why go to the bother of trying to use the feedline to make up for that 'shortness'? {baleing wire is your friend!}
 

cmdrwill

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Well, the SWR readings may be affected due to RF on the coax and you would never know.. unless yo use prcguy's test suggestion.
 

prcguy

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Its not that easy to lengthen a ladder line type J-pole and they are notorious for having feedline radiation which will affect VSWR. A couple of cheap #43 mix ferrites can possibly address two problems, why wouldn't you want to do that??

prcguy
It's pretty simple to lengthen a slightly short antenna. Why go to the bother of trying to use the feedline to make up for that 'shortness'? {baleing wire is your friend!}
 

W5lz

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I didn't know they were "notorious" for having feed line radiation. I wonder why, I've never had that problem.
If that 'J-pole' is fed as they normally are (tapped) then it would certainly require retuning, same for any type of antenna really, not just a J-pole. If it isn't 'tapped' then you would certainly have to re-tune. Simply adding torroids isn't really fixing anything, just hiding the problem.
 

prcguy

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Instead of me telling you about something, I'll let someone else do it. Just because "you wonder why" doesn't mean its not there. This is from W8JI dot com.

J-pole Antenna
Because the J-pole and Zepp are electrically identical in function, and are similar to all other end-fed antennas in problems, pages on J-poles, Zepps, and end-fed verticals overlap.

The J-pole and other end-fed Hertz antennas are prime examples of antenna that can have severe feed line common mode current problems. The coax shield has to be at zero volts potential and have exactly equal and opposite currents to those flowing into and out of the center conductor at the load and source, otherwise the feed line radiates.

When we allow the feed line shield to be part of the radiating system, due to poor feed system design or construction, the system can be unstable. With improper feed line and mast decoupling, feed line and mast length and grounding can affect SWR. Weather changes can affect feed line moisture between the outer jacket and the support for the feed line, and this can change SWR with rain or snow. Even if SWR does not change, pattern can change significantly. For example, just reversing the shield and center on a J-pole feed point can change low angle field strength several dB, without affecting SWR!

Potentially severe common-mode feed line problems of end-fed 1/2 wave antennas vary with feed line length and feed line routing. This is why some people swear by end-fed antennas, while other people swear at end-fed antennas.

The J-pole is a good example of a poorly implemented feed system, because it mixes balanced and unbalanced systems. In the J-pole, an unbalanced end-fed half wave radiator is fed by a balanced 1/4 wave stub. The balanced stub is fed by unbalanced coaxial cable. This creates two improperly treated balanced-to-unbalanced junctions. Additionally, a metal support is often connected to the J-pole antenna, adding a third variable."





I didn't know they were "notorious" for having feed line radiation. I wonder why, I've never had that problem.
If that 'J-pole' is fed as they normally are (tapped) then it would certainly require retuning, same for any type of antenna really, not just a J-pole. If it isn't 'tapped' then you would certainly have to re-tune. Simply adding torroids isn't really fixing anything, just hiding the problem.
 

W5lz

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That balanced "U" when connected to the radiating part of the antenna it is no longer "balanced". Any antenna can have tuning problems so a 'J-pole' is no different. The answer is to simply fix the 'problem'. How? That depends on the problem, doesn't it.
All antenna have two 'parts'. If that 'other' part isn't there, it simply doesn't radiate, the signal never gets to the antenna. Some times that means the feedline is the 'other-half', or radials.
 
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