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Author Topic: 1/2 wave dipole problems  (Read 1215 times)
KC8RCF
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Posts: 12




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« on: February 10, 2009, 06:49:20 PM »

I built a 1/2 wave dipole with a coax choke using LMR-400. the choke is about 18 feet of the lmr coiled on a 2 inch pvc pipe. the wire for the antenna is 16gauge copper wire, 8.4 feet long each side. the choke is about 3 inches from the antenna. total length of coax after the choke is 35 feet. Antenna in located 14 feet off the ground pull tight horizontally. choke and coax are 90 degrees from antenna (vertical)

First problem.

This set-up will not come close to tuning 10meter. it does tune 6 meter pretty well though. what gives?

Second problem.

Even on 6 meter if i raise my power more than 3-4 watts my SWR jumps past 3:1. I'm not one to run my gear past about 1.5-1.7 so this is not going to work.

Can someone please point out my mistake?  what’s going on here? I don’t have an antenna analyzer and the closest person around that does is more than 30 miles in either direction, joys of living in a remote part of Wyoming.

Thanks
Nate
KC8RCF

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W5FYI
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Posts: 1044




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« Reply #1 on: February 10, 2009, 07:11:44 PM »

How does it do in the CB spectrum, say around 27.85 MHz? I'd shorten them a little; make each about 8 feet long, and see how the dipole works on 10 meters. You still may have an SWR a little higher than 1:1, but the antenna length should be closer to resonance at around 29.4 MHz or so (assuming that's where you want to work HF).
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KC8RCF
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Posts: 12




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« Reply #2 on: February 10, 2009, 07:46:32 PM »

i have cut almost a foot off 1 inch at a time and not much happens. it is well over 3 on 11m

thanks
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WB6BYU
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Posts: 13029




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« Reply #3 on: February 10, 2009, 08:17:13 PM »

Did the SWR change when you cut off the ends, or did it stay about the
same?

When you say the antenna "doesn't tune", do you mean that the SWR is
high on the antenna, or that even with a tuner the SWR is still high?

Have you plotted the SWR at 100 kHz points across the 10m band?  Is
it lowest at one of the ends or in the middle somewhere?

I'd start by checking your coax and connectors.  If the SWR didn't change
at all my first suspicion would be a short at at the feedpoint.  You can
check this using a continuity tester on the connector at the rig.  Another
common problem is a poor connection between the shell of a plug
and the shield of the coax inside (in which case the SWR might vary
as the antenna is adjusted.)

Insulated wire does tend to shorten element lengths.  If the SWR has
been decreasing as you prune the antenna,  you might just have to
prune it a bit more.  If it has been going up, try lengthening it instead.
If it remained constant then the problem is probably elsewhere.

I presume, of course, that you have the internal tuner bypassed in your
rig?  If not (presuming it has one) that can really mess things up.

You might also check the minimum rated bending radius for the coax
cable you are using:  2" PVC pipe may be too small.  Usually this becomes
a problem over time as the center conductor migrates inwards (especially
in hot weather), so it may not be a problem yet if you just made the
choke.

How much tension did you need to get the antenna horizontal while
supporting the weight of the coax?  What are you using for a center
insulator?  End insulators?  Support rope?  How far is the dipole from
large metal objects?
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W5FYI
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Posts: 1044




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« Reply #4 on: February 10, 2009, 08:18:20 PM »

If you can find where the antenna is resonant now (e.g., its lowest SWR, or by using an antenna analyzer), multiply that frequency by the overall (tip-to-tip) antenna length, then divide that length by the desired frequency and you will get the new length for that frequency. Let's say your antenna is now 16.8 feet tip-to-tip, and the lowest SWR occurs at 27.7 MHz. You would multiply 16.8 × 27.7 to get 465.4. Now, dividing 465.4 by the desired frequency, say 29.4, the result is 15.8 feet. By trimming half a foot off each leg, you should be able to increase the level of resonance (to get lower SWR) at the new frequency.

Of course, other factors may influence your results. If the antenna is near metal siding, gutters, or heating ducts, it may be coupling to them and skewing results. In that case, it may take some trial-and-error to trim the antenna, or it might need an antenna tuner to lower the SWR, or you might want to move the dipole away from the affecting environment.
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KC8RCF
Member

Posts: 12




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« Reply #5 on: February 11, 2009, 06:50:20 AM »

 the choke and coax are supported independent of the antenna.

 the antenna swr does not change enough on 10 meter to detect. on 6 meter the swr does change with tuning.

 i'm running a 746pro with the antenna tuner turned off. with it on it will tune but then it will tune about anything.

 i will check the coax bend limits. 2 inch may be to small. the connectors will be checked again. before i put it up i checked for a short and found none.

thanks
nate
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WW5AA
Member

Posts: 2088




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« Reply #6 on: February 11, 2009, 07:36:24 AM »

I think the problem is in the choke. The low height of the antenna should have enough ground loss to show a low SWR. Since it's not very high, I would try it without the choke. In most cases there is no need for a choke with a resonant antenna.

73 de Lindy
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WB2WIK
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Posts: 20547




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« Reply #7 on: February 11, 2009, 02:36:25 PM »

LMR-400 is not suitable coax for winding choke baluns.  

It's probably already damaged by winding it on a 2" diameter form; its bend radius is 4".

Why use LMR-400 for a 35' feedline and a choke balun at 28 MHz?  Makes no sense.  Kind of like using a fire hose to fill the fish tank.

I'd use RG8X or RG58A/U for this.  

WB2WIK/6
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KC8RCF
Member

Posts: 12




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« Reply #8 on: February 12, 2009, 10:45:19 AM »

i have lmr-400 and nothing else. so i used lmr-400. again remote part of wyoming. 30 mile drive for RG-58 is not really worth it.
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K7KBN
Member

Posts: 2765




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« Reply #9 on: February 14, 2009, 01:39:36 PM »

Well, as stated earlier, you've seriously compromised your coax by winding it in a 2" diameter coil.  If the manufacturer specifies a bend radius of 4", that means a coil diameter of 8", minimum.  

Doesn't really matter how far it is to the store.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
Member

Posts: 13029




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« Reply #10 on: February 14, 2009, 04:43:33 PM »

The likely failure mode of too tight a bend is (eventually) a short circuit.
Check the coax with an ohmmeter as a quick check.

The choke coil itself shouldn't have any adverse effect on the antenna
even if it isn't optimized.  Try straightening the coax and see if the
antenna works.
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KC8RCF
Member

Posts: 12




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« Reply #11 on: February 17, 2009, 07:31:58 PM »

found one bad connection and problem seemed to go away. took the choke out anyhow. thanks for the help.

nate
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