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Author Topic: 6 meter slim jim 300 ohm tv lead  (Read 2977 times)
VOODOO42
Member

Posts: 114




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« on: December 12, 2010, 03:08:23 PM »

Got me a yaesu 8900r on its way and this will be my first quality vhf rig. I stumbled on an old midland 2 m a couple months back and made a 2 meter slim jim antenna from 300 ohm tv lead. Acording to the meter in the radio and my mfj grand master the thing produced a low swr. I am planning on setting that same antenna in my attic and running 20' coax to the basement. I made a 6 meter slim jim today and if all reads well with it, I am hoping to just run it down through the house also. I am wondering how well this set-up might work. Has anybody tried this before with a 6 meter antenna? I plan to do some research on some of the tri-band base antennas but I don't have the knowlege or the money now to buy anyway. The slim jim offers me a cheap deal for now and If I have to, I can certainly slide it into some pvc and string it up outdoors. Any Ideas?

Thanks
kj4iyl
winston-salem nc
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WB6BYU
Member

Posts: 13239




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« Reply #1 on: December 12, 2010, 04:11:33 PM »

The "Slim Jim" has no inherent advantage over a standard J-pole:  they are the same antenna.  All that
means in practice is that it doesn't really matter how you connect the ends of the twinlead on the
radiator - you can connect them together at the top, bottom, both or neither (as long as you don't
connect it to the top of the short side of the matching stub.)  If you are short on twinlead you can
use regular wire for the radiator (the part of the twinlead above the notch) and just use twinlead for
the matching stub (the portion below the notch.)

When you put the J-pole inside PVC pipe it will need some readjustment because the dielectric constant
of the PVC around the matching section is different than air, which changes the characteristic impedance
somewhat.  The tuning will also change if the twinlead flops around inside the pipe - the best solution
I've seen is to use some pipe insulation foam to keep the twinlead centered in the PVC.

How well it works will depend a lot on your local 6m repeater and/or the distance to the stations you
plan to work simplex.  If there is no local activity on 6m FM, it won't appear to work well at all.
But there is nothing unusual about building a J-pole for bands other than 2m, even if dimensions
are more difficult.  In most cases you just cut a half wave radiator, make the matching stub a bit
longer than a quarter wave, and experiment with the tap points for the feedpoint and a short across
the bottom of the matching stub to get good SWR.  My first twinlead J-pole was for 15m, and the
theory hasn't changed since then.
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VOODOO42
Member

Posts: 114




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« Reply #2 on: December 12, 2010, 04:34:43 PM »

The "Slim Jim" has no inherent advantage over a standard J-pole:  they are the same antenna.  All that
means in practice is that it doesn't really matter how you connect the ends of the twinlead on the
radiator - you can connect them together at the top, bottom, both or neither (as long as you don't
connect it to the top of the short side of the matching stub.)  If you are short on twinlead you can
use regular wire for the radiator (the part of the twinlead above the notch) and just use twinlead for
the matching stub (the portion below the notch.)
When you put the J-pole inside PVC pipe it will need some readjustment because the dielectric constant
of the PVC around the matching section is different than air, which changes the characteristic impedance
somewhat.  The tuning will also change if the twinlead flops around inside the pipe - the best solution
I've seen is to use some pipe insulation foam to keep the twinlead centered in the PVC.

How well it works will depend a lot on your local 6m repeater and/or the distance to the stations you
plan to work simplex.  If there is no local activity on 6m FM, it won't appear to work well at all.
But there is nothing unusual about building a J-pole for bands other than 2m, even if dimensions
are more difficult.  In most cases you just cut a half wave radiator, make the matching stub a bit
longer than a quarter wave, and experiment with the tap points for the feedpoint and a short across
the bottom of the matching stub to get good SWR.  My first twinlead J-pole was for 15m, and the
theory hasn't changed since then.


I like the idea of the foam inside the pvc to keep it from moving for an outdoor antenna. I just wonder how well the antenna will work with 13' of twin wire running vertically in the house. I guess the best thing to do is set it up and try it out for a while and see. My tower is 130' away from the shack which would mean (preferably) lmr 400 coax at .90 cents a foot + lightning arrestor which all adds up to more money. the indoor slim jim will be the cheapest way out. I am sure the 2 meter will be fine in the attic and we will experiment with the 6 meter.













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WA3SKN
Member

Posts: 5477




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« Reply #3 on: December 13, 2010, 01:18:32 PM »

FM is fairly forgiving.  It is nice to have the antenna as high and in the clear as practical.  Making single band antennas with a 20 foot run to the attic is fine.  You might consider adding an antenna switch at a later time... as funding permits.
73s.

-Mike.
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