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Author Topic: Help with vertical for 80m  (Read 880 times)
N3NEP
Member

Posts: 13




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« on: May 20, 2013, 05:25:28 PM »

Hi, I have an LDG S9v43 I have built a copper coil for 80m from a design my friend made and is using. He gave me a radio works T-g4 line isolator but it has a brown spot on the plastic.  When I connected everything up using my mfj 259b analyser I could not get the swr below 14:1.  I have a dx engineering un-43 r UNUN.  I am only intending on using the antenna for 80 & 160m would the unun work in place of the line isolator?  I was given the unun and it is brand new so if it will work I would rather use that than have it sitting around and spending money on another insulator.  If the unun will not work for my application maybe you could offer an alternative to the radio works.

Thanks
N3NEP
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WB6BYU
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Posts: 13117




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« Reply #1 on: May 20, 2013, 07:09:57 PM »

A typical 43' vertical would have an impedance (ignoring ground losses)
around 12-j500 on 75m.  That's 12 ohms resistance and 500 ohms capacitive
reactance.  You can cancel the -j500 with a series coil, but that still leaves
you with 12 ohms.  Without the un-un, that's best case SWR of about 4 : 1.

If the un-un has a perfect 4 : 1 ratio, it would step the 12 ohms DOWN to
3 ohms, for an SWR of about 16 : 1.

Depending on your ground system, the ground loss could add 5 to 50 ohms
to the R value, which will improve the SWR (though at a corresponding loss
of efficiency.)

In this case, removing the un-un will improve the SWR considerably.  The
same goes on 160m, where R is perhaps 3 ohms:  you don't want to step
that down any further.  (In practice the un-un is not perfect, and the
shunt reactance may serve to increase the R value that you have to match.)
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N3NEP
Member

Posts: 13




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« Reply #2 on: May 21, 2013, 01:32:25 AM »

Thanks, so the unun will not be something I can use in this case.  I will have to find a line isolator to replace the radio works T-4G.
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GW3OQK
Member

Posts: 136




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« Reply #3 on: May 21, 2013, 02:36:33 AM »

I agree with BYU's comments. I am preparing an ATU for a similar antenna in a telescopic pole. The tuner/matching unit goes at the base of the vertical and connects to ground rods and some buried radials.

For 80m a series L of about 7.5 uH and a parallel capacitance of about 1500 pF.

For 160m a series L of about 51 uH and parallel capacitance about 6000 pF.

The L is in series with the antenna and the C across the 50 ohm coax. This is of course theoretical and I wait to try it soon. I have a roller coaster L and ex radio variable Cap. I don't see how ununs can properly match non-resonant antennas for they don't tune out reactance. This arrangement should resonate the antenna, tuning out the reactance, and convert to 50 ohms, and I'll tune it with SWR meter at the feed point.
Andrew
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N3NEP
Member

Posts: 13




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« Reply #4 on: May 21, 2013, 04:43:10 AM »

I found this item on the dx engineering website and it seems it would do the trick.

Feedline Current Choke, Maxi-Core, 1:1 Ratio, 1.8-60 MHz, 50 ohms, 2 kW, Two SO-239s, Aluminum Enclosure

Thanks
N3NEP
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WX7G
Member

Posts: 5954




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« Reply #5 on: May 21, 2013, 06:02:46 AM »

A feedline choke (balun) is not needed with a ground mounted vertical. Connect the loading coil between the antenna and the coax inner conductor and adjust it for lowest VSWR. Ground loss resistance will dominate the input impedance on 160 meters and possibly on 80 meters. You should expect a VSWR below 3:1 at resonance.

The loading coil can also be used for matching by connecting it between the antenna and ground. Connect the coax shield to ground and the inner conductor to whatever turn on the coil gives the lowest VSWR. This is an iterative tuning procedure and by going back and forth between the number of coil turns for loading and the inner conductor connection point a 1:1 VSWR can be achieved.
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