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Author Topic: Short 75m dipole using Unadilla 75M trap?  (Read 6591 times)
K1WJ
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Posts: 460




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« on: February 29, 2012, 01:57:25 PM »

Want to look at making a 75m Inverted V using unadilla 75M trap.

Center support will be no higher than 30ft - retractable pole.

In MI had a 75m In.V. total length was 119ft - no room here in back yard.

If using a Unadilla trap for 75M - how long wires would I need before & after trap?

Would like to be 20-25ft each total or shorter - each leg.

Only interested for 75m in 50-400 mile range.

Anyone made a short trap dipole using unadilla 75M trap?

Lowest band use of antenna would be 75M - mono band - no tuner.

Thanks K1WJ
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K3WEC
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Posts: 260




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« Reply #1 on: February 29, 2012, 04:14:57 PM »

What you need are loading coils, not traps.   
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KQ6Q
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Posts: 991




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« Reply #2 on: February 29, 2012, 04:18:41 PM »

a 75m trap is a parallel circuit designed be a high impedance at 75m. To shorten your antenna, you could use a 40m trap partway out, - or more effectively,  just a insert a loading coil partway out each leg of the dipole. You could calculate a value, or just string up the length of wire that you can use, and wind a pair of loading coils on PVC. Check for resonance - if it's higher than you want, make the coils a bit longer, if too low, take off a turn or two. You could do this with some short lengths of PVC pipe and wire from home depot. Once you've got the coil size right, coat it with something to protect from sun and WX.  There are probably formulas for the loading coil size etc, but sometimes it's more fun just to experiment.

Have fun, save money !

Fred, KQ6Q
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WX7G
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Posts: 6210




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« Reply #3 on: February 29, 2012, 05:30:01 PM »

I looked and didn't find the Unadilla design guide. The would tell you how long the antenna will be.

The MFJ-17758 is a 40/80 meter dipole that is 85' long. List price is $90.
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AA4PB
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Posts: 12985




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« Reply #4 on: March 01, 2012, 05:15:44 AM »

Here's one such device:  http://www.spiromfg.com/parts.htm

Try google for "dipole shortener".

As you can see, shorteners (i.e. loading coils) typically reduce the total dipole length by about 50%.
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N4UM
Member

Posts: 480




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« Reply #5 on: March 01, 2012, 06:50:15 AM »

As mentioned earlier, you need a loading coil - not a trap - to operate the antenna only as a shortened 75 meter dipole.  If you download the free demo version of EzNec you should be able to calculate the inductance of the loading coil depending upon the overall length of the antenna and how far out from the center you wish to install the coils.  The further from the center you place the coils, the more efficient the antenna - BUT the coils need to be larger when you move them out further.

I ran some quick calculations to give you an approximation.  It you make your antenna as am inverted Vee(45 feet on each leg) with the apex at 30 feet and the ends at ten feet up, you would need at 55uH coil on each leg placed about 11 feet from each outboard end.  Estimated SWR would be about 2.5:1. at 3.85 Mhz.
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K1WJ
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Posts: 460




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« Reply #6 on: March 01, 2012, 06:53:29 AM »

Thanks guys - learned more about traps - I understand now.

I'll work this out - 73 Dave
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WB6BYU
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Posts: 13482




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« Reply #7 on: March 01, 2012, 07:13:22 AM »

While not the most efficient design, one approach I've used for a simple antenna is to
use whatever length legs will fit the available space with a single coil in the center.  Then
I wind a few turns of wire around the coil and connect that to the coax.  The main coil
sets the resonant frequency and the link determines the resistance at resonance.  I used
this technique on several 80m verticals that I needed in a hurry and it was fairly easy
to set up (presuming you have access to the coil to adjust it.)  I used T-200-2 cores for
the coils (QRP), but a chunk of coil stock or something wound on PVC pipe will also work.

This has the advantage that you have only one coil to adjust to find resonance, and the
impedance step-up is easily set by experiment by unwinding turns from the link.

A dipole with loading coils in the center of each side of the wire will be more efficient,
but you can use that for part of your loading and the center coil for the rest.  You need
enough coil in the center for the link winding to work properly.
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N4UM
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Posts: 480




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« Reply #8 on: March 01, 2012, 09:51:54 AM »

Sorry, didn't realize you wanted to make this thing really short.  I redid an EzNexc simulation using an inverted vee with apex at 30 feet and each end up 10 feet and out 22 feet from the center (leg length 28 feet).  113 uH coils placed 7 feet from each end would result in an SWR of about 2.34 at 3.850.  As another station suggested, you might consider also using a tapped center coil to get a better match in conjunction with outboard loading coils. 
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N4NYY
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Posts: 4818




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« Reply #9 on: March 04, 2012, 06:46:30 PM »

I just built a Unadilla trap dipole for 20-40-80. That 75M trap is only going to work if you run a 160M dipole. You will need a coil as others suggest. A 40M trap would turn it into a 40-80M dipole.
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