[SECC] Matching T Vertical for 160 meters

Wed Jul 18 08:36:29 EDT 2007

Hey Jeff, Jerry and all...

I don't know if this makes any sense, but I just started poking around the internet and found 
this stuff.  

Take a look at this link:  http://www.epanorama.net/documents/wiring/coaxcable.html  it 
contains information regarding coaxial cable.  The chart displays sort of screwy, but pay 
particular attention to the capacitance/ft values of the different coax.  Notice that RG213 has 
a 30.8pf/ft value.  This would be approximately 71.5 feet to equal 2200pf.  Now I can't 
imagine a contestor not having a piece of coax laying around.  Look at this page, because it 
list several common coax.  

For the inductor, I would try to find a wrecked Butternut veritical and use those coils.  

However, here is an article that I found on physical construction of inductors that I thought 
was very interesting...   http://www.arrl.org/tis/info/pdf/9708033.pdf  

I have run out of time this morning to play with this, I could not find anything to determine the 
actual number of turns, the diameter of the inductor and the size conductor for 9.8uh as Jerry 
has suggested .   I bet there are some in the SECC and ACG that can tell (me) us where to 
look.... hi hi

Oh well, been overnight in Montgomery and this gave me some entertainment--thanks!  

cul

Jim/KC4HW

On 17 Jul 2007 at 22:31, K4SAV wrote:

> 
> For a 130 ft long Tee antenna, 35 ft high and not allowing anything
> for droop, it should resonate at 2.07 MHz. Radiation resistance is
> about 8.2 ohms. With a ground radial system consisting of 32 quarter
> wave radials, the feedpoint impedance at 2.07 MHz should be about 16
> ohms. At 1.85 MHz the impedance should be about 70 -j79.
> 
> One way to match this is with an LC, the C across the coax input and
> the L in series with the antenna. C = 2200 pf, L = 9 uH.At 1500W, the
> voltage across the C is 203V and current is 5A.Voltage across the L is
> 895V and current is 8.7A. Power loss in the inductor is 45 watts for
> an inductor Q of 200. Two to 1 SWR bandwidth is 50 kHz. This is a
> large value of cap, so maybe another type matching network would be
> better.
> 
> Including the matching network and ground losses, antenna gain is -3.4
> dBi at a take-off angle of 26 degrees. Ground loss is 607 watts for
> 1500 watts input.
> 
> The matching network is only an estimate. There are a lot a factors
> that cause the actual required values to be different from the
> calculated ones. Actual loss in the radial system versus the assumed
> value is a big factor. I assumed an equivalent ground loss of 8.5
> ohms. Actual wire lengths used for hooking up the matching network is
> another. Antenna droop was also not included in the calculations.
> 
> The way I do these matching networks is by measurement. I have a small
> variable cap and a variable inductor. I take those and an antenna
> analyzer and sit down at the base of the antenna and vary the LC
> configuration and the values until I get a match. Then I measure the
> values. Then I replace them with the appropriate size L and C. I do go
> back to the simulation to calculate voltages and currents on the LC to
> see what those ratings need to be.
> 
> A measurement of the antenna impedance using an analyzer might work OK
> if there are no strong radio stations close to you.Usually with a big
> antenna like this, AM radio stations cause big problem with the
> analyzer on 160. I have a 1 KW station on 800 kHz, 8 miles away, that
> messes up my analyzer on 160.If you get a good reading of the
> impedance, you can use that to calculate a matching network. The
> readings you get won't directly translate into matching network values
> (unless the R reading is 50 ohms).
> 
> Jerry, K4SAV
> 
> ku8e at bellsouth.net wrote: 
> 
>       I recently switched my center fed zepp from one cut for 40
>       meters to one that is 130 feet long.
>        It is about 35 feet high (35 feet of 450 ohm line) in the trees
>        and a flat top.
> 
>        I was thinking by shorting the feeders to use it on 160 meters
>        as a T vertical. Problem is that I don't 
>       have an extra antenna tuner lying around to stick out in the
>       yard at the bottom to tune it. Does anyone have an idea what the
>       feedpoint impedence is for this type of antenna ?
> 
>       Also, I wonder if anyone has built a matching network using the
>       method described in K3KY's article on tuning bobtails and half
>       squares ?  See http://www.angelfire.com/md/k3ky/page40.html . He
>       basically uses a length of RG8 or RG213 for the capacitor and
>       winds a coil for the inductance.
> 
>        I bought a MFJ 259 B Antenna analyzer in Dayton this year. It
>        measures inductance and  capacitance. If I adjust for the
>        lowest SWR by temporarily putting an antenna tuner at the feed
>        point or the antenna and then measure the C and uH using the
>        259B will that give me the exact values I   need for my
>        capacitor and inductor ?
> 
>        Jeff
> 
> 
> 
> 
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> 
> 
> 
> 

Jim/KC4HW
http://www.AlabamaContestGroup.org
http://www.AlabamaQSOParty.org