Topband: What have I done?

[Don Kirk]

Hi Carl,

Does not appear anyone has answered you, so let me give it a quick shot
while between flights in Japan.  I'm somewhat confused with your final L
network configuration because you mentioned one capacitor, and then a
second capacitor without mentioning if you still had the first capacitor in
your configuration, etc.  You also did not mention what L network
configuration you used (see below).

Therefore I started from scratch to see what a 64 foot vertical wire would
look like at 7.050 Mhz to see what kind of L network would be required to
convert its impedance to a pure 50 ohms resistance.

Using EZNEC demo, I approximate a 64 foot vertical wire on 7.050 Mhz has an
input impedance of approximately 1274 +j1436 ohms when just using a ground
rod with medium soil (because this antenna has such a high input impedance
it really does not matter what I use for ground conductivity).

Then using an online L network calculator I come up with the following two
different L network configurations you could use to match to a 50 ohm
resistive source.

*L network with inductor between bottom of 64 foot vertical and ground.*
L = 10.18 uh
C = 59.9 pf

*L network with capacitor between bottom of 64 foot vertical and ground.*
L = 8.5 uh
C = 67.6 pf

Not sure what L network configuration you used, but does it sound like I
closely replicated what you have based upon one of the above mentioned L
network configurations?  If not, then please better describe your
configuration.

Please advise so we can better answer your question, and hopefully I have
not messed something up with my quick analysis/approximation.

Don (wd8dsb)




On Sat, Feb 13, 2016 at 4:53 PM, Carl Braun <Carl.Braun at lairdtech.com>
wrote:

> I have revised my question to correct the length of the antenna in question
>
>
> Topbanders
>
> In the past, I've posted some questions related to making my 80 meter
> verticals (64' tall ground mounted) work on different bands.  The pair is
> spaced at ¼ wl on 80m and then I switch in base loading to resonate the
> same pair on 160m which are then effectively spaced at 1/8 wl.   Now I want
> to make them work on 40m which would have them resonate as ½ or 5/8 wl and
> would have an effective spacing at ½ wl.
>
> So this is what I did...I installed an L network off of the vertical
> effectively tuning a 64' stick of aluminum to 7.050.  It took me a couple
> of tries as the first 1500pF (monster) variable capacitor wouldn't get down
> low enough to get me flat.  I had another 50-150pF Johnson variable cap
> available so I tried that and got the antenna to tune to 46 ohms at j+0.
> Inside the shack I see 1.0:1 Vswr from 7.000 to 7.290...wow!
>
> I measured the cap and it came to 70pf.  My cheapie Chinese meter doesn't
> register anything on the L scale but I have 7 turns on a 3 ½" inductor. The
> full coil is 33 uH at 25 turns so I estimate 8-12 uH of inductance.
>
> But what have I done? Have I resonated a ½ wl antenna or have I resonated
> something else like a 5/8 wl antenna with the added inductance?
>
> I plan on treating them as ½ wl spaced phased verticals on 40 and feeding
> them with equal lengths of ¾ wl feedlines for broadside phasing and then
> adding an additional 2/4 wl to get my 180 degree shift for end fire.
>
> Please share your comments.
>
> thanks
>
> Carl de AG6X
>
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>