Topband: What have I done?

[Carl Braun]

Thanks for the response and the calculations Don. Lots of offline responses to my query.

The antennas use 90 radials 40' to 150' with the average at 66'

Your option B is the way I configured the L network and your numbers are very close to mine.

The second variable cap was set aside to use the smaller one.

I'm going to try and roll my own inductor and apply a 70pF cap to gnd or variable cap to resonate

Wish me luck

Thanks

Carl AG6X

Sent from my iPhone

On Feb 14, 2016, at 11:56 PM, "Don Kirk" <wd8dsb at gmail.com<mailto:wd8dsb at gmail.com>> wrote:

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<mailto: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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