Topband: Fwd: Capacitor for Inverted L

[Jim Bennett]

Although I'm far from being an antenna "expert", I second Wes' comments about matching a 160 meter Inverted L.

I recently made my first ever Inverted L and am very, very pleased with the results. Mine has only 30 feet or so of vertical rise, and the remainder of the "hot" side runs pretty much horizontal. Being very space constricted, along with having to be stealthy due to HOA and CC&R issues, I was only able to get two radials installed. Both are 16" off the ground. When I first installed the antenna and hooked my MFJ-259B to it, it was resonant at about 1.700 MHz. I pruned back the radials several feet and left the "hot" wire alone. Doing so got the antenna to resonate at 1.840, with a 60 KHz 2:1 bandwidth. I feed it with 115 feet of RG-8 and have a huge coax RF choke at the feed point. With this "crummy" antenna setup I've worked the east coast of the USA during the recent California QSO party, in addition to easily working the guys at T32C. This has been with 100 watts from an Elecraft K3. The auto-tuner in the K3 has no problem getting a match up in the phone part of the band. No external matching network needed here...! :-)

I put my final measurements into EZNEC and the far field plot shows the antenna being omni-directional (expected), but with a fairly low take off angle - about 25 degrees. I fully expected that I'd wind up with an NVIS cloud warmer, but the on-air results pretty much compare with the modeling data.

Jim / W6JHB

Begin forwarded message:

> From: "Wes Attaway (N5WA)" <wesattaway at bellsouth.net>
> Date: October 13, 2011 10:23:41 AM PDT
> To: dj7sw at kcag.de, topband at contesting.com
> Subject: Re: Topband: Capacitor for Inverted L
> 
> The question of "matching" an Inverted-L comes up every now and then, and
> under some circumstances it may be necessary to install some kind of
> matching network, particularly if you want to operate across the entire
> band.
> 
> However, for most situations I do not think a matching network is necessary
> because the SWR will likely be easily "controllable" by simply adjusting the
> length of the horizontal wire.  Even if you don't want to try this the
> native SWR of a normal Inverted-L will be less than 2:1 for about 100KHz of
> bandwidth.  Feedline loss is negligible on 160 and, in any event, most
> transceivers and amps can easily match whatever impedance you encounter.
> 
> If you model a simple Inverted-L you can see that adjustments of a few feet
> for the horizontal wire will put the low SWR point just where you want it in
> the band and the radiation pattern is not changed to any meaningful extent. 
> 
> So, for most people, fooling with a matching network won't buy you anything
> but a lot of trouble and it adds another place for things to go wrong.  In
> my opinion, the best thing to do is to put up the antenna, check the SWR,
> make a length adjustment if you think it is necessary, and then go get on
> the air and make some QSOs.
> 
> If you want to spend some time "fiddling" then my recommendation would be to
> spend it on your ground radial system, not worrying about a matching
> network.  As your radial system improves the matching will change, but you
> can still make horizontal wire adjustments as well.
> 
> ----------------- Wes Attaway (N5WA) ------------------- 
> 1138 Waters Edge Circle, Shreveport, LA 71106 
>    318-797-4972 (Office) - 318-393-3289 (Cell) 
>        Computer Consulting and Forensics 
> -------------- EnCase Certified Examiner --------------- 
> 
> 
> -----Original Message-----
> From: topband-bounces at contesting.com [mailto:topband-bounces at contesting.com]
> On Behalf Of Lup Schlueter
> Sent: Thursday, October 13, 2011 9:49 AM
> To: topband at contesting.com
> Subject: Re: Topband: Capacitor for Inverted L
> 
> Hans Gatu SA7AUY had a very cool idea for a selfmade variable power 
> capacitor.
> It has about 300pf, but as you will agree, there is no limit for 
> higher values.The material
> will surely agree every flash over!
> 
> See self:   http://www.heathkit.nu/heathkit_nu_AMU.html
> 
> 55 Lup, DJ7SW
> 
> 
> 
> Am 13.10.2011 13:05, schrieb Gary Smith:
>> I used to use an ugly, beat up 500pf vac variable at the base of my
>> 160M inv-L&  used the MFJ SWR Analyzer to adjust it. The CAT-5 cable
>> I used for wire was brought up over a tall tree and chafing from the
>> wind blown branches wore through it 1/2 times a year. After the
>> antenna came down the last time I used a more rugged wire and cut it
>> 135' long and have it connected to a radial plate which serves as a
>> common tie-in for my separate 160, 80, 40, 30 meter vertical wires
>> but also my butternut HF9V which I use for 20&  up.
>> 
>> The 160M ant will not cover the whole band but as I rarely go above
>> 1.9 Mhz, it's perfect and needs no base loaded cap.
>> 
>> YMMV.
>> 
>> 73,
>> 
>> Gary, KA1J
>> 
>>> I plan on adding an Inverted L soon, and have seen a few websites that
>>> recommend about 1000pF at the base to tune it.  What voltage cap should I
> be
>>> looking for if I am going to run about 600W?
>>> 
>>> -- 
>>> *Clint Talmadge - W5CPT - *
>>> _______________________________________________
>>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>> 
>> 
>> _______________________________________________
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>> 
> 
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
> 
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK