The AM Forum

I'm building an L network for my L. The L works fine.  I recently obtained a BHVC (Big Honkin' Variable Cap), 500 PF, widely space air plates. I think its one of the old EF Johnson jobs.

The coil is one of those 40 uH give or take ones similar to the widget MFJ sells to use with their tunas on verticals, which is "okay" for legal limit. Aside from the obviously bad Q due to the small wire diameter, 1 wire diameter spacing, etc., my gut feeling is this air coil is a prime candidate for arcing. Am I correct?

The option is to buy a good hunk of No. 8 bare copper and wind my own coil on a 4" PVC form. For my application, perhaps the vaunted 3/8" in copper refrigerant/brake line tubing is overkill??

Tnx.

Philip

Philip,

Coil problems are usually due to overheating from excessive current instead of arcing.  This is often seen on small coils with plastic spacers when the plastic gets hot and melts while emitting noxious odor.  160 is the band that brings many otherwise OK tuners to their knees because of excessive loss which converts to high heat.

I would use small diameter copper tubing for your coil, wind it over a convenient size form but then remove the form so it is self supporting.   Some PVC does not react well to being in an RF field, it will get hot.  Build a coil suitable for the legal limit because you never know when you will be running the legal limit if you aren't now.  If you need more capacitance use one or more large (as in physical not necessarily capacitance as you want your variable to be as much of the equation as possible) doorknob caps suitable for RF.  This does NOT include Ceramite brand which do not do well with RF current beyond the QRP level.  If your capacitor doesn't have enough spacing for future power level, go with a vacuum variable and Allen Bond at Max Gain systems is a great source for this and other quality RF parts at reasonable prices.

I used a Gates roller inductor and a vacuum variable for my legal limit reversible L network.  A "tap" switch from a BC-375 allows reversing the L to match either high or low impedance loads.  This is used with an 80 meter horizontal loop to work all bands with my vintage gear, the tuner and loop will also work on 160 by feeding it as a top loaded T.

Of course another option for the L if this is for a single band is to cut the L slightly long so that it is inductively reactive and then just use a series capacitor to tune.  I use my Hy Gain 18HT in this form on 160.  I didn't like either of the Hy Gain approaches for adding 160 so I added a small stub at the top of the main tower with a 40 meter trap to prevent de-tuning 40 meters and then I ran a wire from the stub to main tower as the horizontal part of the L.  I cut the wire a little long for resonance at the low end of the band and I used a BC-375 tap switch (yes, I love those heavy duty switches!) to select 6 different lengths of RG-11 used as capacitors.  This provides coverage of the entire band at reasonable SWR and it has no problem accepting 1500 watts output in any mode in contest duty.

   Hey Phil,

    Play around with this.. You will see that even
at 10Kw "Voltages" are manageable.  There's no arcing
in 50 ohms.

    http://www.random-science-tools.com/electronics/dBm-Watts-volts.htm

    Shift it over to 600 ohms and it becomes a concern.
Set a custom value, say 450 ohms and Voltages are
reasonable.  I assume your "L" is for 160 off the Valiant?
Dunno what the "Z" would be fer that. Resonant antennas
reduce the paperwork!

    Have Fun, Glad the Valiant is back...


/Dan

The L is for all band 160-10. Its cut to about 140' total length, 75' high, 65' long. However, will not run high power with the Valiant, but will do legal limit with the Flex + Amp.

The Valiant is back, but still need to install some new coarse loading caps. Right now I'm clip leading a doorknob to get it to load.

P

Philip,

If you are going to use this for all bands just pick up a beefy roller inductor and be done.  It has been too long since I ran the numbers when I was designing my L tuner but I think 500 pf may not be enough on the low end so you still may need some padder capacitors switched in parallel with your variable depending upon the antenna load.  You will likely need a switch to reverse the L for some bands.

   Nuthin' wrong with that, do the same thing here
for the ART-13's.  External Loading as God intended.

   All Band 140' L  => (Crap Shoot).  Pass the "Graph
paper" please...


/Dan

MMANA GAL was spitting out 470 PF with around 8 uH of inductance to tune this thing. The most L I'm needing anywhere thru 20M is 12 uH.

Any band above 40 is just going to be tuning network practice with a 75 foot high L.

A lot of energy will be wasted on useless angles.

It's fun to use small junkbox components to tune and later replace one component at a time with large parts.

Leave the cliplead stuff on the Vally Ant. Others have managed this way for decades.

I've used Ls like this for years and had great results. This one replaces the one that was vaporized last summer. Yep, the patterns are funky from the simulations I've run. Worked UK on 10m using this aerial and the Valiant just fine.

PHILIP

You can work the world on a bed spring when conditions allow. Carrying on a meaningful QSO on AM requires all ducks in a row.

Yep. People have worked 5 band DXCC from a mobile. Doesn't mean I'm going to use a mobile antenna any time soon. I worked Scotland on 160 meter SSB while mistakenly tuned up into my 80 meter dipole. When the band is open, almost anything "works."

The idea that one antenna can work effectively across all HF bands is flawed.

I made an L. Works well but this one isn't pretty. I often use the white or black insulated wire from 12/2 house wiring. Is easily up to the job and the spacing comes naturally. I have it also on my linear now , using it to replace the copper tubing I first used. Wound it on  a length of PCV for 80m and tapped it for 40m recently. (Pi network). On the L I have it taped every three turns and in both use a gator clip to select.  Currently (this week) I am using my L to match my ARC5 to my Linear,s unturned input. I also use if on my loop modded TNT for AM and my Hb transceivers. In mine I put a light bulb socket in parallel with the coax output so I can screw in a simple bulb (I use a 4.5 watt ,a 7.5 watt or a 60 watt bulb) and instantly have a dummy load available for tuning and testing. It has proved a very versatile and handy device. My only regret is I should have built it prettier.  Built from my junkbox on wood cost was zero.
At $200 it still would have been a bargin.
don

Shades of  VYZet.

Here are a couple of photos of mine during the development phase.

Yep, you sure can work the world on bedsprings....but it meets the design criteria:

1. Low profile to keep K4SMN happy.
2. Low profile to keep HOA happy.
3. Decent performance on 160-17m (bands I frequent). The sucker is a blow torch on 40.
4. 15 and above I consider "occasional" use bands.
5. Only one HF antenna to worry about. I'm too lazy to erect more.

I must have chatted a good 1/2 hour with that chap in the UK. Nice signals both ways.

I'm happy, Mamma's happy, HOA's happy. Win-Win all around.

P

Actually, I do have a motor inductor, which I think was pulled from a military transmitter. However, not sure what power it can handle. I'll take some photos of it when I get home.

Sage advice from an SK legend.

http://amfone.net/Amforum/index.php?topic=20369.msg146289#msg146289

You obviously haven't met Dr. Sheri, K4SMN…….

 ;D

As brilliant as I may or may not be, I'm outclassed in all categories. Reminds me of the Poles attacking the panzers during the Blitzkrieg on horseback with sabers.

http://www.qrz.com/db/K4SMN?ref=1144778313


FEELEEP

As Billie Crystal said in Princess Bride, "Have fun storming the castle."

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By the size of the guide rods assuming they're 1/4 in. Stock, I'd say your coil is good for  200 or so watts easy if unmatched and anywhere from 500 to Ikw pep matched .
No heavy full time carrier load much over 100 to 150 though.

Hmm, two roller contacts, one engages half of travel on phantom guides.
I'd revise it so both rollers contact the same coil turn. One will only be slightly advanced from the other and the median inductance between the two should occur. 

Your coil will also be good up through your favorite ham bands, but I'd add an auxiliary, larger dia. Wire and more turn spaced coil if you want to work ten.

I think I paid 50 for it at the Orlando Hamcation around 6 years ago.

I had a feeling the coil was a little small and that I would be unable to use it for the high powered network I intend to build.

Thanks.

Philip

Philip,

When building a tuner always build for the future so make sure you have the power capacity to handle the legal limit and enough inductance and capacitance to handle a wide range of antennas.  If you have to buy or build an inductor don't make it just large (inductance level) enough to handle your current needs.

Yessir.  Trying my best, but takes forever to get things done with the other demands on my time.

I am SOOOO burned out on autotuners. They seem to auto-destruct like the CRM114 discriminator.

Philip