coil calculations

[w5omr]

For Mobile operation, I'd like to wind my own bug-catcher coil.

How much inductance would I need for 80m (down to 3.5MHz)?

My current bug-catcher is advertised at 85uH... an on-line coil calculator says it's 142uH (4" diameter, 7" long, 8tpi).

As it is, with a 5' whip, the lowest frequency reached is ~4.1MHz.  Sure, a longer whip would make it work, and I've got 'extensions' to make it nearly 6' long, but the 75m taps (what y'all call the 'ghetto') are only about 1 or 2 turns north of the bottom of the coil (tapping from the bottom of the coil).

I've also got a Henry Allen #440 Bug Catcher coil and it's advertised as 40uH, but the calculator says 56uH.

I wonder how much "mutual inductance" could be realized, and how much more than ~200uH could be had if the two coils were to be connected in series? 

Is there a formula for calculating Mutual Inductance?

[KA2QFX]

I built an antenna 25 years ago that plays real well.  It's a big coil that's tapped at 86uH for 75 (3.900) BW is 9KHz
Details are here:http://home.comcast.net/~msed01/ant1.html The page desperately needs updating.

The antenna has a 4 foot base shaft and  96" whip. The coil structure is 17" overall but the coil is only about 7 inches tall, 5" diameter, #14 motor wire, about 4 TPI except for 20m which is double spaced.  It resonates on 20~75m with no capacity hat using up to 70% of the coil.  With a capacity hat of 19" radials (1, 2, 3, or 4) just above the coil the whole L will cover a good section of 160.

I strongly recommend the use of a broadband matching transformer at the feedpoint. It eliminates the variable of a reactive tuning network shifting the resonant point of the antenna.   Th antenna always looks resistive and is easier to shift tap points. IMHO.

Mark

[WA1GFZ]

http://www.rfpowersystems.com/

Mark,
Check out "Product 7". The designer of the Erbtek MRI amplifiers turned me on to using single shielded teflon wire as low Z coax. Product 7 has a table of the impedance of different wire sizes. I used #24 as 25 ohm coax when I modified the amps down to HF. It has a higher breakdown voltage than plastic coax.
The price of 25 ohm coax is crazy.

[W4AMV]

On the mutual L, if you set the coils up series aiding and take a measurement, then set them up series opposing, take a second measurement. The L total is L1+L2 +/- 2M. You have 2 measurements, 2 equations and the L1 and L2 values are know. Solve for M. On the 25 ohm coax, place 50 ohm pieces in parallel.

[KA2QFX]

Frank,

The coax I used in the un-un is 100% teflon. 2 parallel pieces of 50 ohm have worked well for 25 years. But for QRO it would be nice to get something a little heavier. I have a fair amount of #12 silver plated with teflon insulation. I'll measure it up and see if it might be in the vicinity of 25 ohms. If so I'll slip a braid over it, wrap it up tight and see what impedance it ends up. I might get lucky.  Thanks for the tip. 

See you at Deerfield I hope.

Mark

[WA1GFZ]

Mark,
Check the table I think #12 is under 10 ohms. The table is good information for building transformers. It is a good idea you have dropping the feed Z to the mobile antenna. I need to build a high power transformer that will take a pair of RG303 runs. 25 ohms to 50 ohms.

[w5omr]

On the mutual L, if you set the coils up series aiding and take a measurement, then set them up series opposing, take a second measurement. The L total is L1+L2 +/- 2M. You have 2 measurements, 2 equations and the L1 and L2 values are know. Solve for M.

That's good info.  Thanks.

[w5omr]

In my situation, how I'm feeding the antenna is with one of those truck-stop special 'heavy-duty' mounts - SO-239 on the bottom, grounds to the mounting bracket, big nylon/teflon insulating washer with a shoulder to keep things centered (and non-shorting) then a 2.5" mast, a 7" coil, and 6'2" mast.

At the feed-point is a smaller coil, shunted to ground for impedance match. 

I know that maximum Field Strength readings do not correspond to minimum SWR, so what I do is set a tap, roll my rice-box VFO around (at 5w, because of the automatic SWR fold-back protection circuit) and look for maximum Field strength reading.  Determine the frequency, move the tap up or down, depending on where I want to be, and then work with the impedance matching coil at the feed-point.

The think I like -best- about using the shunting inductor, is that the antenna is now at DC Ground potential.  Internal (ignition/computer/etc) noises are minimized.  I've got a 3" wide ground braid attached to the mounting bracket (I spread the braid out, and it's behind all four mounting bolts) and that run goes down from between the cab and the bed of the truck, to the common mounting point on the frame, where the cab and bed are bolted to.

My issue is that the Henry Allen #680 (4" in diameter, 7" of coil @ 6tpi) 85uH coil, doesn't have as much inductance as my original Bug-catcher coil.  Problem with the original coil is, it was mobile for 10 years.. and it's hit many of tree and brush, and fought several battles with a large Oak Tree near the Battlefield of the New Orleans battle, and finally succumbed to that low-hanging branch. (It was probably used to hang the British were they to stand trial)

So... the 85uH coil and a 6'2" whip gives me about one turn from the bottom of the coil (tapping from the bottom) for 3.880.  That's cutting things pretty close.  I suppose I could just adjust the whip for resonance on 3.8880, and go from there, but here's the big question...  Does having more than enough coil available for the lowest frequency make -any- difference on the effective radiated signal from said antenna?  Is it like having 4x the audio power required to modulate a rig to 100%, vs just being resonant with the coil, or going by the 'rule-of-thumb' only needing 500w to modulate a kW carrier?

Best Regards = 73
-Geoff/W5OMR

[w5omr]

To further the grounding conditions in my vehicle,
I also ran some ground strap from the block of the engine to the frame, from the block, wrapped around the ECM and secured to the firewall, which continues to the negative post of the battery, to the nearby fenderwall/body.  I get very low to no ignition noise on all bands, except for 15m, and even there the noise is gone with any kind of signal.

[KA2QFX]

I don't know if having a DC path to ground on the mobile antenna makes a difference in noise, but my un-un xfmr is a DC path to ground. If you look at my photos you'll also see I use an SO-239 type mount as well. Filled with silicone grease the connections are quite weatherproof. 

Mimimum SWR ALWAYS occurs where the system is resistive (resonant). Maximum current occurs where Z is lowest.  Assuming radiation resistance is a fixed function of antenna size all that's left is antenna reactance to reduce maximum current.

But here's the thing about using a reactive matching network. In order to use an inductor in parallel with the antenna to produce a match that's resistive, the radiating portion of the antenna must contain some capacitive reactance to cancel out the matching coil's inductance. Hence, you cannot achieve maximum antenna current because you have some additional reactance adding to the radiation resistance, even when you're matched up perfect and resonant. 

Using the broadband transformer to match the antenna's feedpoint there's no reactance and the antenna itself is tuned to resonance and resistive, without any reactance.  Feed current is proportional to radiation resistance alone.   It's a lot easier to change frequencies/bands when you don't have to worry about retuning any matching network.  I don't know why every mobile antenna (below 20 meters) isn't fed this way.

I've tried every other method and this beats 'em all by 6dB minimum. With 100 watt (PEP) mobile xcvr my signal reports on 75m are consistently about 10-12dB below a similar station running a full size dipole.   40 meters is incredible.  Hearing is believing.

In answer to your question about more inductance than you need, no. Excess inductance, which is usually shorted out, will absorb some of the energy from the active part of the coil and waste it in the R of the shorted turns. There's actually not much energy lost to speak of but it's not lossless.   It might serve to decrease the Q a bit which may be desirable, but I don't think there's any advantage aside from the ability to tune down the band a bit further. 

I have about 40% excess coil on 75 which I use on 160 with a capacity hat to achieve resonance.  My Q is 300 with the excess shorted out and 320 with them open. But If I leave the coils open the resoant frequency of the antenna is highly unstable when moving.  It's actually unusable.  I can't say why and didn't care to investigate further. Short 'em out, everything's stable as a rock, +/- bridge framing and semi's, but that's quite tolerable.

Mobile HF is really ALL about the antenna.

Mark

   

[WD5JKO]

You mobile guys need a decent power supply as well:

http://www.youtube.com/user/meade916#p/u/13/FmCHU5x-oPo

16V at 1000 amperes should do it!! 

Jim
WD5JKO

[w5omr]

You mobile guys need a decent power supply as well:

My amp only draws somewhere near 60~70amps and I've got -a- 110amp alternator on the truck.  I'm sure I'm ok ;-)

[WA1GFZ]

Mark,
Last week I had one of our Biconical EMI antennas apart and found it had a coax 1:1 transformer. I'm sure this helps to force a balance and polarity of this short  cage dipole that operates 25 to 200 MHz. I need to pull the TX biconical apart and see if it has the same 1:1 ratio. I would think it would work better with a 2:1 on the input at the low frequency end. We always have a problem making a big field below 100 MHz. Our solid state amps don't like the VSWR and shut down. 2 dozen 4CX250s in TWA configuration drive the heck out of it though.

[WD8BIL]

We always have a problem making a big field below 100 MHz.

That's why I use an EMCO Biconilog, Frank!

[WA1GFZ]

Bud,
how much power does it take to give you 200V/M field? How is the VSWR?
We drag out the 406 when it is time to motivate. We actually have 2 of them and I'm thinking of building a combiner. The poor type N connector will be begging for mercy with that crappy VSWR. We have a LPDA about 20 elements but it is pretty big. I usually have move it away to get a good field.

[KA2QFX]

Frank,
Yep, I find those transformers do a lot to clean up the patterns of small antennas.  I'm surprised you don't have a well defined impedance profile and adjustable match for the antenna given the power you're putting into it.  24 250Bs?! Man, I only run two on 75.  I'm surprised you don't melt the little antenna

[KA2QFX]

Frank,
This isn't your antenna with the link coupled tunah is it? No wonder your amps balk. That tunah needs a teeny-weeny bread slicer to play right.    

I know hams are cheap but this is pushing it.

[WD8BIL]

I wouldn't know, Frank. Telecom's NEBS qualification only needs 10V/M and I usually do pre-compliance at 20 V/m.
But I did play with it soon after the system was up and running a few years ago and a 57V/M field at 30Mhz took 407 watts at 3 meters.
The EMCO is good to 1KW and the VSWR at 30Mhz is @2:1.

[WA1GFZ]

We usually play at 200V/M, MIL-STD-461E/F and DO160 E/F
We have the EMCO with the egg beater elements. 24 4XC250s takes out the N connector about once a year.
The VSWR is so high that we burn through the coax outer jacket if it rests against ground. There is only a Balun on the input so VSWR sucks at 30 MHz.
Actually the coax radiates pretty well. I would love to build a matching network for it.

[WD8BIL]

I hear ya. Problem with building a matching network in a compliance lab environment is calibration. Unless you have a real good relationship with a cal lab they're hesitent to certify a "homebrew".

I have a CDN I built for conducted immunity testing on ethernet lines that they charged $1K to certify the coupling factors on even though I did the curve!

[WA1GFZ]

Yes, the calibration empire.
We got our backs against the wall with the DO160 600 volt spike generator.
Boss got pissed and told me to build one. 6- FQA11N90 FETs in parallel making over 1800 watts peak power. It will run all day long. He wants me to build a bigger one at some point. calibration at use sticker
I would love to build some lightning generators and get into the million watt peak power league.

[Steve - K4HX]

What does any of this EMI and calibration stuff have to do with the original question on coil calculations? If you want to chat about EMI work, start a thread in the QSO section. Thanks.

[WD8BIL]

Yes master!

[WA1GFZ]

HUZ it is very related. Making a small antenna efficient is a real art. The EMI world has been trying to make an efficient antenna in the 20 to 200 MHz range since dirt was invented.

[WD8BIL]

He's right, Steve. Although I have to admit, the relationship to the original question is kinda like 2nd cousin once removed.