Measuring Ladder Line With An Antenna Analyzer

Testing a ladder line for an electrical half wave length will yield meaningless results with a MFJ 259B Antenna Analyzer.  What works with 50 ohm coax will not work with any other impedance coax or ladder line (unless it’s 50 ohm ladder line).  The analyzer is a bridge circuit where the other three legs of the bridge are 50 ohms.  The fourth component must also be the electrical equivalent of a 50 ohm resistor for a meaningful readout. But not to fear.  It can still be accomplished with just a little work.  The steps to find an electrical half wave length for a ladder line are identical to that of a 50 ohm coax.

Using the same technique as would be used with a 50 ohm coax first find the electrical quarter wave frequency .  With the far end open and the near end attached to the analyzer tune for a dip at the lowest frequency. Going through several dips to get to the lowest one is normal.  Both resistance and impedance should dip as close to zero as possible. If impedance is zero but resistance is high keep tuning until both are zero. Note the frequency.

Now find the next frequency up where both resistance and impedance dip to zero.  This frequency should be triple the first frequency.  This will be another odd multiple of a quarter wave and it should be the three quarter wave frequency.  Make a mental note that the difference of these two frequencies is the electrical half wave frequency. For example the first frequency was one quarter wave length and the second frequency was three quarter wavelengths. The difference would be two quarter wave lengths or one half wavelength.  How do we get to meaningful dimensions?  We need the velocity factor.

The MFJ 259B has a built in advanced mode function called distance to fault.  The distance to fault will be the electrical distance to the end of the cable. It will be longer than the cable itself because the insulation slows down the electrons (by an amount known as the velocity factor!) To find the distance to fault hold down both the mode button and the gate button at the same time until advanced mode appears on the screen.  Press the mode button repeatedly until the distance to fault mode appears.

Repeat the steps above but this time enter the data into the analyzer.  Go back to the lower dip frequency and press gate.  This will become the 1st data.   Next rotate the frequency knob to the next highest dip and press the gate button again.  This is now the 2nd data.  Press the gate button a third time and the distance to fault will appears.

We need to know the physical length of the cable so measure the total length with a ruler.  Velocity factor is the quotient of the distances measured electrically by the analyzer and the actual measured length. Divide the physical length by the electrical length and the quotient is the velocity factor.  An example would be .88.

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Verticals vs Dipoles

“Dipoles are better than verticals.”

If you’ve heard this before you won’t be surprised but I hadn’t and it really shocked me.

For years we had been taught to put up dipoles for stateside contacts and verticals for dx.    “They” said dipoles have 2.2 dB gain over isotropic and verticals are better low angle radiators.  Both are at best half truths.  More accurate thinking would be to put up a vertical only when a dipole can’t be erected at the proper height or length.  Below is an explanation of why dipoles should normally always be the first choice.

“The dipole is the basic building block of many antennas. A dipole does NOT have 2.2 dB gain over an isotropic radiator when the dipole is placed over earth. The dipole has about 8.5 dB gain over an isotropic radiator! Always remember this when you see antenna models over earth given in dBi. If the model over earth shows a “gain” of about 8.5 dBi, the model effectively has the same gain as a dipole.” – http://www.w8ji.com/antennas.htm  (Tom Rauch)

I “fact-checked” Tom’s comment by using EZNEC to model a dipole one half wavelength above average ground. Gain numbers are in the same ball park as Tom’s considering he was modelling at 145 feet above ground and I was using a more attainable 33 feet.  In this case the model does seem to agree with Tom.

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For a rigorous explanation see Joel Hallas, W1ZR, in QST, November, 2015 p44, Antenna Gain, Part 1:  What Do The Numbers Really Mean?.

Verticals ground mounted over average soil conductivity with adequate radials don’t have any gain.  This model shows gain of 0 dBi. In other words you’d be giving up 8db of gain by using a vertical instead of a dipole.

(I’m planning on inserting a model of a quarter wave vertical with ground radials over average ground here as soon as I can find someone with NEC4.  NEC2 supposedly can’t model ground interaction very well and that’s half of a vertical antenna)

Next we model the identical antennas but this time we look only for the gain at a take off angle of 15 degrees.  We chose 15 degrees because many DXers believe this is the optimum take off angle to work the most countries.

From Jim Brown, K9YC:

Ah, some say – but the vertical doesn’t do nearly as well at the high angles that support short distance propagation. Yes, that’s true – but: 1) Don’t forget inverse square law – field strength falls as the square of the distance, so stations at 800 miles are 6 dB closer than stations at 1,600 miles and 9 dB closer than stations at 2,300 miles! You don’t need as much signal to work those closer stations.

In closing the soil determines how well a vertical will work.  The one time verticals will outperform dipoles is if they’re over salt water or on a salt water beach.

40 Meters – Practical Example

As promised above here are the two models using EZNEC v.6.0 (NEC2-based because I gave up on finding someone with a NEC4 version).  Comparing a dipole at one half wave height to a quarter wave vertical on 40 meters both over average soil we get the following graphs.  The vertical is on the left. The shapes are surprisingly similar but the gain is not.

 

Here are the same graphs larger so the numbers are easier to read.

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The green dot marks the 15 degree take off angle.  The gain at 15 degrees is -1.48 dBi on the vertical and 4.92 dBi on the dipole.  This is a difference of 6.4 dBi.  In other words to get the same signal out requires 4 times the power on a vertical.   Fifty watts into a dipole is the equivalent of 200 watts into a vertical.

This gain comes at the price of directivity.  A vertical, of course, has a 360 degree omni directional pattern.  A dipole has a 78 degree beamwidth.  The dipole is the winner if it is pointed in the direction of the DX.  If the DX is off to the side of the dipole the vertical will be the winner.  Here is the same dipole showing the azimuth pattern at 15 degrees take off angle.

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The front to side difference is 13.93 dBi which we’ll round to 14 dBi.  Now the vertical that was 6 dBi worse off the front is 8 dBi better off the side.  This is a power multiplier of 6.31.  It would take 316 watts into a dipole to equal 50 watts into a vertical when the DX is off to the side of a dipole.

What is the conclusion?  Both antennas are needed.  Two dipoles at right angles would work, too.

Perfect Antenna for April 1

Too bad it’s August 1 instead of April 1.  I ran across this ideal antenna while modeling some other antennas in EZNEC.  It would be too good to be true except on that one day of the year.   Of course, it’s true.  What’s not to be trusted?  It is modelled in EZNEC to prove it.  The name of the antenna is SUPERGAIN.  As can be seen in the view ant window below it is a triangular loop.

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So far so good. It’s a simple wire antenna.  Here’s a look at the FF Plot to show the pattern and the gain.   The gain figures are in the notes below the pattern.  Gain of 23.85 dBi and at a fairly low angle, too.  I’ll round that out to 24dBi.

Click to enlarge

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This antenna is modeled over real ground by the way, not free space. What a fantastic dx antenna.  I’m pretty sure “fantasy” is the operative word here.  But let’s take this at face value for now.

What could be done with an antenna with 24 dB of gain?    We could put 1 watt in and have an effective radiated power (ERP) of over 200 watts.   We wouldn’t need linear amplifiers any more. We wouldn’t need towers and beams or any other antennas. If we used a typical transceiver with it’s 100 watts the ERP would be 20,000 watts. Or a qrp-er could use 5 watts and have an ERP of 1000 watts.   Hmmm.  This is a really nice antenna and don’t forget from the plot all that radiation is going out at fairly low angles.

What is fooling EZNEC or why haven’t we heard about this wonderful contraption before? What’s going on here?

Dipole replaces mag loop

The AEA Isoloop was working as designed and making contacts but this dipole is blowing the doors off the loop.  I’m actually seeing dx from Europe and this afternoon I worked France.  That is the first QSO with Europe since we moved into this condo.  Disappointingly the Isoloop never was able to achieve that goal.  Tonight I worked European Russia with the dipole.    I love it.

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Constructed of electric fence insulators mounted on the lanai screens, I stuck hookup wire in the insulators and a MFJ 1:1 current balun in the middle.  I had to hunt for the insulators but I found them at Murdoch’s Farm and Ranch Supply. Amazing what one can do with wire antennas.

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