Sunday, July 20, 2014

Why 65.5 feet?

I've had a question or two about why I chose 65.5 feet for my PPD length. (For that version anyway)

Well, quite truthfully, it was coincidence. I ordered two 100 foot rolls of #26 wire. One measured 101.5 feet and the other about a foot longer. I cut the longer one to match the other, so they both equalled 101.5 feet. A bakers dozen I figured.

This length formed the basis of the MiniBAC antenna I wanted to build. Along the way I wanted to build a 130 foot or so quad loop for Field Day so I had to cut down the two wires to somewhere around 65 feet a piece and join them in the middle at the top. As I was about to cut the wires I realized that my pole being 36.5 feet extended would allow my rig to be about a half a foot off the ground to keep the jewelry clasps from binding up in the S-Biners at the apex in doublet mode. This was back before I realized the importance of stressing the pole.

So, as I cut only 36 feet off the ends, what was left was 65.5 feet per leg.

Now these jewelry clasps are kinda neat. I can use male/female pairs to join sections of wire together mechanically as well as electrically. They are made of Sterling Silver and can be found at Hobby Lobby.

So there you go a full-sized 101.5' per leg MiniBAC or a shortened version, a 65.5' per leg version. When the shortened version is deployed on my stressed 36.5' mast, the apex clips end up at midpoint making for a 32.75' radiator and equal riser Tx line. Hmm, a 40m doublet.

The 101.5' version up 30 feet under a stressed pole yields a 143 foot doublet. Not stressing the pole much, say 35 feet of riser line, you could get a 133 foot doublet. But with the 700-800Ω riser line, it matters not. The feedpoint impedance will be transformed to something else anyway. The BLT will take care of the match.


Saturday, July 19, 2014

Just Another Day in the Park

Today I walked over to what my family calls the "Purple Park". It is just east of my home QTH and is easily accessible on foot.
I decided to take along the 23 foot cheapie end support poles too. All that I had is shown here. 
I setup the 65 foot per leg wires in the WVØH PPD fashion and proceeded to call CQ. I started out on 20m and raised several stations and a Tuna Tin 250mW station out in California. 
Then I QSY'd up and then down the bands. 15 was dead, so was 17 meters. By the way, I forgot my 3G iPad so my plan to post live updates was dashed.
Then along came a sparrow and he had friends. They all decided to gather on the aerial. "Shoo". I said but they decided that I wasn't that threatening.
The wire is pretty close to half and half, I have a red mark where mid-point is and I could see that the apex clips were right at those points. 32.75 feet in the riser and 32.75 feet for the radiator, per leg.
But all in all 20 was the best and actually worked an XE2 on 30m. Cool.
There are tuning solutions that don't require the parallel capacitor, this is one of those times.

Forecast high temp was to be in the 90s but it feel that hot sitting next to the stream and having a breeze blow over the stream to me. For about 4 hours of op time it was super fun. I didn't feel hot until I got home and looked at the thermometer, 94°F! But this is Colorado. 

73 everybody.

Wednesday, July 16, 2014

Attaching a spike to the MFJ-1910 pole

The MFJ-1910 pole is a good support for wire antennas but can be difficult to secure at the base. I obtained a 3'x3/4" mason spike and two rubber plumbers union joints (boots) from the local hardware store. The union joints are positioned 2 feet apart on the base of the pole.
I used the hose clamps that secure the union joints to the pole in such a way as to create a cradle for a 3/4" x 3' spike to nest. 
Once those rubber boots have been adjusted the spike can be secured to the boots by attaching another SS hose clamp over the entire assembly.
To put up the pole you start by extending one section (the tip top) of the pole first, the drive the spike in the ground 12 inches deep with a 4-lb sledge hammer, then slip the assembly over the spike. Tighten the two clamps securing the pole to the spike leaving the four around the boot alone. When done, loosen the two clamps, slip off the pole, remove the spike from the ground and slide it back into the cradle for storage.

Saturday, May 31, 2014

WVØH Park Portable Doublet Analysis

Since putting together this antenna system I have noticed a fairly good match with the 4:1 balun on 20 meters. I ran the numbers and here are results providing that good match.

I took readings with my uncalibrated AA-600 and 4:1 balun across the HF band and see that the input to the line is 139-j384Ω on 20m. Now this is with the 4:1 balun causing the readings to be normalized to 200Ω. So for practical reasons, I will stay in that 200Ω system for calculating convenience.

The first thing I would like to point out is that 90° transmission lines tranform impedances by the following formula set. The antenna Z I have here is just an example.

Knowing the input impedance of the antenna and line we can insert them into the second formula and arrive at an answer. But like all things in this world exact values are rarely achieved because of the antenna impedances are location dependant because of varying soil conductivity, etc. causing shifts in the readings. What we can see from this experiment is a general range or location of solutions that lie in certain quadrants of the Smith chart. To get close enough to the center is enough to gain an understanding of the sensitivity and direction we would like to go.

The transmission line is approximately 800Ω and the line input Z = 139 - j384Ω (at 200Ω system impedance). My earlier back yard measurements revealed that an 8 inch spacing yields a characteristic impedance of 725Ω, so when it is greater than that, ~10-12 inches, 800Ω is close enough for the math.

Zantenna = 533 +j1474Ω (AA-600 with 4:1 balun behind our line)

Now, the line is approximately 20 feet long and we need to remove it mathematically to see what the antenna input impedance is as well. I generated a spreadsheet table showing the length of various fractions of wavelengths for a line slowed down by 6% due to imperfect effects of the wire and insulation on that wire. My arrival at 94% velocity factor is from an actual measurement.

So a 20m quarter wavelength piece of that line is physically 16.5 feet long. Since we are at about 20 feet I would like to know how electrically long 20 feet will be. So in the last two columns I took that 20 foot piece and calculated the how long (in wavelengths) that piece of feedline is for each band. At 20m it is approximately 0.3 wavelength and I converted to electrical degrees (with each 90° wrap removed for simplicity on the Smith chart) in the last column.

This yields 109° for 20 feet at 14MHz given a velocity of propagation of 0.94.

Adding the antenna impedance, line impedance, and length into the Smith chart one can see that the resulting input impedance is approximately centered. 

Since the load can vary a certain amount out in the right quadrant by a somewhat large numerical amount, the amount of area travelled on the Smith chart is relatively small. This means that the other end of the line back at the input varies in the vicinity of the blue VSWR circle. That is why, when the wind blows on the antenna or the open wire line dances around a bit, the actual VSWR doesn't change that much and is close enough to the center to provide a good match. Again, all of these values are at the 4:1 or 200 ohm level through the balun.

The blue circle represents VSWR values of 2.2:1, any value inside that circle is under 2.2:1.

Next, I will run the EZNEC model and see if the input Z matches what I have measured.

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Tuesday, May 27, 2014

The WVØH Park Portable Doublet

This post will show how to assemble a very simple and effective portable 50 foot doublet. The length doesn't matter as you could just as easily use two 50 foot pieces and end up with a 60 foot doublet, this is just what I had laying around the shack.

The idea came from Bonnie Crystal and her MiniBac antenna system, where nearly every precious watt of your rig will be radiated. I don't use spreaders distributed along the feeder as they inevitably tangle the wire.

I use a 20 foot Crappie pole but just about anything can used. The height doesn't matter much but you'll need to remove any thin tiptop section if it has one. Next, obtain a 100 foot roll #26 PVC stranded hookup wire and cut it into two equal lengths. 45 feet per leg is just what I had but it turns out that the input impedance is close to 200 ohms, a good match with a 4:1 balun on 20m. I use a my home brew balanced line tuner.

The beauty of the design is the built-in open wire feeder without taking any extra feedline with you. The insertion loss of 20 feet is roughly 0.08dB at 14MHz. (I measured -0.4dB of 100 feet #26 wire spaced 8.25 inches at 14.143MHz).

The two wires clip onto two Nite-Ize plastic key ring clips called S-Biners. Theese allow for a quick setup as you can just clip them onto the wires and go. Since they are plastic they don't add much weight to the top and the wire slips nicely through them when you tension the final installation.

I used some 95# paracord available at Hobby Lobby in their craft department and formed an apex that separates the wires 8 to 10 inches.

Another nice thing about using a fairly wide stance is that you get the benefit of high Z line where the loss using smaller gauge wire is less per unit length because the I^R losses go down as the Z goes up. Also, when the wind blows creating movement the characteristic impedance doesn't change that much because any Zo change from 10 inches down to 6 inches is smaller than any charge from say 1 inch down to 1/4 inch.

I used #1 snaps but would get the next size bigger when I get more next time.

The whole mast can be supported by one giant screwdriver. I had one that can be gripped with two hands and thought that it would make for an excellent base support for the pole. I remove the bottom cap from the pole and slip the open end over the screwdriver shoved in the ground. The photo below shows an orange and black handle sticking up next to the pole base. It's probably 7 inches long.

I attach banana plugs to the wire ends to interface with the MiniBalun BLT then cinch both ends of the wires off to the ground or other available supports to put a slight bow in the pole to maintain a taut open wire feeder. With 20# Dacron fishing line I can wind 75 feet plus the 45 feet of wire onto the Irwin chalk line reels available at Home Depot.

 I just use one stiff soda straw at the bottom to maintain the desired spacing distance.

That's it! One quickly deployable park portable doublet.

Tuesday, February 18, 2014

BL2 Cal Fixture for AA-600

I recently purchased a Rigexpert AA-600 and wanted to measure balanced lines, antennas, etc. The MiniBalun and the BL2 were great candidates to create an attachment that would allow a quicker method to cal out the balun while attached to the AA-600.

I used a DPDT center off switch to do just that.

The cal fixture can be used to measure just about anything balanced, including the short in my house telephone wiring with the TDR function of the analyzer! I found a short about 8 feet away from one of the phone jacks. I'll repair that later.

This is a photo showing measurement configuration to measure my 20m QRP trap. Results are within 1% of calculated!

The tan column is calculated values and the E/F columns are without calibration or fixture, just a BNC binding post to an N/M to BNC F/M adapter. The G/H columns are the measurements with the cal fixture. I measured the resonant point by a different method I will talk about later. The Z=Q*Xl method is the calculated value hanging out in the I column.

Saturday, February 15, 2014

Magnetic Loop RF Safety

While it is okay to determine the electric field strength it is not the dominant safety factor. Please use the magnetic field strength formula as this is the dominant field when this close to a magnetic loop.

Page 67 of FCC OET 65 shows the table for maximum safe RF levels for a controlled environment. These levels are from IEEE C95.1-1991.

For example, on 20 meters use 4.89/14.000 = 0.350 A/m maximum limit for a controlled environment, such as when you are pedestrian mobile with a mag loop.

I've added a row in my AA5TB loop calculator spreadsheet (C34 as shown below) to calculate the magnetic field strength. 

C34 =(C33*(E11/2)^2)/(2*((E11/2)^2+C35^2)^1.5)

My 4 foot 1/2" Heliax loop with 5 Watts input yields a 0.248A/m field strength using the loop diameter of 4 feet. So one loop diameter distance away on 20m is considered safe for a controlled environment using 5 Watts. 

This is my rule of thumb for my operations, keeping at least one loop diameter distance between me and the closest part of the loop when using 5 Watts.

Myron WVØH
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