Dipole Tuning and an Ugly Balun

So much has happened radio-wise in the past two weeks, and I’ve been quite delinquent in updating the blog. This post won’t even get to Field Day, nor my new transceiver, but I’ve got to start somewhere. So let’s get to it!

Using the MFJ-207 antenna analyzer I picked up at the SMCC Hamfest a couple weeks ago, I’ve now set up tuned, worked with, and taken down a 40m inverted-V four or five times now. Since space around my apartment is limited, I’ve been walking the 15 minutes out to the shores of Lake Michigan. The lakefront has ample trees and open space to erect an antenna without being too much in anyone’s way.

The first day I set up the inverted V was the most exciting, and went something like this. I took my dipole-kit out to the lake, which contains 200′ of thin nylon rope, a dipole center, a couple equal lengths of 18-guage speaker wire (35-ish feet each), two weights filled with rice to throw into trees, and three 8″ pegboard hooks. (The hooks make excellent ground-spikes to tie off the ends of the nylon rope. I picked up a bunch at Menards when they were on sale for 20 cents each.)

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The good ole dipole center. No balun here, just wires from the center conductor and shield to either protruding wire.

In addition to the antenna parts, I’ve been taking along a small SLA battery and powerpole adapter, a 25′ bit of RG-58 Coax, a homemade “ugly balun” (see below), the MFJ antenna tuner, a radio, headphones, a pad of paper and a pen. The heaviest parts of the pack by far are the SLA battery and the the two coax items. I’ve also sometimes brought a portable camp chair so I don’t have to sit on damp grass… but lying in the grass in the summertime isn’t bad either.

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The view over the lake just after tearing down the antenna for the night. I love the summer.

The process of putting up the antenna is straightforward. I unpack the kit and lay out the parts under a suitable tree. I throw one of the weights with a rope as high over a branch as I can, then tie the dipole center to the loose end of that rope. I lay out the two antenna wires roughly on either side of the center, and attach both wires, the ugly balun, and the coax to the dipole center. Once everything’s attached, I haul on the rope to raise the dipole center up as high as it will go, and tie the working end of the rope around a stake just under the center of the antenna. I walk the wires out as far as they’ll go (each now has maybe 5′ of nylon rope attached to the end), and tie them to stakes as well. The whole process takes between 10 and 15 minutes and is getting faster every time.

The first time I put up the inverted V was enlightening, to say the least. I attached the ends of the antenna wires directly to the ground stakes, and hooked up the MFJ tuner to the other end of the coax. The MFJ207 is essentially just a wideband oscillator with a 50-ohm output, with a built-in SWR meter. You tune across the frequencies of interest (in five separate ranges, see the MFJ 207 Manual for technical details) and read off the SWR on the analog meter. In this case, I was hoping to see a nice dip around 7MHz. Though that’s what the picture below shows, it was quite a process getting there.

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The dial frequencies on the analyzer turned out to be fairly accurate, at least as measured with my cheapie Ebay frequency counter. The analyzer has an RCA port on the top next to the BNC specifically for frequency measurement.

The first time I put put the antenna, it showed a clear dip… but a little under 5 MHz! To be sure this wasn’t just an issue with the calibration of the analyzer, I plugged my cheapie Ebay 8-Digit Frequency Counter (“Cymometer”) into the frequency-measurement port on the top of the analyzer. It read 4.87MHz. Dang. My antenna was definitely too long.

As a starting estimate of how much to trim the antenna, I did a quite back-of-the-envelope calculation of how “electrically long” it currently was. Taking the speed of light to be roughly 3 x 108 m/s and dividing by our resonant frequency of 4.87 MHz gives a wavelength of 61.6 meters, which means each quarter-wavelength wire of the dipole is about 15.4 meters long. To work on the 40m ham band (7.0 to 7.3 MHz here in the states), let’s say the antenna wants to be resonant around 7.1MHz. Similar math to the above gives a wavelength of 42.2 meters (surprise!), or about 10.5m per wire. By this math, the wires were about 5m too long.

However, there are many factors that can cause an antenna’s resonant frequency to be lower than it would be in free space, and one of them is proximity to the ground. Not only was the antenna’s center only 6-7 meters feet above ground, but the ends of the antenna were literally touching the ground.  Given these factors (and the fact that while I had cut the wires longer than necessary, I didn’t think they were that much longe), I decided to be conservative in the my trimming. What’s more, I left the ground stakes where they were, and replaced the trimmed sections of wire with nylon rope. In other words, the angle of the inverted V didn’t change as I trimmed the wires, only the length of the wires themselves, and therefore the height of the ends above ground.

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One end of the antenna – you can see the orange nylon rope staked into the ground at the upper left and tied to the antenna wire in the lower right.

With an eye toward being conservative with my trimming, and knowing that the math would have me trim 5 meters off the antenna in free space, I instead opted to trim 5 feet off each wire instead. This left each end of the antenna about 2′ off the ground, instead of touching it. After this brief surgery and re-attaching the nylon rope, the MFJ meter proclaimed the antenna resonant at 6.46 MHz. Now that’s better!

Another quick snip of 18″ on each end got the antenna resonant around 6.75 MHz, and another 15″ off yielded resonance at 7.097 MHz. For these measurements, I tuned the analyzer for lowest SWR, then measured the frequency with the frequency counter. In future, it seems like the dial measurements will be good enough for daily use, but for dialing in the antenna the first time, the frequency counter was very helpful.

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The lowest on the trimmed antenna was around 7.097 MHz, neatly in the middle of the 40m US amateur band. Success!

To get an idea of the useful bandwidth of the antenna, I also swept the frequency up and down around the resonant point and took some frequency measurements. The antenna had better than 2:1 SWR between 6.922MHZ and 7.282 MHz. Pretty much all of the the 40m band, and certainly the CW portion that I plan to most immediately be working.

All in all, I consider the several outings this week to be great successes. The MFJ is surely worth the $15 I paid for it. I was battling some dodgy coax the first couple times out, which made it hard to take SWR readings without the needle jumping all over the place. A little Amazon order of some RG-58 fixed that. Now, I’ve got a working, tuned, successful antenna.


One more thought: the so-called “ugly-balun” or air-core choke. Mine is simply 25′ of RG-58 close-wound on 10″ of 1.5″ PVC pipe, secured with zip-ties and duct-tape.

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The “ugly balun.” It’s at least kinda pretty…

My understanding of its usefulness is still rough (and there is much information, data, and heated discussion about it), but here’s what I think I know:

When using a dipole without a proper center balun (that is, an actual transformer that forces equal and balanced voltages on the balanced side from a single voltage on the unbalanced side), the dipole essentially becomes a “tripole”, with the outside of the coax shield possibly becoming part of the antenna and carrying RF. I say possibly, because it will vary with the length of coax between the transmitter and antenna feedpoint. This is undesirable, because we’d like the antenna itself to be the radiating/receiving part of the system, and not be depending on the physical arrangement/length of the coax itself.

One solution would be to install a proper balun (sometimes called a voltage balun) at the center of the dipole. This forces the voltages to each leg of the dipole to be equal, thereby eliminating common-mode current on the outside of the coax. However, a choke can also reduce current on the outside of the coax. For this reason, the coke is sometimes called “current balun” or “choke balun,” though strictly speaking it isn’t a balun at all.

Here’s a quote from one of the links above:

A choke is not a balun in any way. It’s given that description, ‘balun’, because it can do something a balun can also do, get rid of unwanted currents (CMCs) on the outside of the coax feed line. It can’t transform from a balanced to unbalanced state.
– ‘Doc

There’s lots of other useful information in that same thread, which I find myself returning too and musing over. It seems, long term, that a ferite balun is really the way to go for choking off stray RF, but in the meantime, adding the air-choke to the antenna feedpoint does seem to help somewhat. I’ll continue throwing it in my pack and experimenting in the meantime.

Hear you on the air!

73

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Six Meter Club of Chicago (SMCC) Hamfest 2016

This past Sunday morning, bright and early, I made my way out to my second Chicagoland hamfest, the Six Meter Club of Chicago‘s annual hamfest in Wheaton, IL. It was quite different in feel and size to the De Kalb Hamfest I attended a couple months ago – not better or worse necessarily, but definitely different.

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Apparently, theatrical tie-line is also “Nylon-core Antenna Rope.”

Where the De Kalb hamfest was spread out over several long winding paths and four or five buildings, all of the tailgaters in Wheaton were compacted in one central parking lot, which was already filled and thrumming when I arrived at 8:02. With the thermometer and humidity spiking by 9am, it was nothing like the foggy March morning of the last Hamfest.

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Even the morse keys need shade on a hot day.

In terms of sellers, I would say the Wheaton hamfest had perhaps a third as many actual vendors (including The RF Adapter Guy and all his wears and a few others) but three times as many tailgaters. Many tube radios to be found in the parking lot, same a last time, and maybe five or six folks with a healthy collection of CB gear and some antique television sets.

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If you need tubes, there are three or for folks who would love to unload theirs.

I picked up a few little things early in the day – some more trimmer capacitors, a couple used hand mics and panel-mount connectors for the same, but nothing was really catching my eye. There were more parts-dealers at Wheaton then out in De Kalb,  but no one really had anything special that caught my eye. I did find a couple twins to the air-variable capacitor that I picked up, with and without casings, but no split-stator types, which is what I’m hunting for now. Not many enclosures or antenna parts either.

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A very similar variable capacitor to the model I picked up in March; I think this was the same gentlemen who was selling them in De Kalb as well.

Just as I was ready to pack in for the day, a gentleman announced that the Antique Radio Club of Illinois auction was about to start. This proved to be the most bountiful part of my morning, and had a fascinating structure. Lots of equipment, lots of it Boat Anchors, was laid out in rows along the side of the main exhibition building. After allowing a few minutes for folks to wander through and poke at things, we began the action proper, which they called “bidders’ choice.” Basically, if you wanted to bid on something, you held it up for the gentleman in charge to see, and he’d throw out a starting bid, say $5. Anyone else who wanted that item could volunteer a higher bid, much like a regular auction, but there wasn’t much bidding happening. No item went above $20. It was during this period that I snagged an antenna analyzer on the cheap (see below)

Then, once there were no items left that anyone wanted to bid on, the gentleman in charge announced, “Everything left is $5. Grab what you want, and get your $5 to Rudy.” Rudy did well for himself at this point, and maybe half of the items vanished. Once the dust had settled, the gentleman in change once again proclaimed, “Alright, now everything left is a dollar!” Well, for just a dollar…. and once that was concluded, anything left on the ground was set loose for free! All in all, I snagged an MFJ-207, a big analog multimeter, an antique transistor radio, and a hustler resonator for under $20. Not too shabby! I must remember to stick around for Auctions in the future.

After that, a quick trip to Fry’s and Menards capped my morning, and I was home by 2pm. Another great, friendly, fruitful hamfest.


So, here’s the final haul:

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  • An MFJ 207 Antenna Analyzer. The big score of the day! I’d be eyeing an identical unit in the parking lot marked at $70, but to score this one, working, for $15 at the auction really made my day. All it needed was a new battery.
  • A Micronta Analog Multimeter. With settings for AC and DC voltage and current measurement, as well as resistance, it’s a neat unit with a six-inch analog meter movement. It’s in pretty good shape, but I’m sure it’ll need new batteries.
  • An Arvin 9562 Transistor Radio: Apparently made in the late 50’s, it’s got an attractive wood case and a big ole internal six inch speaker. I was planning on gutting it and using the case, but it turns out the thing actually works – it turned on while I was carrying it in from the car and scared the bejeazus out of me.
  • A Hustler 40m Resonator: I took a flyer on this, since it was free. It’s designed to go on a  mobile vehicle mast and turn it into a 40m antenna that’s only 6′ long. We’ll see what it gets gutted and turned into.
  • Two hand-mics, both with 5-pin connectors
  • Two 5-Pin Panel Mount Connectors to go with the above microphones
  • Two baggies of Trimmer Caps of a couple different sets of values.
  • A Pack of Binding Posts, probably will end up used as grounding logs
  • A Small Metal Box – can never have two many enclosures!
  • A two-pack of NTE110 germanium diodes (from Fry’s electronics, on my way back home).

Another swell time hanging out with hams. And with ARRL Field Day coming up this weekend, I’m sure I’ll have more ham stories to tell. In the two days I’ve had it, I’ve already put the antenna analyzer to good use out by the lake, but that’s a story for another post.

See you at the Fest!

73

Antenna Analyzer, Part 1

Today, I received a package in the mail from a fellow ham who was getting rid of some gear. Specifically, he’d put together most of the parts to put an Arduino-controlled VFO on one of the cheapie Chinese Forty-9er kits, like Farrukh Zia did in a QST earlier this year (his kit is available online.) The package I received had the Zia board, an Arduino Nano, an AD9850 DDS module, a rotary encoder, and the Forty-9er kit itself, already assembled. I do want to someone assemble the kit as described in QST, but for the moment, what I’d really like to make is an antenna analyzer.

The scope of what constitutes an “antenna analyzer” is wide – from my rinky dink setup at the bottom end to 4-figure solutions at the high end. Really, what I’ve put together is andirectional coupler with 50-ohm terminations. The fancier analyzers will tell you not only the actual impedance of the antenna (or anything else on the output port), but also the magnitude (and in some cases the sign) of the reactive component of the impedance. Neat stuff! For my purposes, a simple Arduino-driven device that measures relative SWR will be good enough for my purposes. (Dan, KB6NU, has a helpful write-up of some of the mid-range options.)

I’ve been looking at K6BEZ’s plans for an Arduino-based analyzer for a long time – really, the key piece I thought I was missing was the AD9850 – and this seemed like a good time to give it a try.

I had an SWR Bridge Kit from KitsAndParts lying around, and I thought this would be a good time to put it to good use. The kit is simple directional coupler, with a couple diodes and capacitors in each line to hold a peak DC level. The coupler is meant to drive a pair analog meter movements.

To adapt this coupler for use, I first simply attached each output to the Arduino’s analog inputs. This went quite poorly – the charge building up on the capacitors on each side of the coupler has no where to go, and so rather than giving an instantaneous peak voltage, the analog readings just kept creeping up and up. A 68K resistor from each output to ground (facilitated by a small breadboard) solved that issue.

The main issue now is one of scale. The AD9850 Module itself puts out a little less than a milliWatt at most, deceasing with higher frequency outputs. The K2ZIA module has a little two-transistor cascade amplifier to help compensate for this, but that only pumps the output up to 14 or 15 dBm at 1 MHz, with decreasing returns at higher frequencies. Given that the directional coupler has a “single turn” (1 wire) pickup through the center of each toroid, and each toroid has 12 turns, we can surmise that less than 1% of the energy going into the coupler is being diverted to measurement functions. with the rest passing through to the antenna.

This low amount of coupling would normally be desirable, but this time, that means -5 dBm presented to the FWD measurement port, which just isn’t enough power to overcome the forward voltage of 1n5711 diodes in the coupler. (I measured mine to have a voltage drop of 0.323V. That’s which a $50 multimeter, so take it with a grain of salt, but that’s the order of magnitude.)

To temporarily overcome this, I hooked up my little BS170 board from the Si5351-Amplifier to the output of the K2ZIA board’s amplifier, which gives me about 4W output at 1 MHz (and decreasing above that). This gave me enough power to test the coupler (and the K6BEZ software), and both seem to be working fine. I’d like not to build another 5W amplifier just to use as an antenna analyzer, though, so once a shipment of 1n5711’s arrives from eBay, I’m thinking of just replicating K6BEZ’s detector circuit, using a resistive bridge and op-amps.

Hear you on the air!

73