A New 40m Dipole

Up to this point, if I wanted to get on the air, I had three antenna options:

  1. Walk 20 minutes to the lake, spend 15 minutes getting a 40m inverted-V set up in a tree, and hope I didn’t forget anything. This has been netting me good results, but is a bit cumbersome.
  2. Set up the PHF40 loaded-dipole antenna on a tripod in the backyard. Also a bit cumbersome, and I’ve not had any success making a contact on that antenna yet. I suspect the efficiency is quite a bit down from a full size antenna.
  3. Try the random-wire I’ve got strung up around the office.

While it certainly doesn’t feel like it in the August heat, winter is surely coming. In order to ensure I won’t have a long operating hiatus once the snow hits and going outside is impossible, this week I set about putting up a 40m dipole in an inconspicuous place.

I won’t share exactly where the antenna is mounted, but suffice to say all parts of the antenna are about 12ft off the ground – much more of a skywarmer than a DX antenna, I should think.  I tuned this antenna the same way I did with my inverted-V, using just the MFJ-207 and a pair of snips.The antenna ended up being 31’6″ along each leg.

The final antenna is resonant around 7.075 Mhz with an SWR of 1.2:1 at that frequency, with a 2:1 SWR bandwidth of 6.9-7.240 Mhz.

Currently, the feedline is just a piece of RG-58 sneaking in through a window into the apartment. Interestingly, the SWR at resonance shoots up to 2:1 if I bring the metal window-frame down to within about an inch of it. I should note that I’m not using a balun with this dipole, just the usual center-conductor-to-one-wire, shield-to-the-other setup, so it doesn’t surprise me that the coax is coupling to the window. I’ll need to set up some kind of non-metallic passthrough to block airflow with maintaining the antenna characteristics.

After a couple evenings of listening, all signs point to this being a workable antenna! The map below shows the stations that I’ve heard (not contacted) in the past couple evenings. The station in red is one that I was able to contact, but we were fighting QSB and couldn’t really complete the QSO.

Hear you on the air!

73

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

Wind, Antennas, and a First Portable Session

This week I had two radio firsts: trying to put together a halfway decent antenna, and a first portable operating session.

I started by putting together a center insulator a dipole antenna. There are many possible designs online, but I ended up making one with materials I’m familiar with. It’s made of a 1″ PVC cross, four PVC caps, three 1/4″ eye-bolts, some nylon lock-nuts, and panel-mount BNC connector. The whole went together in about half an hour. The PVC connections were surprisingly tight as a press-fit, but they and the eye-bolts are secured with lock-tite and sealed with hot-snot just in case. The BNC connector is soldered internally to a couple pieces of hookup wire, which you can see poking out the sides of the cross. The antenna wires themselves are simply tied to the eye-bolts and wire-nutted to the hookup wire.

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The dipole center-support. This picture was taken after the fact, hence the pre-cut Nylon cord wrapped around it. You can see the two copper hookup wires poking out either end.

The dipole center, a 200′ spool of nylon wire, some “stakes” for securing the antenna to the ground (really, 8″ pegboard hooks, on sale at Menards!), some zip-ties, and a couple of rice-filled socks as counterweight all fit handily into this small plastic case I had lying around:

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I live only about a mile from Lake Michigan and the beautiful lakefront park space that runs along the East side of Chicago. I figured with the sunny-but-chilly weather we’ve been having this week, the lakeshore on a weekday afternoon would be pleasant to work in and not too packed with people.

I couldn’t find a suitable table or bench close enough to trees to set up under, but then I spotted the convenient height of the sea-wall between the outer lakefront trail and the lake itself. I couldn’t have been any closer to the lake without getting my socks wet!

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The operating position. There’s a little ledge just on the other side of this wall that was perfect for standing on.

I strung up a portable, homemade 40m dipole (wires cut in advance) as high as I could could into one of the nearby trees – I only got the center maybe 20 feet in the air, but that’s still better than the 8′ the random wire in my home office is. The ends are tied off to stakes near the ground. This made the setup more of an inverted-V than a true-dipole.

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One of the antenna-wires staked off to the ground. You can see the black coax sloping from the tree to the operating position.

The antenna is fed from ~60′ of RG-58, running into a ZM-2 Tuner for matching, and then via a short RG-58 jumped into the Virgin Receiver. I also brought along the ardiuno-controlled, Si5351-VFO I’ve been working on to allow for the ability to change frequencies, as detailed in my last post. If I’m going to spend the time setting up practically in a lake, I thought it would be nice to actually scan the whole band. Since the VFO rig is still on a breadboard, I was quite spread-out over my little stony operating station.

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Visible here: the coil of coax coming from the antenna; the Virgin Receiver, open to allow the VFO to interconnect; the breadboard with the Si5351 VFO on it; two separate 12V SLA batteries; some nice comfy headphones; and the large red cable-coiler I used to store the dipole wires (a bit overkill).

So now, the moment of truth – I hooked up the batteries, connected the receiver and VFO, did a quick by-ear tune on the ZM-2. I’ve never done something like this before – would it actually be better than my office random-wire? Was it all just a jaunt in the park? In short, would it work?

LIKE HECK IT WORKED!

Especially up in the SSB portion of the band, there were some stations that might as well have been on the other end of a phone call. (This, by the way, confirms that the Virgin can receive SSB and AM as well as CW.) In no particular order, I heard:

  • N8KKR, KA9ZXN and others on the Hams for Christ net on 7.263 (330p Central, Monday-Sat). Not my personal bag, but they seemed a very pleasant bunch.
  • W9DCQ and W4LWW having a nice QSO, between the two of them and some other operators I couldn’t quite copy.
  • KG9O and KK4FZI having a friendly chat.
  • Some other snippets of stations.

The listed stations alone include QTHs in Evergreen Park IL, Columbus OH, Middleton WI, Franklin TN, Grassy Creek NC, and Marion IN. None of them terribly far away in radio terms (the Tennessee locations is farthest at ~500 miles), but a great confirmation that the antenna was clear and working. With a height above the ground of less than 1/4 wavelength, the ideal dipole should have a high takeoff angle and be fairly omnidirectional, and that seems plausible based on results.

In sadder news, I seem to have broken some part of the WSPR functionality of my VFO in consolidating that functionality from two separate programs into one. 1W into the antenna and several repetitions netted be exactly zero receptions, either into this antenna or my office random wire. So it’s back into the code I go. While I’m at it, it would really be worth packaging up the little VFO into its own enclosure – having all those wires flapping about is a bit worrisome for transport!

All in all, a tremendously successful, if very chilly, day by the lake. Now that I know what I’m doing, I suspect it would be able to have the station set up and listening in about 10 minutes, and about the same for tear down. With the days getting warmer all the time, I’m sure I’ll be out there again soon.

Hear you on the air!

73