Spider Antenna – Circa 1985

On the heels of the Wolf River Coil DIY tripod rebuild, comes the Spider Antenna. Invented and patented at the time by Fred Smitka, K6AQI, out of California, the Spider Antenna was created and sold in the early 80’s. It is by design a mobile antenna designed to take advantage of the ground plane a vehicle can provide. But since I had just reworked my tripod for the WRC, I made a few adjustments so that I could also connect the Spider Antenna and take advantage of the radials I had already created for the WRC. And it works! Sort of. More on that in a moment…

Where the WRC is a bottom loaded vertical, the Spider Antenna is a top loaded vertical. This blast from the past was picked up in a Craigslist buy several years ago. I was looking to upgrade a dipole from my then G5RV Jr. When I arrived to purchase a DC-CC fan dipole I was greeted with a pile of various antennas. There were a couple of wire dipoles, a rigid Cushcraft D3 20-15-10 rotatable dipole, what looked like at least a pair of “chimney sweep” MFJ antennas with all manner of missing parts, and maybe one other antenna worth of parts I never did figure out. But, there was also a cardboard tube that contained this odd looking antenna system that is known as a Spider Antenna. The seller was more than happy to see the entire pile leave. Their trash became my treasure.

So what is this thing? In simple terms, the Spider antenna is a top loaded vertical. Unlike the coil antennas we are accustomed to seeing, where you have 1 coil, and then you tap the coil for the band you wish to operate on, the Spider Antenna has a 5′ metal mast, with a fitting at the top for up to 4 individual coils. There is a separate coil for each band. The coils come in bands 80-10. The one catch is that 80, and 40, can only go on top. So you cannot for instance have 40 and 80 on at the same time. But you could do 40, 20, 17, and 15 if you like, or whatever combo you wish. What was really cool about this find was that it was a complete set of coils for 80-10 meters. bands!

To tune the antenna, over each coil is a ferrite that you slide up or down the coil until it is resonant where you want it within the band. As the bandwidth can be a bit narrow on 40/80, you will have to adjust to which end of the band. What is also remarkable is that after 36 years, this antenna still seems just fine. The antenna is rated to easily be able to handle 200 watts. And I even found a review online from Gordan West that claims he ran a Magnus 800 watt amplifier through the Spider Antenna and it worked just fine.

The entire antenna is of quite high quality construction. Normally, as we have experienced, plastic coatings break down over time. The materials on the Spider Antenna have held up very well. Each coil is of rigid fiberglass construction.

Setting it up in the back yard and using my Nano VNA, I was able to bring adjust each coil in just a few minutes. I hooked up my KX3, but was only on battery so output power was quite low, but I was able to receive quite well. I will have to connect to the TS-440 and do an A/B test and post an update.

Son of “LancePod”

A couple years ago now, our local craft person, Jonathon, W6BID put together a club build for a tripod for the Wolf River Coils SB1000. The WRC TIA (Take It Along) kit t is an excellent, economical, portable, base loaded, vertical for pretty much any band. The community around this antenna has spawned all manner of modifications, whip options, radial options, and in this case a DIY tripod. The “out of the box” tripod, while mostly adequate, was tippy in windy conditions. Several of us have also adopted either the MFJ-1979 or Chameleon SS-17 stainless steel whips for better performance on the lower bands. The problem being with the longer, larger whip, the base needs more stability. Enter W6BID…

For a club meet, W6BID organized kits for us to assemble in a weekend garage project. Each of us taking turns drilling holes, and assembling our new WRC tripods. The original kit consisted of a piece of plate steel, a 3/8″ thru-hole, stainless steel antenna mount, 6 L brackets that Jonathon had welded to the plate ahead of time, 3 sections of 1/2″ steel square tubing, and some bolts here and there. All that and a quick coat of spray paint resulted in one stout tripod base. It has served us well!

But some issues crept in over time and after multiple uses. For starters, this thing is chonky! And secondarily, since the plate is steel, oxidation formed in between the plate and the stainless steel antenna mount creating resistance with the radials. This was finally identified as to why SWR readings were so out of whack from when the last time it was used.

So back in the shop it went for an update.

To address the chonk, I drilled holes down each of the 3 legs at 3/4″ intervals, and then turned the leg 90 degrees, and repeated at offset 3/4″ intervals. The end caps which had fallen off long ago, were replaced, but this time filled with hot glue before being re-attached. The base metal plate also had large holes drilled where I could safely remove material. This reduced the weight considerably, but still providing a solid stable base for the antenna.

To solve the oxidation issue I used a product called DeoxIT L260Cp that is a lithium grease that is infused with copper particles. Once the steel surface was cleaned back down to bare metal, a dap was applied to the surface and everything reassembled. Other changes made along the way…

  • Much experimentation with radials
    • Out of the box are 3 33′ radials made of black silicone coated wire.
      • Pro – Fairly quick to roll out.
      • Con – Performance was “Ok”.
    • Doubled to 6 33′ radials
      • Pro – Increased performance significantly
      • Con – When out in the field, you are taking up a circle nearly 70′ across. When camping this is valuable site space. The black wire was hard to see in the grass. Gopher/ground hogs actually tried to drag radials down one of their holes one night at a camp site.
    • Swapped out wire radials for 33′ tape measures
      • Pro – Fast to deploy. Easy to change the length if you want to “tune” the length for the band. Super fast to roll back up. Bright yellow tape is easy to see when deployed so people are less likely to walk through your radials.
      • Con – They rust. They are heavy. They are bulky.
    • Swapped out tape measures for 9 8′ radials constructed from a bright yellow silicone wire, and added quick disconnects.
      • Pro – Very fast to deploy. Bright yellow for visibility when camping or portable. Shorter length is way way less prone to tangling. Very lightweight. Compact. Works just as well as the 6 33′ radials.
      • Con – Not quite as efficient on 40 or 80 meters, but is still below 2:1.

Overall this has been a great project that has now gone through a couple iterations resulting in a very portable HF antenna solution that has performed exceptionally well. I just recently also picked up a padded tripod bag to store it in, and have a 50′ length of ABR Industies ABR240-UF (Ultra Flexible), with 5 ferrite beads shrink wrapped to the end. This entire solution can be in place in moments, is easy to tune, and with the larger SS17 whip is an efficient antenna system.

A Few Items from Tonight’s Tech Net

On tonight’s tech net, the main items of discussion centered around coax, antennas, power supplies and batteries.  All things that we as hams have to deal with.  Due to the length of my explanations here, I’m going to skip batteries for now, and just throw down a few thousand words about coax and antennas:

Coax

Coaxial cables come in many different types and sizes.  One question was why are there so many different sizes of coax?  Without getting deeply into the mud, I suggest that there are broadly two main reasons:  loss and power handling capabilities.  Let’s talk about power first, since it’s pretty straightforward.  The more power a coaxial cable needs to handle, the bigger it must be.  Part of that is like battery cables, the center and outer shield have to be beefy enough to handle the power.  Another part is that the more power going through a coax cable, the higher the voltage difference is between the center conductor and the shield.  Just like with high-voltage transmission lines, as the voltages get up into the thousands, the two parts of the coax must be moved further and further apart to keep them from arcing over.  I have seen coax in use at a 50,000 watt TV station that is larger in diameter than my arm (and I’ve been working out!).

Regarding loss, the general rule is that the smaller the diameter of the coax, the greater the loss is going to be.  Also the higher the frequency we’re using, the greater the loss is going to be. In the Wi-Fi consumer electronis industry, we routinely use 50 ohm coax cable smaller than the lead in a pencil because of the need to be able to snake the cable in and around very tight spots.  We accept the fact that the loss of this cable in the 2.4 and 5.0 Gigahertz WiFi bands is substantial but, due to the short lengths we usually use, it’s acceptable.

Below is a quick list of coax cables you are likely to meet as a ham.  There are hundreds of types, so this is a very pared-down, ham-friendly list.  RG-174 is often used by QRP (low power) operators, because it is small and light, and easily carried in a back pack or go kit.  RG-214 and RG-8 are a bit over 1/2 inch in diameter, and like LMR-400, are your best bet for carrying VHF and UHF signals for 50+ feet without too much loss.  In fact, please use LMR-400 rather than either RG-8 or RG-214 for VHF+.  RG-214 has a double shield, and is quite expensive, but good for use in repeater cabinets.  RG-8 and its cousins are good for 2-30 MHz use, and is much cheaper also.  Note that these thick cables can also withstand 5,000 volts, and RG-6 (75 ohm cable TV) is only good for 350 volts.

So why did we settle on 50 ohms for most coax?  Actually, 72 ohms is best for least receiving loss of very small signals, and that’s why the cable TV industry like it.  It turns out that 50 ohms was calculated back in the 1930’s (and re-calculated many times) to show that it was the best impedance for transferring power.  For receiving, 72 ohms is still best (and some hams use 72-75 ohms for that reason), but 50 is a good compromise.  It also turns out to be a happy value, close to the natural impedances of certain types of monopole and dipole antennas.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Antennas

Taking the simple example of a steel wire antenna poking up from the center of a car’s roof, if it’s about 19 inches long, we can say that this is a quarter-wave antenna in the 2 meter ham band.  It radiates (and also picks up when receiving) signals from directions generally best represented by imagining throwing a big, inflatable doughnut over it, like the ring-toss game.  The pattern favors the horizon in all directions out from the car, with some pickup also from maybe 30 degrees above and below the horizon, as this drawing shows:

This 1/4-wave antenna has a slight gain (meaning you get more power or received signal than you put into it!) compared to an isotropic antenna.  Isotropic is a theoretical pattern described by a beach ball–it both transmits and receives well in all directions.  Our 1/4 antenna gets about 1.2 dB (abbreviation for decibels, which relate in logarithms of the powers of 10, so 3 dB is a doubling or halving of power) more gain at the horizons only compared to an isotropic antenna, because we squished that beach ball with giant hands down in the middles, top and bottom.  It’s all the same amount of energy that we started with, but we’ve squirted a larger amount of it in a direction that’s more useful to us!

Speaking of squishing a beach ball, now let’s squish it down in the middle even more by changing the antenna to a 1/2-wave type.  A half-wave antenna simply squirts even more energy at the horizons (so the signal goes further across a city, for instance), and even less up or down.  We also lose more of that fat, doughnut shape, and it starts to look a bit more like a frisbee, but (and here’s the big deal) more of it squirts out towards the horizons, which are the useful directions we really want our signal to go.  Here is an image showing “Unity”, or about like a 1/4-wave antenna, then two more antennas that have 3 and 5 decibels of gain over a dipole (that’s the little “d” after the “dB”, it’s the thing we are referencing ourselves to).

 

That’s all antenna gain is–we refocus the energy from that theoretical beach ball in directions we care the most about.  A beam antenna becomes more like the reflector on a flashlight.  Held in the open, a flashlight bulb somewhat weakly puts light out in all directions.  However, that same bulb, now placed in a flashlight with the reflector right behind it, can really put out a strong beam, but only in one direction.  Dish antennas are very close to flashlight reflectors, and accomplish pretty much the same task: they reflect back any energy in a single (well, mostly) direction, causing increased signal in that direction.

So, broadly speaking, gain-type antennas create this wonderful gain because they redirect the antenna’s energy in the directions we care most about.  Make sure you walk away from this little tutorial with one important thing in your head:  Gain antennas don’t create any new energy.  If they did, the Law of Conservation of Energy would be violated, and would probably get quite upset about being violated in such a rude way.  No new energy, simply the same amount aimed where we would like it to be most effective.  The universe maintains its balance.  No dark matter was harmed in the making of this movie.

Aha, so “gain” is actually a misnomer, isn’t it?  There is nothing gained in one place that isn’t also equally lost in some other,  thus maintaining nature’s balance.  Maybe instead of calling them “gain” antennas, we should start a campaign to call them “redirecting” antennas.  Well, maybe it would catch on…

As a final thought, we can make gain antennas either by making them a certain length, or by stacking multiple antennas, or by brute-force reflectors (like a dish).  It turns out that certain lengths of antennas have more gain than others.  A half-wave has 1.2 dB more gain than a 1/4-wave.  A 5/8-wave wire has 3 dB more gain than a 1/4-wave wire.  Stacking several 1/2-wave antennas has more gain than them all!  The only “trick” to getting all this gain is that you have to make each new length or added antenna happy by matching it to the exact impedance of the rest of the antenna system, and that can get tricky, because it will rarely be 50 ohms. Not super-difficult, just tricky.  The reason everyone likes a 1/4-wave antenna is because,given a decent ground plane to work with (the car’s roof, in this case), it happens to come out to a nice, round 50 ohms characteristic impedance.  That means we can connect any old 50 ohm coax, and we’re done!

What if we want the gain and directionality of the 1/2-wave antenna?  OK, here’s a new challenge:  The 1/2-wave antenna, at its two ends, are thousands of ohms in impedance, instead of just 50 ohms (but you can find 50 ohms in the middle of that 1/2 wave antenna–makes sense?).  Darn, now what?  Well, we have to create a (hopefully simple and small) matching network to transform 50 ohms to several thousand, and connect it up to our half-wave antenna.  The trick (and the reason I get paid for this!) is to lose as little energy as possible, and make it for cheap and simple.  I can tell you that the matching network will probably involve an inductor (small coil of wire) that has a tap on it about 1/4-1/2 turn up from the bottom), and a capacitor to resonate with the coil near the design frequency.  Depending on how big and lossy those parts are, we could easily lose most of the gain.  You won the battle, but lost the war, so to speak.  (RF engineers’ joke:  What do you get if you end up with a poorly-designed matching network?  Answer:  New antenna company on eBay with offshore mailing address!)

OK, haha, thanks.  So adding a bunch of 1/2-wave, 5/8-wave, or even 3/4-wave antennas together will give you more and more gain, offset by whatever losses you have in your matching networks (and there’s always some energy lost).  A vertical stack of these antennas to make one high-gain, 20 foot-tall antenna of maybe 9-12 dB is called a collinear array.  All the while, that pattern you see above gets flattened more and more out towards the horizon.  Is there a practical limit to the size of a collinear antenna?  Yep, eventually, the losses in your matching networks at each junction of new antenna elements will add up to give you diminishing returns.  You will have less and less of the original energy available down at the antenna connector.  The array also becomes more and more mechanically unstable until finally someone shouts “Jenga!”  Again, you’re welcome.

Ham Radio 360 Podcast features local hams

George, KJ6VU, has a great podcast and accompanying web page with some interesting gear and DIY projects going on. They are presently doing a group build of a 1-30 MHz antenna analyzer based on the Arduino platform. Have a look and/or listen at www.hamradio360.com

 

I mentioned this on tonight’s tech net at 9 PM.  While you are perusing that website, have a look around at a few other very interesting things.  For instance, see George’s presentation from this year’s Dayton Hamvention, where he introduced the very handy portable end-fed antennas from his new company called PackTenna.

 

The presentation does a very good job of showing you how to wind some simple baluns (actually they are un-un’s) to get your 50 ohm feedline matched to the very high impedances of either a resonant, end-fed dipole, or a random-length end-fed antenna.  Or…you could buy one ready-made from PackTenna for $89.  If you go to www.packtenna.com and look around, be sure to read the QST magazine “test drive” of his antennas.  Highly recommended.