I’ve been threatening to replace my 40m/30m fan dipole for several years now. I replaced the coax a year and a half ago, but the antenna had been up so long that I was having several other problems with it. For one thing, the antenna had stretched enough that the resonant frequency was below 7 MHz, and up in the phone portion of the band, the SWR was above 3.0, and the tuner in my rig will only match SWRs 3.0 or less.
I also wanted an antenna that I could use on 80m. At times during the winter, the 40m band is unusable here. I guess it’s because the band goes so long that wherever the skip is, no one is either awake or it’s out in the middle of an ocean. I don’t have a huge lot, though, so I have to make do with a short antenna for 80m.
One antenna that looked promising is the Cobra Antenna. This antenna is heavily advertised on QRZ.Com. There are three versions of this antenna available:
- a 41-ft. antenna for 40m – 6m ($99)
- a 73-ft. antenna for 80m – 10m ($115)
- a 140-ft. antenna for 160 – 10m ($125)
The interesting thing about this antenna is that the elements are made from three-conductor flat cable. The conductors are connected in series so that the overall length of the RF path is actually three times the length of the element. So, for example, the length of one of the elements for the 73-ft. antenna is 73/2 x 3 or about 110 feet.
The K1JEK site is a little skimpy on technical details, but a HamUniverse.Com article has more information on it. Apparently, information on this antenna first appeared in the June 1997 issue of 73 Magazine. Like many articles that appeared in 73, it was long on story, short on details. The HamUniverse article is decent, though.
I had read the 73 and HamUniverse articles several years ago, and it looked like it might be a good solution for me. I even went so far as to purchase some three-conductor rotator cable from Radio Shack (RS P/N 15-1150) to build the antenna. The conductors are 20 ga., and while the cable is light, it looks to be pretty strong.
The Radio Shack cable looks exactly like what the K1JEK Cobra is made with, but unfortunately Radio Shack no longer sells this cable. I did Google around a bit this morning and found a couple of other cables that might work, but more on that in another blog post.
Building the antenna
If you’ve been reading my blog, you know that one of my hot buttons is paying a lot of money for an antenna that you can build for half the price or less. I don’t begrudge the antenna makers their dough, but if you build an antenna from scratch, then you’ll certainly learn more about it than if you just screw things together.
With that in mind, I went shopping for something that I could use to make the center and end insulators for my version of the Cobra. Someone on some amateur radio forum somewhere mentioned slicing up a cheap cutting board. The ones that I found at Kroger, however, were kind of thick, and besides, they cost six bucks!
My next stop was the Dollar Tree store. There, I found exactly what I needed, and it only cost a buck! Using a Dremel tool, I sliced off the handle, then cut the board into three parts. I then drilled holes to run the rotor cable through. I’m really pleased with the way this turned out.I next cut two, 36.5-ft. sections of rotor cable began actually making the antenna. To be honest, I copied the K1JEK design. I even used blue crimp terminals. After crimping all the wires together, I added a touch of solder to each of the terminals that connected two wires. Then, I screwed everything down to the insulators.
About 11 am Saturday, my friend Thom, W8TAM, came over. I still hadn’t quite finished the antenna construction, so he kibitzed while I finished putting it all together. About 11:45 am, I’d finished, so, of course, we took a break for lunch. Thom went home to eat, but also to pick up some antenna supplies, including his roll of Dacron rope and his spare (!) Johnson Viking Matchbox tuner.
I ate some lunch and then spliced together the two lengths of 450 ohm ladder line that I had. The Cobra instructions note that you should have at least 81 feet of ladder line, and because I’d been using pieces of my 100-ft. roll to make J-poles, I only had about 75 feet left. I did kind of a crude Western Union splice and added about 25 feet from another hunk of ladder line that I had from a previous antenna project.
Getting the antenna in the air
Thom returned about 12:30pm, and we hauled everything out to the backyard. The first thing that we had to do is to take down the 40m/30m fan dipole and the 17m dipole that was also attached to the support rope. After the antenna was down, I coiled up the coax, inspecting it as I rolled it up. I noted several spots where a varmint had bitten into it.
Now, it was time to raise the Cobra. Thom was in his element here, making some very good suggestions about how to hang the antenna. For example, a tree near the rear of my property supports the far end of the antenna. I had just thrown a rope over one of the upper branches and tied it off to one of the lower branches. Thom suggested that we install a pulley to make it easier to raise and lower that side of the antenna. Fortunately, I had one in my “antenna box” and installing the pulley took next to no extra time.
Finally, we discussed how to get the ladder line into the shack. Previously, I had just let the coax lay on the ground. That’s a no-no with ladder line. What we ended up doing is routing the feedline around a pine tree and then over to the house, where we wrapped the excess length around a trellis attached to a small deck that I have. My wife hasn’t noticed that last little bit of handiwork, but she’s been a saint so far when it comes to my antennas, so I don’t think she’s going to mind all that much.
For test purposes, we just ran the ladder line in through a partially-opened basement window. I’m going to have to do something about that and have been reading K5VR’s advice about how to do this. I purchased some brass hardware yesterday for the feedthrough.
Antenna tuner troubles
I had planned to use a Dentron Super Tuner that I had sitting on the shelf to tune the antenna. It has a built-in 4:1 balun, and thought it would do the job. WRONG! We couldn’t get a match anywhere using that antenna tuner.
(I have since come to find out that we didn’t follow the setup instructions properly. Although the Dentron Super Tuner instructions are admittedly sparse—they consist of a single page of text and a second page of schematic—they clearly state that when you connect ladder line or open-wire feedline to the tuner that you also have to connect a jumper wire from one side of the ladder line to the single-wire terminal on the back. We failed to do this because we not only did not read these instructions, we joked about reading the manual. Moral of the story: RTFM!)
Fortunately, Thom had brought over his spare Matchbox. It’s an interesting story in itself how he came to be the owner of two Matchboxes, but again, more about that later. The Matchbox is a cool tuner because it’s designed to accept a balanced input. No balun needed!
With the Matchbox, I was able to get a good match all across the 40m band. Next, we tried 80m. Unfortunately, I wasn’t able to get a 1:1 match down in the CW portion of the band, but I did get a 1:1 match in the phone portion of the band. I’ll need to play with that a bit more, maybe shortening the ladder line.
Being a big 30m guy, I next wanted to see how it would do on 30m. The interesting thing about the Matchbox is that the switch that controls the tap on the coil is marked 160-80-40-20-15-10. Those are, of course, the amateur radio bands that we had when the Matchbox was manufactured.
To see if I could get a match on 30m, I first set the switch to 20m No luck. When I set the switch to 40m, though, I was able to get a 1:1 match on 10.110 MHz. I was in business!
The proof of the pudding is in the transmitting
Of course, the proof of the pudding is in the transmitting, so I tuned the Matchbox to about 7.030 MHz and called CQ on 7.031. Almost immediately, I got a call and had a nice QSO. Then, I decided to try to make a phone contact. After retuning the Matchbox, I called CQ on 7.240 MHz and made a couple of QSOs up there. The first station was kind of weak, but the second was a solid QSO.
I also tuned it up on 30m and made several QSOs, including CN40KD in Morocco. I tried making more, but my CQs went unanswered. I attribute this to all the activity in the CW Sweepstakes contest that was going on that weekend. I was getting SNR reports of 40 dB or more on ReverseBeacon, andn I don’t recall reports that high ever using the dipole.
Overall, I really like this antenna. I think that the signal reports that I’m getting are somewhat better than the signal reports that I was getting with my 40m/30m fan dipole, and of course, I can now operate 80m. I’m so geeked by my success with this antenna project, that I’m thinking about building a hex beam in the spring.
Stay tuned (pun intended).
Kirk Seifert says
Dan,
I bought one of these antennas several years ago. It hangs in the trees here on the mountain. Was labeled a doublet and had those exact dimensions. Except it has 100ft of ladder line. I use a 4/1 balun. The wind and elements split the wire so it looks like 3 separate wires at several locations. Cheap wire! It tunes with my TS2000 built in tuner on most freqs.
I alternate with coax switch with 5 band vertical and really don’t see much difference, typically. Some rough propagation days there is a significant difference in favor of the doublet which is up 45 ft and vertical is ground mounted.
I like it. Good luck with the project.
Kirk W5KRK
Dan KB6NU says
Thanks for the information. I hope mine weathers a little better.
MJ WO9B says
Nice article. I too am a fan of antennas fed with ladder line. I own a Dentron Jr tuner and use it successfully with my ladder line dipole. It has a built-in 4:1 balun. From what I can see on the web, the Dentron Super Tuner also has a 4:1 balun.
If all is well with the tuner, it should function well. A jumper is required to be installed between the single wire and one of the balanced line terminals. Mine has been flawless.
Well good luck and good DX. The simple wire antenna’s work surprisingly well.
Dan KB6NU says
Thanks for the info. I did not connect a jumper wire to the single wire terminal, so I think that was where I went wrong. This is absolutely a case of where I did not RTFM.
You’re absolutely right about simple wire antennas working well. I think it’s all about efficiency, and it seems to me using ladder line really increases the antenna efficiency.
Frank WA8WHP says
FB! I am still playing with some ideas with a “Slinky” antenna. I think two metal slinkies per side should get me a resonance on 80 meters. When I can get some help I plan on doing some work on the relation of stretch and resonance, maybe adding height above ground to the mix.
Dan KB6NU says
I had a Slinky antenna for a while, and yes, you do need two Slinkys per side to get on 80m. I was feeding it with coax and adjusting the number of turns to get it to be resonant on a particular band. What might be interesting is to simply stretch it out to its maximum length, feed it with ladder line, and use a tuner to tune it. That way you could get multiple band operation without having to lower it and retune it when you want to change bands. I don’t think that I’ve heard of anyone trying this before.
On the other hand, why not just build an antenna like mine? With so much metal in the air, the Slinky antenna is going to be pretty heavy and sag quite a bit. You’re also going to need a lot of rope to support it.
Bart W0IIT says
Your homebrew Cobra antenna brought to mind my first exposure to George Dobbs, G3RJV, of QRP fame. It was at a forum held at the Dallas/Ft. Worth hamfest. He walked to the microphone and his very first words were, “You Americans!!! I just walked past a vendor out there selling G5RV antennas and there was a line of guys buying them, paying money for an antenna anyone can make at a fraction of what he was charging.”
At the time I was using a 80 meter dipole and using a KNWD AT-230 to tune it up on any frequency I wanted to work with my TS-530s. I swore right then and there never to purchase a wire antenna. I see simple wire dipole type antennas selling in QST for over a hundred dollars——duh!!!!
Now, I have no problem with a handicapped ham buying a pre made wire antenna but I really wonder where his non-handicapped ham friends are????
Vern Snodgrass says
Its not about the antenna…
Jeff says
Hey! I just bought the Cobra — arrived here just a few days ago. I plan to put it up this weekend and see how it plays on 80m. 73. Jeff
Dan KB6NU says
Interesting. I guess great minds think alike. I’m liking mine so far. I’ve been able to tune it on 80m, 40, and 30m, and I think it’s outperforming the 40m/30m dipole that I had up previously. Not by a huge margin, but maybe an S-unit or so. Let me know how it goes. What antenna tuner are you planning to use?
Jeff says
4:1 balun has been in use for awhile now — using the internal tuner in the TenTec Eagle. Will let you know how it performs. 73. Jeff
Darrell says
Good deal on your homebrew cobra. However, buying an antenna isn’t necessarily bad. Those who sell these antennas aren’t necessarily making a killing as many would think. They are earning money which helps to support themselves and their families. Not everybody wants to nor is inclined to build some things that they use. If nobody bought prebuilt anything then there would be no economy and therefore few or no jobs.
That said, I’m certainly not opposed to saving a bit by homebrewing and also learning from it if you’re so inclined. I’ve done both, built and bought prebuilt. with me, it pretty much depends on my needs at the time.
Anyway, please don’t feel that buying antennas is bad and just making the makers richer. Often times there is much more to building these things in quantity than you think, and the makers usually don’t make a bunch of money as everyone seems to think.
Dave Dennett says
I built this antenna last summer as a backup for my yagi and to get on 160 in the city lot.
The results have been great on 10 thru 80. 160m not so much unless the band is really in good shape…but better than no antenna at all on 160m. I used 210′ of 14 ga stranded insulated copper and zip tied it every 10′. Inv V fashion with apex at 55′, ends at 10′. Im only using 50′ of ladder line to a 4:1 balun at the base of the support, then 75′ of LMR400 to the radio. Yaesu FT3000 internal tuner will tune 10-30 fine as well as CW portion of 80. For 40, 80 phone, and 160 I use an old Drake MN2700.
Greg Seckinger NU4M says
I met Dr Raymond Cook W4JOH while working at a tv repair shop. I was called to his home in Valdosta, GA for a service call. We both were hams and he showed me his antenna he used for 160M, He said the idea came to him in bed one night so he could talk to his friends on 160M. He finally came up with the correct dimensions and it worked. Not sure what other frequencies he was interested in but he was proud of his antenna and his Packard Car. He is still around. He is somewhere around 95 yrs old now.
J A F O says
Hi Greg,
Have you any dimensions to share regarding Dr Cook’s unique 160m antenna design?
The best I’ve come up with for a normal (1/8 acre) city lot is a HAngLO, a vertically hanging 2:1, 1/2 wave rectangle fed at the top center and separated at the bottom center with a foot or so of Dacron cord.
It’s longer than it is tall making it 40′ high x 80′ long, and usually hung between a couple good size trees, or fiberglass poles stacked & guyed.
It’s fed with balanced line and also makes a fantastic ~20° angle-TOA 40m antenna, broadside with ~2-3dB gain.
Anyway, I’m sure others would also be interested in reading about Dr Cook’s design, if you have any details, please share!
73
Anthony Bowyer says
Anyone know the approximate distance between the attachment points on the insulator (for the elements)? I’ve considered putting one of these together, but haven’t seen any details about separation distance. My assumption is that it’s not too critical, but a ballpark figure would be helpful. If they are very close, seems they would act more like one thick conductor. Too far and other issues come into play.
Dan KB6NU says
There are a couple of pictures on the Cobra Antenna home page, k1jek.com. Mine is built pretty much just like that. They’re a couple inches apart, but I also suspect that it’s not critical.
Alexandre Bertoletti says
This antenna has a direct relation with the classic multiband antenna that must be cut into half-wave relation prara the lowest operating frequency however shortened with a linear load, very good because not lost heat energy load coils.
This antenna has the regular impedance of a dipole about 70 ohms in both the feed point as the end of the line abertaque goes to the antenna tuner.
The Balum 4: 1 is not suitable for this antenna due to the fact that its impedance is approximately 70 ohms, the ideal will be to connect the line open in a Balum 1: 1 Current guanela, with this you will very easily sintonisar your antenna to any simple internal radio sintonisador or any other external quality lossless.
Very good antenna although it can be small compared to a regular half-wave antenna.
I have helped classmates, best regards 73
PY2ALE ALEXANDRE BRAZIL
Lonnie White says
I’d like to know how far apart each run of wire is
Dan KB6NU says
The wires are right up against one another—think three-conductor ribbon cable, except the wire is a heavier gauge. I’m not sure how wider spacing would affect the performance of the antenna.
David Dunn KG5JCK says
Dan, I got mt license in 2015 and built this antenna. My uncle sent me the specs to build it and I was able to find rotator cable from Radio Shack. I have a 30ft pole attached to the side of my house and use a pulley to raise the antenna. I have 100ft of 450 ohm ladder line that goes down to a 4:1 current balun. The extra ladder line I have it coiled up and hanging on the side of my house by the balun. I am able to tune 80m to 10m using my MFJ-994B Auto tuner and have had excellent results. I have worked 109 DXCC’s since January 2016. This is a great antenna to build for a newbie and does not cost very much.
73
Eric Thompson says
Hi guys-
I bought and erected the Cobra UltraLite Jr in 2008 (73 ft horiz section, 80ft ladder vert section), as a flat top, and had good performance with it.. The only issue I had was what to do with the super long ladder vertical section.. The center of the antenna was about 20 ft over the top of the house. This meant that I had about 60ft of ladder line to deal with. I used a 1:1 balun at the end of the 80ft ladder section. I contacted K1JEK by telephone and told him about my installation and asked if I could reduce the length of the 80ft ladder line, and he said no (not with the UltraLite model).. The best thing to do would be to lay out the excess ladder section in a very large and loose figure-8 pattern on top of the roof. So I followed his advice and used the antenna for another two years.. I also used this antenna for a few 50.26 MHz, fsk441a meteor contacts..hi… 73, N6SPP
Dan KB6NU says
Hmmmm. I don’t see why you couldn’t do that. I used a random-length piece of ladder line (what was left over from a previous antenna project), and it seems to work fine.
David Ryeburn says
It could be that the reason for the long feedline is to increase loss so that apparent SWR at the bottom of the feedline looks better.
David, born a skeptic
VE7EZM and AF7BZ
Dan KB6NU says
Ladder line has a lot less loss than coax, so I don’t think it’s that. The length changes the impedance at the transmitter end of the line.
Glen Sansoucie says
Just found this after I remembered the name (“cobra”). I built one of these from a type written description handed to me at a club meeting in 1999 or 2000. It had to be the 140’ version as I worked 160-10 on my TS-440 back then. I had an MFJ Roller Inductor tuner with a built in balun that did the trick for me. At the time I bought two rolls of 450 Ohm ladder line and used one on the antenna. The other roll is in the same box in the same location as I put it back then. I had great luck with that antenna, it came down years later when the 75’ ash that I had it connected to came down in a storm.
I’m trying to decide on my next antenna for my “ham revival”. My 107’ Random Wire is FB, but we always want more…
N1XF
henry justice says
K4VSI My 70 ft Cobra Lite with 50 ft ladder line with a 4:1 balun, performs extremely well on 80/40 meters with very low swr without an antenna tuner. NICE! Nearly completely flat over entire 40 meter band and over phone portion of 80 meter band. 2:1 swr on 20, flat on other bands.
Dan KB6NU says
Cool. I’ll have to try that here.
Ron says
Reminds me of the MorGain from the ’60’s…just squashed.
Dan KB6NU says
Hmmmm. That name kind of rings a bell. Do you have a link for more information on the MorGain?
Bryan says
SO, I am thinking of building something like yours, Has yours survived up to this summer?
I have a random wire end fed, but the 9:1 balun only handles 100 watts.
Llike many others, the supports I have for the ire antennas are not that high. They are only 15 feet over my concrete roof. So I have no idea what to do with the excess ladder line. I figure it wont like to be close to the rebar reinforced concrete roof.
Is there a place to get the 3 conductor wire? Or should I just zip tie 3 THHN wires or DX antenna wires together and hope for the best?
Bryan
Dan KB6NU says
These are all great questions.
Dean F. Mason says
Will it still work well if you put it up as an inverted V?
Your not gonna belive this but i bought the 40 Meter one for $5 at a swap meet.
Dan KB6NU says
Wow. Great deal!
I don’t see why it wouldn’t work as an inverted vee.
Cary Geeslin WA4JHE says
;I’ve owned two of the K1JEK Cobra Ultralite Senior antennas for a several years now. The first one I put up lasted until it was taken out by a big storm with high winds. The antenna broke at the center insulator from metal fatigue. I also had to repair the stock 450 ohm ladder line two times due to metal fatigue as well. The stock ladder line is 18 gauge solid and is very prone to breaking due to wind constantly bending it. The second one I put up I replaced the stock ladder line with some existing 14 gauge stranded 450 ohm ladder line I had from a old 135 foot dipole. I found that 84.5 feet provided an acceptable match for my Kenwood TS-590S autotuner on all bands 160-6 meters. That said, I have found that the antenna excels on 75/80 and 40 meters. Is good on 30 but not as hot on the higher bands. Perhaps the angle of radiation is too high. I have mine up about 70 feet at the center in a horizontal configuration.