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NI4L’s 7 Band OCF Dipole

Posted by on June 29, 2014
W2AU Balun in place

NI4L’s 7 band OCF dipole in place

This review of NI4L’s 7 band OCF dipole was prompted by my needing a quick deploy multiband antenna for both field day, and for RVing.  Last year on Field Day I was able to use an OCF dipole for the first time, and I enjoyed the quick bandswitching it allowed, so I thought I’d try and find a good OCF dipole for use while in the field.  After looking around I found NI4L’s site.  Interestingly enough I found it while on eBay, not with Google.  Having looked over his antenna selection I decided on the 7 band OCF version, as that would cover most of my needs while in the field, not need a tuner, and was not too large.  Although I really want a Fan Dipole, (less common mode because the system is inherently balanced up to the coax), for speedy deploy, and the up/down setup/tear down of RVing I decided on the OCF dipole, less wire to play with in getting the antenna up/down.  His web site is very difficult to locate on Google.

NI4L Arrival Package

NI4L Arrival Package

The antenna came in a nice package, which arrived only a few days after ordering it.  I called and placed the order via telephone in order to speed things up a bit.  If you do decide to order an antenna from NI4L, check his eBay ads first, the prices differed a bit from his web site when I checked, and you might be able to save a few bucks.  In any case, what caught my eye was that NI4L is using Flex-Weave cable for the main antenna line, another reason is that NI4L is also using an off the shelf W2AU Balun, and UV protected sheathing for the Flew-Weave.  I have had good luck with the W2AU balun so I decided to purchase this antenna.   It arrived a few days later in the package you see on the left and above.

 

The Unboxing:

The Unboxing

The Unboxing

Prior to unbagging this antenna, my first thought was it would be a tangled mess when I slid it out of the bag.  It was not..  I was pleasantly surprised to see that the antenna wire was plastic covered as well as being Flex-weave!  Flex-wave is basically coax shield turned into wire, only better.  It is 136 strands of #36 wire meshed like coax shield.  You can tie this stuff in knots, and then untie it and use it.  It refuses to take a kink.  it really is amazing stuff.  The military uses it for field radio, and you just can’t kill it.  I have one of those Embassy radios for EMCOM, and it has a Flex-weave antenna with it, it is perhaps 20 years old, and the antenna is still easy to work with, and has no kinks in it.

The antenna itself:

The wire has a good feel to it.  It is very flexible, and it feels like rubber to the touch.  The covering is UV proof, and as per NI4L’s web site weatherproof, and capable of being run through trees with low abrasion.  So it looks like some thought has gone into this antenna.  You can see from the photo below, it came out of the bag just fine.  I was a bit worried it would explode out of the bag, and be a tangled mess.

OCF Dipole opened up

OCF Dipole opened up

After looking at it for a moment, it became obvious I was to just flip over in order to splay out both sides of the antenna, this is good news.  The antenna will be deployed it on a military style extendo pole for field day.

The end insulators are that white plastic like nylon dogbone type.  Maybe someone will leave a comment on what they are made of.  I looked and was not able to find out what they were made of, although I know I have seen that information on the net before.

 

 

The End Crimps:

OCF Dipole end insulator

OCF Dipole end insulator

My next area for inspection was how were the wires secured to each other, and to the rest of the antenna.  Crimp on connections were used, as opposed to solder on, for most of the connections.  Contrary to the ARRL Antenna book, and many others, (the Air Force among them), it is believed that a connection in an environment which is both corrosive, and subject to vibration, a crimp is better, than soldering.  The thought process is as one heats the wire, the solder wicks up the wire, causing a very sharp interface between the non-soldered wire, and soldered wire, allowing for a flex break caused by metal fatigue as the antenna blows in the wind.  I flexed the antenna cable right at the crimp point and felt no evidence of metal fatigue starting.  The amount of “roughing up” I did would have broken a soldered joint.  Also, note that the crimps are only used where mechanical strength is needed.  Later you will see where some solder was used, and it was used in the correct place, where electrical connections were needed.

NI4L's OCF Crimp

NI4L’s OCF Crimp

Sometimes vendors won’t crimp things tightly enough, and you can get a wire creeping out of a crimp.  Look at the crimp on the left, (left click the photo for a larger view), I doubt that anything will be slipping out of that.  The metal used to make the crimp, is pressed tightly enough, but not too tightly as to weaken the wire.  At first you might think this is bad, because it will not allow you to tune the dipole…  Remember this antenna is an Off Center Fed, (OCF), dipole, and if you tune it for one band, you detune it for another band.  An OCF Dipole is a very finicky item to tune, so it is best you do not change things.  All the crimps on this antenna look like this one, I saw not one shallow crimp.  I believe this is a very good sign, as it implies a reasonable amount of quality control in place during manufacture of this antenna.  I let the antenna sit outside, not mounted, but on the ground, for the past few months so I could get a look at the crimps, antenna hardware, and antenna cable, after a few months of Oregon rain, snow, and freezing.  The crimps look exactly the same as new…  No corrosion visible at all.  Same for the wire, and the rest of the antenna.

Balun connection

Balun connection

Looking at the connection to the center balun, you can see some care and thought went into this setup…  The crimp is good, and the actual wire is connected to the balun via only one weather proofed soldered connection.  That then loops though the support eyelet, then to the actual connection.  Why the loop?  If the wire were to go directly to the lug, you would create a flex point at a spot where metal fatigue would tear things up in short order, this way, teh loop reduces the amount of flexing placed on the wire at the lug/wire interface.  Again, some serious thought went into this design.  Click the photo to the left, and look carefully at the antenna wire to copper wire interface, hidden under the heat shrink tubing.  Note the slight amount of goo squeezing out from the heat shrink on both ends– weatherproofing.  Look at the lay out and imagine it swinging in the wind, bending metal, and having rain  drop off…  The entire loop assembly also serves as a drip loop to keep the balun from getting all the water into it, at a point where the case is penetrated.  Everything runs away from the balun.  As I mentioned before, some serious thought went into this design.

The Balun:

W2AU Balun Weep holes

W2AU Balun Weep holes

The use of a W2AU Balun was a good idea on NI4L’s part.  The balun is good for this sort of antenna, it helps balance the feed line, of this inherently unbalanced antenna.  If you look at the photo of the balun, you will see the weep hole at the bottom.  This allows condensed water to drain out of the balun, and onto the coax connector, be sure you cover the coax connector with either coax seal, or rescue tape to keep water out of the cable.  Don’t cover the weep holes.  In theory water should run downhill, and not into the connector, however I worry about water touching the connector and wicking into the cable.

The W2AU balun is rated at 2KW PEP, or around 1 KW digital, 1.8 MHz to 40 MHz., at less than 3:1 SWR.  You probably want to only push it to 500 or 600 watts if running digital.  I am sure it will do better, but I tend to down rate everything when it comes to that level of power and 100% duty cycle.  Looking up the ratings of the balun, I noticed it is not rated for six meter operation, and that might be why the SWR is 3:1 on six meters, as opposed to the 2:1 or lower on all other advertised bands.  I assume the designer has tested the antenna on six meters, I did not however, beyond the short SWR test shown later.

Clear View of W2AU

Clear View of W2AU

Remember, if you are using a tuner, you can run the antenna and balun into danger areas…  If for instance you operate on 15, you could, using a tuner, feed a full KW to the balun, running beyond the stated 3:1 ratings.  We were running 100 watts, and had no problems running 15 meters.  Don’t run this antenna at greater than 3:1 SWR, and using higher powers.

Remember your tuner only makes the transmitter think it is seeing a 1:1, the antenna remains mismatched no matter that your tuner says.  I always bypass my tuner and then check SWR.  That tells me what I am working with.

I expect the power and SWR limits, comes from the W2AU balun, after all, the antenna is just wire.  If you look carefully at the top of the balun, in the clear view to the left, you can see that the two screw eyes create a spark gap, Unidella says this will protect against lightening.  Unidella states a 600 pounds pull apart limit for this balun as well.

Test Conditions:

W2AU Balun in place

W2AU Balun in place

The 7 band OCF Dipole antenna was erected at 25 to 30 feet, on a hill side, using a military style push up mast, with a few customization on it.  The short end was placed at about 40 feet off the ground, tied to a tree limb, using about 10 feet of insulated rope.   The long end was tied to a tree, again, using insulated rope, and with maybe five feet of distance between the antenna and the closest limb, at around 35 feet.  This put the antenna overall height at about 35 feet off the ground all along the path it took, which was straight.

The feedline, (RG8), was run up an insulated non metal pole, wire tied to that pole, The extra feed line was coiled up in about a two foot diameter coil consisting of maybe 10 turns and left on the ground at the pole base.  All in all, 50 feet or so of feedline was used.  The W2AU balun was hung from the top of the pole, and the entire setup looked quite professionally done.  The feedline was run into a tent for field day.

SWR Measurements:

SWR of NI4L's 7 Band OCF Dipole Antenna

SWR of NI4L’s 7 Band OCF Dipole Antenna

The SWR measurements were taken at the end of the feedline, not at the W2AU balun feed point.  I apologize for the poor photos of the SWR chart, but I just got my AA-30 analyzer, and discovered it does not save for later review, so I had to take a photo of the screen to get this…  I added the band designations in black along the bottom.  The AA-30 does not cover 6 Meters, so I used my MFJ-259B to check SWR on six, it was 3:1.

As can be seen the antenna does cover the 7 bands called out in the advertisements, all without a tuner, and all under 2:1, with the exception of six meters, which was 3:1.  You can operate this antenna on 15 with a tuner, but it is compromised somewhat.  We also had another OCF dipole up, slightly higher, and I was able to test it.  The only real differences were a slightly better SWR, but that antenna was up another 20 feet so that is to be expected.  All in all, the two antenna’s performed just about the same.  Note the other OCF dipole also did not cover 15 meters, with out a tuner.  This view gives a somewhat deceptive perception of the bandwidth.  The antenna is less than 2:1 over all portions of all bands, and in many cases much better.  The total bandwidth of the sweep on the left is 3.5 to 30 MHz., so you can see how wide it is at the tune points.

Recommendations:

For temporary use, like Field Day, or in an RV, just throw it up.  If you are going to put it up permanently, be sure to weather proof the coax to balun connection.  The design will allow water to run onto the coax, and into the coax if you have not used rescue tape or coax seal on it.  This would apply to ANY antenna, and is not a design flaw of this particular antenna.  I might note that the antenna we tested it against, had a design that actually directed the water away from the coax connection, and on to the jacket, further down the cable, by having a tube the cable connector set in.  However this makes disconnection a real problem…  You need a pair of needle nosed pliers to get to the coax screw ring.  Beyond that the antenna is wonderful, performed well, and I would be happy to use it again next Field Day.

Conclusion:

I would recommend this antenna to anyone I know for a fast deploy, easy to set up and use antenna.  It works well, and we had zero issues with it.  It was raining for part of field day, and the antenna maintained the good SWR readings I found upon install.  It was easy to set up in a hurry, and for use as a fast set up antenna this fills the bill.  We never had to use a tuner, (except for a test on 15), and the antenna continued to perform flawlessly the entire time.  I was a bit worried about common mode, and did detect a touch more RFI noise than I remembered from the previous year, using a FAN dipole, in the same location, as opposed to an OCF dipole.  OCF Dipoles are by nature unbalanced, and hence suffer more noise than a dipole will in the same environment.  If I had limited space, and needed to hide the antenna I would use this antenna.  If has a very small visible footprint, and is easy to set up.  That makes it perfect for this sort of installation.  On the other hand, I have a rather large area, and no issues with antenna visibility, so I will be using a Fan Dipole for my next permanent multi-band antenna.  I may build the fan dipole, but they are so hard to tune, I may just break down and buy one.  However for RVing, and field day this is the antenna I will be using for many years to come.

 

5 Responses to NI4L’s 7 Band OCF Dipole

  1. Wes N7WS

    Nice review. However, I would question the use of a W2AU balun (or any voltage balun for that matter). Iain, VK5ZD, (an excellent analyst) has measured these with not-so-surprising results. See: http://www.vk5zd.com/W2AU/W2AU.aspx

    The W2AU website contains such gems as: “In any event, the matchbox is a tuner and all it does is make the transmitter believe it has an antenna with a lower SWR than it really has. This way it will introduce more power to the antenna than it can typically radiate.”

    I did not know that antennas are limited in the power they can radiate.

  2. Walter

    Wow, found your site from a search about cartridges then located the ham page and a big congrats go out to you. The article on the off center fed dipole is awesome. Good job.
    Walter
    AK4FK

  3. Nelson Farrier

    Nice review. Very thorough to include even the eyelet loops and weep holes. It’s nice to see well thought out products.

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