Going beyond simple SWR tests, measurements of antenna patterns were made of the N9TAX single band 2m rollup 2BCX SlimJim style antenna plus other checks for stray currents that tell the real story of antenna performance.

Performance

The N9TAX 2m antenna works pretty darn well yielding a free space H-plane gain a bit over 2 dBi plus or minus the usual small J antenna gain perturbations. This agrees well with models for J style antennas (traditional J, G2BCX SlimJim, whatever). I was suspicious of the ferrite choke working at VHF, but testing confirms it does mitigate current flow back down the feed line – a surprising and very happy finding. The conclusion of VHF performance is the N9TAX 2m antenna is as good as any standalone dipole in free space with inconsequential interactions with the feed line to spoil things. This is the best an antenna of this length can be and N9TAX nails it.

N9TAX VHF SlimJim Gain Plot

The gain of the Slim Jim antenna matches that of a reference dipole in laboratory conditions.

The N9TAX VHF Slim Jim against a reference dipole.

 

The single band model at its 3rd harmonic

If you can deal with a bit higher SWR at the UHF frequency, this single band model will offer some capability at the 3rd harmonic for 440 operation with reduced broadside gain and significant energy both above and below the antenna’s waist. Not wonderful or optimal, but likely better than an HT’s rubber duck antenna.

Construction

Fantastic. The attention to detail is obvious. The heatshink placement is logical, gives a polished look and makes his weather resistant claims 100% believable. The little built-in Velcro strap for holding the bundle together during stowage is functional and brilliant. Yes anyone can build one of these for much less money, but N9TAX’s value added improvements and features justify the price.

Custom tuning available

The N9TAX web site states this single band model can be custom made for frequencies between 136 to 999 MHz. This is no doubt true as N9TAX has obvious experience tuning these models to perfection although we expect geometric realities will be problematic at the higher frequencies. Keep in mind our testing reveals the ferrite stops functioning as a choke somewhere between 160 MHz and 440 MHz therefore expect copious feed line radiation for anything other than VHF use.

Custom cable/connector choice

Near the top of my favorite features of N9TAX antennas are the options available for cable lengths and connectors. My personal favorite is 16’ of RG58 with an SMA that when rolled up with the antenna portion isn’t much larger than my hand when stowed and deploys into a tree for an extremely effective aerial hence turning an HT into a much more capable communicator. This antenna and an HT can fit into the top flap pocket of a backpack making a portable solution that literally “packs” a punch.

Solid conductor limits use

A ladder line with stranded conductors, such as DX Engineering’s DXE-LL450, would be more appropriate for antennas intended for frequent deployment – aka backpackers. I’d pay $10 more for this improvement if given the option.

Value

For VHF this antenna shines and provides great value for the fair price.

SlimJim high gain myth

While N9TAX obviously knows how to mechanically build an antenna exceptionally well, his “6db of gain” claim (note the lack of dBi or dBd or any statement of gain reference or conditions – i.e. freespace) on his sales web site www.2wayelectronix.com (retrieved August 16, 2016) and zero antenna patterns of any kind, simulated or measured is troubling. In his defense, the whole SlimJim 6 dB over something-or-other kerfuffle is a myth that just won’t go away and he is just another victim of it with his parroting of the obviously nonsensical gain claim of what is essentially a form of folded dipole. Quelling that myth on the Internet is like playing whack-a-mole, but vendors such as N9TAX are not helping. If you are selling antennas, you really need to confirm their performance if you are making performance claims (aka promises) in your sales literature.

SlimJim low gain myth

On the other hand, there is a loud and vocal consortium of so called “antenna experts” suggesting the J antenna and its variants can never better a 1/4 wave ground plane antenna despite crystal clear evidence to the contrary… provided there is, of course, containment of the well known and problematic RF currents on the feed line and conductive mounting structure.

N9TAX SlimJim – a classic dipole where you want it

The N9TAX single band antenna has no conductive mounting structure to worry about. The ferrite, for VHF at least, chokes the common mode feed line RF currents. This yields an end-fed vertical dipole with a freespace gain figure of just over 2 dBi wherever you can hang it. Nice!

Scoring

One star off for not vetting or patterning your product. Another star off for making indefinite gain claims in your sales literature. One star back for producing a customizable, affordable and portable antenna that is an example of an exemplary VHF vertical dipole.

About John Huggins

John is an electrical engineer working in astronomy and aerospace including 33 years in antenna/RF design with experience modeling, manufacturing and measuring past, present and new antenna concepts.

Tags: , , ,

Leave a Reply

You must be logged in to post a comment.