Description
Which radio is bolted into the vehicle, and will it drive this antenna?
This is one of the rare pieces of equipment where the buying decision is settled almost entirely before you look at any performance figure. The ATAS-120A is not a passive antenna. It is a motorized antenna that changes its own electrical length when the radio sends it a tune signal, which means the radio has to know how to send that signal. Yaesu lists the compatible transceivers explicitly: the FT-857 and FT-857D, the FT-897 and FT-897D, the FT-450, FT-450D and FT-450AT, the FT-991 and FT-991A, the FT-100 and FT-100D, the FT-891 and the FT-710.
If your radio is on that list, the antenna is a straightforward proposition. If it is not, the antenna will physically attach and will not tune itself, which removes the entire reason for buying it. The listing contradicts itself on this point in a way worth naming rather than smoothing over. The description says the antenna is designed for the listed radios and possibly others, and tells you to check your Yaesu owner manual for compatibility, then a few lines later states that it is for listed Yaesu models only. Those two statements cannot both be the operating rule. Treat the explicit list as the rule and verify against your own radio manual.
The second part of the decision is the vehicle. This is a mobile antenna built for automotive use, roughly five feet tall when extended. That has consequences for garages, car washes, low branches and parking structures that people tend to discover the expensive way. Measure your garage door opening before you fit the mount.
The decisive specification is the compatibility list, not the band coverage
Yaesu states coverage from 40 meters through 70 centimeters, which a feature bullet restates as 40 meters through 450 MHz. Those agree, since the 70 centimeter amateur band runs from 420 to 450 MHz. What matters is what is not in that range. There is no coverage of 60, 80 or 160 meters, so if your operating interest sits on the lower HF bands this antenna does not reach them at all. For a mobile installation that is a common and reasonable trade, because a physically short antenna on 80 meters is a very inefficient radiator regardless of how clever the matching is.
Efficiency is the honest caveat on any five foot antenna used on 40 meters. A quarter wave on 40 meters is about 33 feet. At five feet you are operating an electrically short antenna with a loading coil, and the motorized coil is what makes the match possible. A good match is not the same thing as good radiation. The listing publishes no gain figures, no efficiency figures and no radiation pattern data for any band. It publishes only that the antenna tunes. Anyone expecting HF performance close to a full size antenna should set that expectation aside now.
What the motorized section is doing when you press tune
The antenna is a two piece design: a motorized lower coil section topped with a stainless steel whip. Yaesu describes the mechanism as a trombone section that lengthens and shortens to tune the antenna when the radio tune button is pressed. Mechanically this is a driven slug moving inside the coil, changing inductance until the radio sees an acceptable match. It is quick and it is automatic, and it is also a moving mechanical assembly mounted outdoors on a vehicle, exposed to road salt, rain and vibration. That is the part that wears out, and it is the reason to think carefully about where on the vehicle you mount it.
The listing gives physical dimensions of 37.3 by 2.4 by 3.3 inches while the description and a feature bullet both state a maximum extended length of approximately five feet, or 60 inches. Those are not the same antenna unless the 37.3 inch figure describes it retracted or in its carton, and the listing never says which. Plan clearance for 60 inches above the mounting point.
Living with it: the mount, the cable and the parts that are not included
Two items you still have to buy are stated plainly. The antenna needs a suitable SO-239 threaded mobile mount and a coaxial cable, and neither is in the box. The antenna base itself carries a PL-259 style thread, which is the mating half of the SO-239 socket, so the two statements are consistent even though they read at first as though they conflict. Budget for the mount, the cable and, on most vehicles, a proper ground plane connection, because a mobile HF antenna without a solid ground path to the vehicle body will not perform as intended no matter what the tuner reports.
One specification is simply impossible. The item weight is listed as 0.01 ounces. A motorized five foot antenna with a coil, a motor and a stainless whip weighs on the order of two to three pounds. A figure like that usually means the field was never populated correctly at import, and it is a good reminder to treat the automated fields on this listing with suspicion while trusting the manufacturer text.
The licensing point deserves stating plainly. This is transmitting equipment for the amateur radio service. Operating it requires an amateur radio license in the United States and in most other countries, and radio frequency exposure guidelines apply to any transmitting antenna mounted on a vehicle where people sit close to it. That is a compliance matter rather than a preference. Anyone buying this without a license should understand they are buying a receive only accessory until that changes.
When a different antenna makes more sense
If your operating is VHF and UHF only, you are paying for HF capability you will never use, and a simpler dual band antenna will do better on those bands with none of the complexity. The Comet CA-2X4SRNMO mobile antenna review and the Tram 1180 dual band antenna review both cover that route with no moving parts to fail.
If the installation is at a fixed location rather than on a vehicle, a base antenna is the better engineering answer, because height and length are no longer limited by a car roof. The Diamond X30A base antenna review and the Tram 1481 fiberglass antenna review cover fixed installations, and the rest of the range sits in the radio antennas category.
It is worth saying clearly that this product is filed here because the retailer category path places radio antennas under television and video accessories. If you arrived looking for over the air television reception rather than amateur radio, this is not the product you want, and the TV antennas category is where to go instead.
Who this suits and who should look elsewhere
It suits a licensed operator running one of the listed Yaesu transceivers mobile, who wants band changes handled by pressing a button instead of stopping the car to swap resonators. The listing shows 69 ratings averaging 4.6 stars and a rank of 1,206th within radio antennas, and the product has been listed since March 2011, so there is a long track record behind it.
It does not suit anyone running a different brand of radio, anyone whose interest is the lower HF bands, anyone who wants maximum efficiency rather than convenience, or anyone without a license. It is also the wrong product for television reception, despite where it sits in the category tree.
