    
Tech833
Moderator
Username: Tech833
Post Number: 1810
Registered: 8-2002
| | Posted on Friday, January 29, 2010 - 3:54 pm: | 
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K40 BCMAX antenna review
‘Trucker’ style antennas are those having a 3/8 inch, 24 thread stud and typically fall into one of two categories- Helical fiberglass, like the Firestik, and base or center loaded metal. Fiberglass antennas offer flexibility and vibration resistance, as well as being inexpensive. They lack power handling capability and bandwidth (for those who want to travel outside the CB band).
Most of the non-fiberglass antennas designed to be used on trucks have strength and durability in mind, as well as some kind of power handling capability. The K40 BCMAX takes strength, durability, and power handling to a whole new level.
Some people believe the K40 BCMAX is a Monkey Made MM9 knock-off. I compared both antennas next to each other, and they are certainly not as similar as you might think. It might be more accurate to call the K40 BCMAX a Viper knock-off than a Monkey Made copy.
The K40 BCMAX is made from very large, robust materials. The lower parts of the antenna are stiff aluminum. The upper part of the antenna is a large, possibly chrome plated brass casting that holds a very thick, tapered stainless steel whip with a K40 exclusive radiused tip. The oval-shaped aluminum coil material is welded to the antenna lower and upper shafts and not wound around a form, but exposed on all sides in air to help with cooling during high power use.
I experimented with the K40 BCMAX on a few different vehicles to measure the bandwidth. I had no trouble at all tuning the K40 BCMAX for the 10m amateur band so I could legally use up to 1500 watts of power for some real-life on air testing.
In all of the installations, a low SWR was quick and easy to obtain. The K40 BCMAX is not picky about coax cable length like some other ‘trucker’ style ungrounded type antennas. A ‘flat’ 1:1 SWR in the center of the CB band would give a flat SWR on the outer edges in every installation I tried. The K40 BCMAX has a very low Q coil, which increases the overall bandwidth of the antenna compared to the popular helical wound fiberglass antennas. The less than 2:1 SWR curve in one sweep was just under 3 MHz. wide.
When at highway speeds, the K40 BCMAX does not bend at all until your speed exceeds 55 MPH. Even at 65, the very tip of the antenna was only deflected a few degrees from vertical. The antenna looks exactly the same sitting still as it does at highway speed! Keep in mind, if the K40 BCMAX strikes an object like a low tree branch, it WILL break something, probably your mount! I put a 4-inch heavy duty spring under the K40 BCMAX and tested again, but the heaviest duty springs available still aren’t stiff enough to keep the K40 BCMAX vertical above 45 MPH. At 70 MPH, the K40 BCMAX will be almost completely horizontal. This is a BIG antenna! But, the best part is, the antenna is quiet at highway speeds- no whistling.
On-air testing showed tremendous performance improvements in both transmit and receive over a short Firestick. The performance boost is more noticeable when tuning into the 10m amateur band. The K40 BCMAX does not require retuning for receive into 10m, but the Firestik gets really deaf outside the CB band and was almost useless on 10m, even for receiving only. In band, signals were typically 2 s-units better on transmit and 3 s-units better on receive. On the range, gain was measured at -2.9 dB compared to a ½ wave dipole antenna. This is good considering the Firestik I tested measures -5.1 dB (that’s negative 5.1 dB)! But the performance difference is also because the K40 BCMAX has much better overall efficiency than a helical fiberglass antenna.
For those with super-power in the cab, the K40 BCMAX will handle what you can dish out. Running full legal limit on the 10m amateur band of 1500 watts, I was not able to raise the temperature of the antenna in any measurable way. I am sure the K40 BCMAX could handle several thousands more watts than I was able to test it with.
If you still want to add a 4” heavy duty spring to the K40 BCMAX, you need to shorten the whip by about 1-3/8 inch, in my tests. There is plenty of travel in the whip mounting stud, so you can make this adjustment easily, and repeatedly. As a comparison, the Monkey Made MM9 is not tunable by sliding the whip up or down.
The fit and finish of the K40 BCMAX antenna is very good. The welds are not perfect, they are a little globby, but still better than I could do. The 3/8 – 24 threads at the base are a stainless grub screw threaded into the base piece, which is threaded into the aluminum lower shaft. All threads are well cut, and all parts fit flawlessly. The stainless whip is held by two huge socket head screws. There is no way this whip will come loose! The whip is close to ¼ inch thick at the base, and about half-way up begins to taper down to its radiused tip. The round tip eliminates the need for a static ball, which could easily be damaged. The brushed aluminum finish of the lower and upper shaft and coil easily accepts paint if you wanted to coat your K40 BCMAX.
I have always been a K40 fan, and was afraid of what might happen to the company when they sold it a few years ago. It was also unclear if K40 would be putting out any new products under the new ownership. But, the K40 BCMAX is indeed a new product, and carries the K40 guarantee and continues the fine K40 tradition in a very high performance, high quality antenna. And, the K40 BCMAX gets a LOT of stares from passersby.
-Tech 833
Your radio 'Mythbuster' since 1998
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