Batboard

I seem to remember discussion about using a commercially manufactured 5/8 wave high band (2 meter) antenna as a 1/4 wave low band (6 meter) antenna. The math works out. I am looking for information from anyone that may have experimented with this configuration. The 5/8 wave high band antennas are longer physically than the base loaded low band antennas that I've seen, and common sense dictates that full 1/4 wave antenna at any given frequency will perform better in the same mobile installation verses a loaded 1/4 wave antenna resonant on the same frequency.

Your thinking of the Larsen NMO150.

Having been a long time 6 meter geek for the last 40 years or so, I think I have tried about all the antennas that have
come down the food chain. This covers the spread from a 6 meter halo to full size ball and spring mount antennas.
What I have settled on is the Larson W540 whip on their NMOQ base. This base is nothing more than a mount for
the whip and a spring finger contact to the NMO base on the vehicle roof. There has been some model number
changes over the course of time. The Larson catalog has this thing they labeled NMOQ52C if my memory is
still with it. This gives you both the W540 whip and the base mount for the whip.

I have never had to prune the whip to make it work with a low reflected power. Just install it and go. Make
sure you solder the coax shield to the PL259 connector shell. I have seen many installations where who
ever made up the coax connector, just jammed the shield into the connector and walked away. This does
not provide a very good ground to the coax and causes all sorts of issues. I always have taken a file and
cleaned off the nickle plating on the reducer. Then tinned it. Then fold the shield over the reducer and
put the coax into the connector. Solder the center pin and solder the shield down. It will take a good sized
iron with some heat to melt the solder. You don't want to apply the heat for a long time. This will cause the
center conductor insulation to melt and short out the coax.

Hope this provides a little insight to the antenna selection.

Jim

W2MB wrote:I seem to remember discussion about using a commercially manufactured 5/8 wave high band (2 meter) antenna as a 1/4 wave low band (6 meter) antenna. The math works out. I am looking for information from anyone that may have experimented with this configuration. The 5/8 wave high band antennas are longer physically than the base loaded low band antennas that I've seen, and common sense dictates that full 1/4 wave antenna at any given frequency will perform better in the same mobile installation verses a loaded 1/4 wave antenna resonant on the same frequency.

I'm fond of that antenna too. Works very well without cutting across a number of bands with acceptable return even at higher freqs (though it does become more critical). Excellent general purpose antenna. I also agree whole heartedly with Jim about ensuring good ground at the connector especially at lower freqs where insufficient ground plane exists on the vehicle. You will get all kinds of stray energy floating around making noise for you if you don't.

I used the Larsen NMO50 base-load paired with a Syntor X9000. It worked great for TX on 6 and surprisingly well for RX across the entire 29-54 VHF-Lo range.

Whatever you end up using, run ground braid to your hood and trunk lid. One run will help, but two on each corner at the hinges is even better.


edit: Some of the old heads will disagree with me on this, but I use and love crimp PL259s. RF Industries RFU-505-ST.

I'm currently using a hood mounted Larsen NMO150 with the whip untrimmed for the 42-50 MHz Maratrac I have mounted in my truck. Frequency range is all within 42 MHz. The mount is the Larsen 3/4" NMO for high freq with the Larsen RG58 with the foil cut to about 12 feet. The connector is soldered properly and the hood panel is 1 inch braid grounded to the firewall at the hinge bolts.

Now then, so far, all is well and stable with this combo on my truck but not so on another. When doing the install, I momentarily couldn't find where I had put the NMO40 coil that came in the package, so-o-o-o- I tried the NMO150 coil just for sh?ts and grins and it worked, so it's still in place, but on another vehicle the same combo is a complete disaster. Since low band VHF can be so finicky about so many factors, I was pleasantly surprised that at operating freqs on my Ranger pickup, the SWR is between 1.4 and 1.8 and the impedance reading shows between 40 and 60, so I ain't trimmin' nuttin', honey. On the other vehicle, the antenna placement is slightly different due to brand differences as well as some body panel grounding, etc, etc, and the SWR just will not come below 2.8.

I just got lucky and pretty likely your mileage may vary, but how will you know without trying?

I just messed with this last week. I have a NOS LM150 (!!!!) on the roof of my E250. Should be the same electrically as the NLA or NMO coils. Center tuned for 147.0. Swept it with my Anritsu SiteMaster and found the SWR at 53 MHz dipped to just under 2:1. I don't know if moving the frequency down the band on hi-band would affect either frequency response or minimum SWR on low-band at 53Mhz.

Dave

Slight thread swerve - NIST is testing metamaterial antennas that are a fraction of a wavelength.

http://www.nist.gov/eeel/electromagneti ... 012610.cfm

Thank you all for your insight!