Mobile anyennas for six and two meter bands

Lenny-

I would get the matching LDG auto tuner. It would work on HF through 6 Meters. That way you could cut the 6 Meter antenna for mid-band and let the tuner match other frequencies.

This assumes the antenna needs tuning. I would check it to see how far off it is at band edges when tuned to 52 MHz. If you are lucky, the FT-867 can tolerate the extremes.

Fred

"Insufficient data." NMO is a connector type, and can be used to support all sorts of different monopole antenna types. How sharply these antennas tune, will depend a lot on just what sort of design they have... some are simple quarter-wave whips, some are shortened antennas with inductive loading (sometimes a lumped inductance right at the base, sometimes a spiral-wound distributed inductor).

As a very coarse rule of thumb, inductively-loaded monopoles will probably tune more sharply than a full-length whip... but a _really_ short whip with a lot of loading is likely to be quite lossy, and these losses may mask the sharpness of the tuning.

The other rule of thumb is "When you need to tune or trim an antenna, you probably want to trim it for the frequencies that you're going to use most."

So, the only real answer to your question is probably "try it and see."

Most two-meter mobile antennas are broadbanded enough that they can cover most or all of the band with an acceptable SWR (some may not tune well down at the low end of the band, which is where SSB and CW are done). You may not need to be concerned about this - put a commercial 2-meter NMO-mount antenna up on the roof and be happy.

For 6 meters... FM tends to concentrate at the top end of the band, and by convention this is vertically polarized. Some magic-banders prefer to have a second antenna which is tuned to the lower end of the band and is horizontally polarized, for CW and SSB operations in those frequencies.

Some antennas for the lower bands have a fixed lower section, and an adjustable upper whip... you can change the tuning "in the field" in a minute or two, and can pre-mark the whip with tuning marks for various frequencies. This might be a nice, inexpensive compromise for 6-meter operation.

Or (as somebody else suggested) install a remote antenna tuner in your vehicle, or a manualy-tuned matching box (might be as simple as a two-way switch with a couple of LC components, to select the upper and lower parts of the band), or go whole-hog and install a small screwdriver antenna for HF.

te police vehicle and so holes were already cut in both the middle of the t runk and the middle of the roof. I have a bunch of NMO type antenna bases a round here that cover these frequencies and I would like to use these anten nas for this. I have used these antennas before but only for one specific f requency. It has been my experience that these antennas tune rather sharply . Since both of these bands are 4 MHZ wide what frequency should I cut the antennas for? Center frequency would be easy but wouldn't an antenna cut fo r 52 MHZ have very poor SWR when used at 50 MHZ? The rig has both low and h igh band outputs. I'm not sure what I should do. Thanks for any input on th is. Lenny

Interesting that you mentioned Dave that some of these base loaded antennas have the bases inductively coupled to the whip. I have had to replace a fe w of these bases for some of my commercial customers who's antennas got sma cked by a tree branch or some other stationary object. So I had a chance to autopsy the broken bases. It looked like a coil of number 10 or 12 wire. A nd these did tune very sharply. This may not be a good idea. The 857 is kno wn for blowing finals so an intentional mismatch would be fool hardy. Since I hang out with the military guys at the ham fests maybe as you mentioned I'll tune the 6 meter antenna for 51 and mark the whip for other freqs. The two meter I don't know.. The car is in the shop now so I can't play around with it but when it comes back I'll do a bit of experimenting.

The other thing is and I hear you about the antenna tuner but doesn't a tu ner match the rig to the tuner rather than the load? And while I can apprec iate protecting the finals but then what does that say for maximum power tr ansfer? Would "you can't have your cake and eat it" be a good analogy here? Lenny

It's a dessert topping _and_ a foot ointment!

An "antenna tuner" or "transmatch" is an impedance transformer. It works both in both directions.

In principle, you can place a suitably-tuned matching network anywhere between the rig, and the load, and then adjust it so that the rig "sees" the nice easy 50-ohm-resistive, no-reactance load that it's designed to work into. Your transmitter will then deliver its intended output power without burning up... it'll "think" it's working into an ideal load, because that's what it sees.

Now, depending on where the matching network is (how much transmission line is between it, and the antenna), you'll need to adjust the matching differently in order to achieve this goal. As you move the matching network away from the antenna, the impedance that you "see" at that point will be transformed by the transmission line (you can calculate this using a Smith chart). You'll need a different setting on the matcher to transform this impedance into a friendly 50-ohm resistive value.

Antenna tuners typically have a limited matching range. Some loads impedances are very difficult to match - they're very high-Z, or very low-Z, or extremely reactive. Matching them may require "difficult" values of inductance or capacitance in the matching network. Or, it may result in such high circulating currents in the matching network that you waste a lot of your transmitter power in losses in the network, or may result in such high voltages that the components "arc over".

So, it really depends a lot on your setup.

Putting the matching network near the antenna (rather than near the rig) does have some advantages in terms of power efficiency. With this arrangement, the whole run of coax from the rig to the antenna is "seeing" a 50-ohm impedance when it looks towards the antenna - the SWR on the coax itself is 1:1 or close to it. If the tuner is near the rig, then even if you tune it so that the rig sees 1:1, you've still got a high SWR on the coax all the way out to the antenna... and this can result in high currents at low-impedance points (losses) and high voltages at high-impedance points (the coax itself could arc over at high power).

For mobile antennas, a fairly common approach is to add a switchable "base loading coil" very close to the base of the antenna... a coil or toroid with multiple taps and a switch is often used. In the frequencies around the one the antenna is tuned for, the _resistive_ part of the antenna impedance often doesn't change very much, but the _reactive_ part changes rapidly on either side of resonance. If you tune the antenna so that the reactance is near zero at the high end of the band, and becomes capacitive as you move down the band to lower frequencies, you can use a switchable inductor (at the antenna base) to cancel out this capacitance and restore a (near-)resonant condition. I gather that a lot of people have done this and been happy with the results.