Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Mobile anyennas for six and two meter bands

I plan on installing my Yaesau FT 867 in my car. The car was a former state police vehicle and so holes were already cut in both the middle of the trunk and the middle of the roof. I have a bunch of NMO type antenna bases around here that cover these frequencies and I would like to use these antennas for this. I have used these antennas before but only for one specific frequency. 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 for 52 MHZ have very poor SWR when used at 50 MHZ? The rig has both low and high band outputs. I'm not sure what I should do. Thanks for any input on this.. Lenny
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Default Mobile anyennas for six and two meter bands

I plan on installing my Yaesau FT 867 in my car. The car was a former state police vehicle and so holes were already cut in both the middle of the trunk and the middle of the roof. I have a bunch of
NMO type antenna bases around here that cover these frequencies and I would like to use these antennas for this. I have used these antennas before but only for one specific frequency. 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
for 52 MHZ have very poor SWR when used at 50 MHZ? The rig has both low and high band outputs. I'm not sure what I should do. Thanks for any input on this. Lenny


"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.



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Default Mobile anyennas for six and two meter bands

On Tuesday, September 18, 2018 at 7:07:49 AM UTC-4, wrote:
I plan on installing my Yaesau FT 867 in my car. The car was a former state police vehicle and so holes were already cut in both the middle of the trunk and the middle of the roof. I have a bunch of NMO type antenna bases around here that cover these frequencies and I would like to use these antennas for this. I have used these antennas before but only for one specific frequency. 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 for 52 MHZ have very poor SWR when used at 50 MHZ? The rig has both low and high band outputs. I'm not sure what I should do. Thanks for any input on this. 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 few of these bases for some of my commercial customers who's antennas got smacked 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. And these did tune very sharply. This may not be a good idea. The 857 is known 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 tuner match the rig to the tuner rather than the load? And while I can appreciate protecting the finals but then what does that say for maximum power transfer? Would "you can't have your cake and eat it" be a good analogy here? Lenny
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Default Mobile anyennas for six and two meter bands


In article ,
wrote:

The other thing is and I hear you about the antenna tuner but doesn't a tuner match the rig to the tuner rather than the load? And while I can appreciate protecting the finals but then what does that
say for maximum power transfer? 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.



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