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Author Topic: 11m Tuner for 2m Antenna  (Read 5098 times)
KJ4PKO
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Posts: 53




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« on: May 06, 2013, 03:50:50 PM »

I know the title sounds strange, so I'll explain. 

I am about to install a VHF radio and a CB in a disaster relief trailer.  I want the setup to be easy to use, and I want to keep external antennas to a minimum due to certain restrictions.  I would like to use a Hustler SF-2 antenna for both radios. I know it will be very inefficient on 11m, but I just want something for communication with non hams during a deployment.

I want to build a permanent value tuner for the CB.  How should I go about this this? 

Thanks,
Sam
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WB6BYU
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Posts: 12974




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« Reply #1 on: May 06, 2013, 09:57:02 PM »

OK, that is a 5/8 wave whip with a series loading coil at the base.

First you need a diplexor to keep the 11m RF out of the 2m receiver and vice
versa
.  With that much separation you might get by with just a series
coil on the 11m side and a capacitor on the 2m side, but a better approach would
be to use series capacitor / shunt inductor / series capacitor on the 2m side and
series coil / shunt capacitor / series coil on the 11m side.  Specific values would
be chosen for a loaded Q of perhaps 2 to 5 on each band - we can figure that when
we get down to details.

You'll need an additional loading coil for 11m, probably around 2uH, likely with one
end grounded and the 11m coax tapped onto it to get a match.  This would go
between the 11m port on the diplexor and the 11m radio, preferably as close to
the antenna as possible.  Actually since the L-C-L circuit for the diplexor can also
work as an antenna tuner we might be able to chose those components accordingly,
but I think a better approach would be to add an extra variable coil and capacitor
to provide adjustable matching at that point.  A screwdriver adjust mica compression
trimmer capacitor should work.

SWR bandwidth is likely to be narrow - perhaps 400kHz at 2 : 1 SWR, depending
on the details of the circuit.  As long as the circuit can be installed RIGHT AT the
antenna mount it might actually be almost as efficient as a base-loaded CB antenna
of the same overall height.
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K1CJS
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Posts: 5807




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« Reply #2 on: May 07, 2013, 09:20:19 AM »

You would be better off making a wire dipole for the CB.  Something that you could toss up into a tree or string between the trailer and a near by support.  The range you'll get trying to use it on a 2 meter antenna will be nil--that is, nobody beyond a few hundred feet will hear you.  Been there, done that.
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K8AXW
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Posts: 3592




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« Reply #3 on: May 07, 2013, 09:39:39 AM »

My opinion:  Trying to do 11m and 2m on the same antenna is ridiculous!  The gear, limitations and procedure for using this gear makes it ridiculous.

I've also found from experience than 11m doesn't like dipole antennas, unless they're strung vertically.  To use a horizontally polarized antenna like a dipole for a primarily vertical polarized service is very inefficient.

If you do try the dipole, string it as vertical as you can.  Opt for separate antennas.  Hams are noted for their ingenuity when it comes to adapting to complicated situations.

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RC286
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Posts: 3




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« Reply #4 on: May 07, 2013, 11:32:07 AM »

I agree that trying to match an 11m rig to a 2m antenna for diplex use would probably end up being a headache.
If you are able to get a match, your usible bandwidth would be limited to a couple channels or so before requiring adjustment
to the tuning.

If space and quick setup are are key features. (which I assume they are) A 5/8wave 4 foot loaded fiberglass whip would be a choice. There are also fold over mounts that can be had from ebay for about $12. They threat onto the stud and allow the antenna to be released and folded for storage. Add one of those wall clamps used to hold mops and brooms to hold the antenna securly in its folded position for storage or transport of the trailer.
This will also eliminate the need for a nearby supporting struction for a dipole if the trailer is deployed on a large open parkinglot or field. Would take litterally seconds to deploy the antenna.


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KJ4PKO
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Posts: 53




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« Reply #5 on: May 08, 2013, 09:53:07 PM »

Thanks for all the replies.  I know it's a less than desirable setup, but I think it will be OK for what we will be doing.  The VHF radio will be a MURS radio.  Most communication will take place on MURS (as most are non hams), the CB is just for communicating with heavy equipment operators, truck drivers, general public, etc.

WB6BYU:  I like the idea of combining the tuner and duplexer circuits.  Would the tuner in series with the VHF radio effect its SWR?

I have never built anything like this, how do I go about designing the circuit?  How do I figure the needed component values?

Sorry, I don't know much, but I want to learn.

Thanks,
Sam
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WB6BYU
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Posts: 12974




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« Reply #6 on: May 09, 2013, 11:44:36 AM »

Quote from: KJ4PKO

 Would the tuner in series with the VHF radio effect its SWR?


It doesn't have to.  The circuit can be designed to match 50 ohms to 50 ohms with
a range of Q values.  It can also be made somewhat adjustable in case the antenna
doesn't present a perfect 50 match in the first place.


Quote

...how do I go about designing the circuit?...


I know there are a number of filter design programs around.  I'll have to ask some
of the other hams here at work what they would recommend.  You can do the
matching by hand on a Smith Chart, and formulas for the common L and pi networks
used to be in the Handbook.  I'm sure some of the other posters on this board can
make recommendations as well.

But for someplace to start, try W9CF's tuner simulator applet available online here:
http://fermi.la.asu.edu/w9cf/tuner/tuner.html

This implements a standard T-network antenna tuner, which is the same circuit you
would use for VHF side of the circuit.  (If you go into the setup window and set the
maximum inductance to 3uH it makes it easier to gauge the small values from the
positions of the knobs, since the readout is only to the nearest 0.1uH.)

For a circuit that does NOT transform the impedance, expect both capacitors to
have the same value.  A standard value such as 10pf or 27pf should work.  (Smaller
values will increase attenuation of the 11m signal, but may have a little more loss.)
In this case, using 10pf for both capacitors, we get minimum SWR at about 0.065uH.
In practice you'd use a pair of fixed capacitors and an air core coil, then spread or
squeeze the coil turns slightly to get minimum SWR with a 50 ohm load on the output
of the network.  (If you need it adjustable, then replace one of the capacitors with
a trimmer, and adjust both that and the coil for minimum SWR.)

The 11m side is a bit more difficult because it uses series coils and a shunt capacitor,
but you can use the same tool just by reversing the sign of the reactance for each
component.  So if the circuit calls for a coil of 1.1uH, for example, you'd convert that
to reactance (~200 ohms) and instead use a capacitor with -200 ohms reactance (which
should be about 33pf if you want to check your math.)  You'd also need to change the
sign of the input reactance. 

Seems to me that the modeled load impedance was about 4-j500 ohms on 11m, so if
you set the frequency to 27MHz and enter 4+j500 as the load impedance in W9CF's
tuner simulator you can see the effect of various component values.  (The Autotune
button comes in handy to get you close, then you can experiment with standard fixed
values for one or both capacitors.)  In this case one of the capacitors is at maximum,
which is a clue that a simple "L" network will probably work as well for this impedance.

Personally I'd probably choose to transform the impedance in two steps, perhaps from
5 ohms up to 15 or 20 ohms (depending on what gives a convenient capacitor value)
and then a second stage from that to 50 ohms.  With two "L" networks, this is the
same as the "Pi-L" output circuit used in some tube transmitter output stages, and
you should be able to find the equations for that, though any design charts probably
won't go down to 5 ohms.   But if you consider each stage separately then it isn't
difficult to do the math by hand.

That won't tell you everything you need to know, but might get you started in some
interesting directions at any rate.
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KA2ODP
Member

Posts: 35




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« Reply #7 on: May 09, 2013, 08:33:55 PM »

Hi Sam,

Might I suggest a different approach?  The Hustler SF-2 antenna uses a 3/8 inch X 24 threaded stud mount.  While a common antenna mount design, especially with CB antennas, there is an alternative.

The industry standard for business band 2-way radio antenna installations is the Motorola NMO mount.  It is a low profile mount that installs in a 3/4 inch hole drilled in the roof of a vehicle, or to a simple "L" bracket that can be attached to the inside edge of a vehicle fender in the gap between the fender and the hood.  In your case, the "L" bracket could be attached to the side of the trailer near the roof line if you do not want to mount it in the thin sheet metal roof of the trailer.

For a VHF antenna I would recommend the Larsen NMO-150.  It can be trimmed for any frequency within the frequency range of 144-174 MHz.  I believe the Hustler SF-2 is limited to 144-148 MHz.  With the MURS channels being in the 151 and 154 MHz range, you could tune the Larsen NMO-150 antenna for a far better match than an antenna designed for frequencies almost 10 MHz away.  It is available with a chrome whip (NMO-150C) or a black whip (NMO-150B).  I don’t think the Hustler SF-2 antenna will perform very well so far from its intended frequency range.

For a CB antenna I would recommend the Larsen NMO-27.  It can be trimmed for any frequency within the range of 27-31 MHz.  It also screws on to the Motorola NMO mount.  While not as fancy as many of the CB antennas sold at truck stops, it gets the job done.  Tuned for 27.185 MHz, it will be centered on CB Channel 19, which is the popular channel used by truckers.  It is available with a chrome whip (NMO-27C) or a black whip (NMO-27B).

I just cannot see trying to use one 2-meter antenna for two different frequency ranges so far outside it’s resonate frequency.  It will not provide a good match on either frequency range, and the SWR will be fairly high.  The high SWR values might end up damaging both the MURS radio and the CB radio.  Plus there is the problem of having two transmitters connected to just one antenna, and the extra equipment needed to keep them isolated from one another.  At that point you might as well just install 50-ohm dummy loads on the output of each radio and call it good.  All of the time and money spent to “adapt” the Hustler 2-meter antenna for MURS/CB operation would be better applied to installing two separate antennas that are resonant for each desired band.

Both the Larsen NMO-150 and the NMO-27 are base loaded antennas.  You simply thread them on the NMO mount and hand-tighten.  There is no need for a wrench, as with the 3/8 inch X 24 threaded stud mount.  You can leave the antennas off until needed, keeping them stored inside the trailer.  A NMO rain cap can be installed to protect the NMO mount while the antenna is removed.

If you are interested, check out the pictures of the Larsen antennas at the Amateur Electronic Supply (AES) web site or at the Ham Radio Outlet (HRO) web site.  The NMO mount with 17 feet of RG-58A/U coaxial cable is Part Number (P/N): NMO-KHF-CX-PL.  The simple “L” bracket is P/N: TMB34 (note that the picture is upside down in the catalog).  The NMO rain cap is P/N: NMO-CAPB.  These antennas, properly tuned, will provide far better results than the single Hustler SF-2.
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KE3WD
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Posts: 5694




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« Reply #8 on: May 10, 2013, 05:02:49 AM »

The OP states right off the bat that this installation is to be used for disaster relief work. 

In that situation, separate antennas is not only the better path for reasons of efficiency of antennas and ease of use, it also would create a much better comm situation. 

For example, while switched to 11 meters and working poorly at it anyway, any calls on the 2 meter band would be missed. 

Couple that with the poor performance that the 2 meter antenna system is going to have on 11 meters and the whole thing just doesn't make sense at all. 

Not when a separate length of coax attached to a vertical antenna made for the CB band can solve the whole problem nicely. 

Couple that with the OP's admitted lack of knowledge on the subject, the wish to design something that the OP can't design and truth be told, the best of us might get something to make an impedance match appear good, but -- that does not mean the antenna system, 2 meters, is likely to work well enough to bother with -- and consider the amount of CONFUSION such a system would create for Disaster Relief Operators when every second counts makes this whole idea something in need of serious RETHINK.


73
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KJ4PKO
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Posts: 53




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« Reply #9 on: May 10, 2013, 05:02:14 PM »

I just came across this.  Do y'all think this might work for what I am doing?  It comes with a "splitter" box.

http://www.ebay.com/itm/Shakespeare-5204-8-Sea-Scout-VHF-CB-6dB-Unity-Closeout-/200750805438

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WB6BYU
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Posts: 12974




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« Reply #10 on: May 10, 2013, 05:49:43 PM »

That should work just fine if you have room to mount it.  Saves a lot of work, too,
and probably works better than what you would have ended up with.
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QRP4U2
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Posts: 106




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« Reply #11 on: May 10, 2013, 07:47:11 PM »

Quote
It's an excellent choice in situations when the number of on-board antennas must be limited and absolute top performance on both bands is not mandatory.

Looking at the language, I would say it is a compromise at best and probably more tuned to CB.
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KE3WD
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Posts: 5694




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« Reply #12 on: May 11, 2013, 06:40:42 AM »

Um, that Shakespeare is called the "Sea Scout" antenna for a reason. 

The "VHF" band there is the Marine Band VHF and it is not likely the thing was designed to work well in the 2 meter Amateur Band. 

Also note that the antenna is a "Close Out" and seriously ask yourself WHY. 

A few feet of coax, a mast, and an 11 meter stick, permanently attached to the 11 meter radio. 

Again, I say that this would be best for the type of important communications, "disaster relief" or EMCOMM. 

Multiple Operators. 

Different skill levels. 

The need for the best coverage you can obtain for each band.

Trying to feed a single antenna from the two different radios is a recipe for disaster in and of itself.  One mistake can take out the finals on either or both radios and then what do you have? 

The single antenna notion here is a very bad idea. 

You've been told.

Twice.


73
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KX8N
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Posts: 543




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« Reply #13 on: May 12, 2013, 12:11:53 PM »


Trying to feed a single antenna from the two different radios is a recipe for disaster in and of itself.  One mistake can take out the finals on either or both radios and then what do you have? 

The single antenna notion here is a very bad idea. 

You've been told.

Twice.


And now three times. There's a reason you can't just stick a repeater on the air for a hundred dollars. You need ridiculous amounts of ridiculously expensive separation between input and output, and you would need that kind of separation between the CB and the 2 meter radios. With the kind of range you would get operating CB with the 2M antenna, you'd probably be better off just standing outside and yelling at who you are trying to communicate with.

Use the 2M antenna you're looking at, and get a magmount 11M antenna. It would cost almost nothing, it would be a temporary installation, and you could mount it on ANYTHING metal that is nearby if putting it on the trailer is a problem. But considering how small a magnet it would take, the extra space would be negligible. Plus everything would be tuned up and efficient. Remember, that CB is only a few watts to begin with.
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KA2ODP
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« Reply #14 on: May 17, 2013, 06:50:19 PM »

Just to clarify a few points:

-There is NO 2-meter radio involved in this project.  But he intends to use a 2-meter antenna anyway.

-The only two radios involved will be a MURS (151/154 MHz) and a CB (27 MHz).

-There will be no missed calls while the antenna is switched to one of the radios, because there is NO switch.  Both transceivers will be connected to the same 2-meter antenna at the same time.  He intends to transmit and receive simultaneously with both the MURS and CB radios connected to the same 2-meter antenna.

-The Shakespeare "Sea Scout" antenna is designed for the VHF Marine band (156/157 MHz) and CB (27 MHz).  While this antenna at least matches the operating frequency of the CB radio, it is still not a match for the MURS frequencies (151/154 MHz).  But it is closer than the 2-meter antenna (144-148 MHz) that was the orginal antenna selected for this project.

-Separate resonant antennas for MURS and CB, installed at opposite ends of the trailer roof, remain the best practical solution for performance and separation.  However, this is NOT what he is interested in doing.

-Since most cargo trailers and RV trailers have aluminum skins, the magnetic mount antenna idea for CB might not work.  There are no details on the specific"disaster relief trailer" being used.  But even if the magnetic-mount CB antenna is just set on top of the parked trailer, it would still outperform a 2-meter antenna being used for CB.  It would just have to be removed before moving the trailer.  But again, he is NOT interested in this approach, either.
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