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Author Topic: "Enough" Power on 2 Meters?  (Read 2502 times)
KC2MMI
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« on: February 01, 2004, 02:32:12 PM »

I'm newly licensed and of course now starting to look at equipment.<G> I'd like to put a 2m or multiband in my car and I'm wondering just how much power is "enough" to get a 2m radio working reliably in different situations, i.e. out of a masonry/steel parking garage, the typical urban bunker? Or, in an urban situation with large buildings around, what is enough power to reach a couple of miles? Or ten miles?

You get the picture. Without considering repeaters, how much power can be expected to get what kind of ranges in open and urban situations? I'll probably be limited to a magmount antenna, and I know that isn't going to help, but that's what I'll have to start with.

Jared

KC2MMi
WCO3103
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AA4PB
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« Reply #1 on: February 01, 2004, 07:11:52 PM »

You are asking a question that can't be answered with any degree of certainty because there are just too many variables. For example, it's one thing if you are sitting on the top floor of that parking garage and it has openings all around. It's totally different if you are on the bottom floor and it's all enclosed with concrete and steel. It also depends on the terrain, the other buildings around you, plus the location and equipment of the station you are trying to work.

Most 2M mobile radios are either in the 25W range or the 50W range. The difference is only 3dB which won't make a huge difference in communications range.
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K7VO
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« Reply #2 on: February 01, 2004, 08:48:30 PM »

I agree with the previous answer.  You will find that penetration is better on higher bands (as in UHF bands) but that range is better on VHF bands (6m, 2m, 1.25m).

You mentioned a multiband rig.  I highly recommend that.  More bands = more people to talk to.  Where I live the busiest repeaters are on 2m but our club has links back on two 440 repeaters that greatly increase the available range.  Our widest range repeaters in this area are on 222, with one repeater, 224.84, as busy as any 2m repeater in the area.  Another, 224.22, is linked to 2m for an ARES net and is often used for DX spotting.  By having four bands in the car (15m, 2m, 1.25m, 70cm) I can take advantage of all of this.

If you can afford it, make it multiband and multimode, like an Icom IC-706MkIIG or a Yaesu FT-857.  Yes, that includes HF, but either when you upgrade or when the expected rule changes come through you will have access to HF.  In the meanwhile you can enjoy 6m/2m/70cm SSB.  During some band openings polarization doesn't matter like it does for local contacts and you will be surprised how far you work.

FWIW, I run 25W on 2m, 1.25m (222) and 70cm (440) and that seems to be very adequate for most situations.  In a parking garage I have to fold down my antennas which means nothing works Smiley

Good luck, whatever you choose.

73,
Caity
K7VO
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WB2WIK
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« Reply #3 on: February 02, 2004, 04:14:36 PM »

Enough power on two meters is achieved when you can work everything you can hear, and pretty much exactly then.  

For me, that occurs around 70W or so, on FM.  I find most simplex base stations, and also most repeaters, run between 50W and 150W output, and 70W seems a reasonable compromise that results in being able to keep contact with most stations I can hear, most of the time.  I use a Yaesu FT-3000M (70W out, single band, two meter only) rig in my car most of the time.

For mobile work, more important than power is receiver performance, receiver ability to ignore noise (ignition noise, especially, from not only my car but others on the road), and antenna performance.  The difference between a very good and a poor 2m whip can be quite a lot...more than the difference between 25W and 75W, anyway.  My strong preference for "max capability" from the average motor vehicle on 2m FM is the Larsen Kulrod (5/8-wave whip) with NMO-150 base and through-hole mount, in the center of the vehicle roof.  I've tried a lot of stuff, and nothing seems to work better than this.  Same whip on a car trunk lip mount is typically 6 dB down -- sometimes more than that.  Some "lesser" whips, including glass-mount 1/2-wave whips, can be 10 dB down from my center-of-the-roof Larsen 5/8-wave.

For working local repeaters, it hardly matters.  I notice the difference when working car-to-car simplex over long distances.

WB2WIK/6
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AA4PB
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« Reply #4 on: February 02, 2004, 05:44:20 PM »

Same whip on a car trunk lip mount is typically 6 dB down
---------------------------------
Excellent point. Making a proper antenna installation improves the transmit by the same amount as going from 25W to 100W AND you get the same improvement in the receive ability.
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KC2MMI
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« Reply #5 on: February 02, 2004, 10:22:31 PM »

The choice of 5/8 wave I can understand.

The NM150 appears to just be a line/impedance matching coil? And that I think I understand. Or at least, I accept.<G> Got to dust the cobwebs off impedance matching and antennas, IIRC if the antenna can be built to the right impedance without any matching coil, it would be more efficient at radiating, no?

I can't be making a hole in my roof, I'll be using a magmount for better or for worse. But I don't understand why your hole in the roof would give stronger signals than a trunk lip mount, IF that trunk lip mount was properly grounded. I'm not familiar with the new lip mounts, all I remember of the old old ones is that they were all junk which made bad contacts.

From the radiation patterns in the old ARRL Antenna Book (new one on order, there must be something in the 600 additional pages<G>) I would expect the trunk lip position to give you very different results only because it would have a "beam" pattern compared to the unidirectional one form the roof mount. Are you sure you are seeing that many dB down, regardless of the position and direction of the other station?

What's missing from this picture?
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WB2WIK
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« Reply #6 on: February 03, 2004, 02:54:15 PM »

The only length whip that "works" without any matching devices is a quarter-wave, which is around 50 Ohms or so all by itself (with a good, and solidly connected ground plane).  But more gain can be achieved by going to larger aperture antennas, which unfortunately aren't 50 Ohms impedance.  5/8-wavelength was settled on many years ago as probably the perfect compromise between performance and achieving a reasonable match using low-loss components, and has become the de facto standard for VHF whip antennas over the years for good reason.  But you cannot directly feed a 5/8-wave whip, as it's not 50 Ohms, and has capacitive reactance.  A series matching coil adding equivalent inductive reactance is all that's needed to make the whip match 50 Ohm coax very well, and if that coil is large in diameter and made of very low-loss material, it will introduce no measurable loss at all.

This is what Larsen achieves with the NMO-150 coil, as opposed to many others on the market.  The coil is silver-plated copper with very high Q and excellent L:D (length-to-diameter) which helps provide the high Q.  It doesn't really introduce any loss at all.

The reason a through-hole mount works better is simply reduced ground losses: The lower the ground resistance, the more power is radiated by the antenna and not dissipated in the signal return path (ground).  It's really as simple as that.  No mag mount can achieve this, although very good ones can come pretty close.  Ironically, about the best mag-mount base I've ever tested is also from Larsen (it's rectangular and ugly, but has nearly optimum coupling capacitance through its foil base and the car's paint, and works very well on 2m).

The reason the middle of the car roof works better than a trunk lip is because the center of the roof is the highest location on the vehicle, has increased LOS and thus increased OTH range via tropospheric scatter, and isn't blocked by other sheet metal parts of the vehicle -- or other vehicles that surround you.  Unless you're driving a 12' tall car, you'll still be occasionally blocked by other, taller vehicles (mostly trucks, buses, etc), but at least with a whip in the center of the roof, you're not continually blocking your own signal.  And of course the center of the roof provides the most optimum ground plane for the antenna; a trunk lip provides a pretty good ground plane in one direction, but almost no ground plane in the other.

Actual empirical measurements indeed indicate an average 6dB difference in radiated field strength as you drive a car in a circle and measure the field at a point a 100 or so wavelengths away at similar elevation; this has been performed an awful lot of times by many experimenters...probably most notably Motorola, who did repeated studies like this back in the early 1960's and published all their results.

WB2WIK/6
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KC2MMI
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« Reply #7 on: February 04, 2004, 12:17:51 AM »

Steve-
 I found the antenna radiation diagrams I was thinking of, in an old ARRL "FM" book not the old antenna book. Actually the radiation pattern from a front quarter panel mount (i.e. factory am/fm antenna) is pretty close to the roof center mount, in some ways even better. I'll be curious to see if the new antenna book shows the same figures, I ordered through ecampus.com who are usually the cheapest source, but slow.

 What do you think of helical whips? Apparently not many are made commercially, and I suspect that is because winding one is labor intensive compared to a plain whip (or whip with a base coil) which makes them unattractive to the manufacturers. THe usual instructions are to use about 1/2 wave's worth of wire, which produces a helical electrically similar to a 1/4 wave antenna. But, with a whip tip and concentrating some of the turns up towards the top, it becomes slightly more top loaded. I've had excellent results from this in the past (and have NO idea where I got the specs from, I need to dig them back up) and have been wondering about winding a 5/8 wave or longer helical the same way.

 The helicals I've done in the past produced 1:1 SWR and excellent communications, which is a good starting point.<G> I suppose I'd need an MFJ meter to actually check a final antenna to see how closely it was matched to 50ohms and whether that needed work. (Probably cheaper to buy than build.)
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KE4SKY
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« Reply #8 on: February 04, 2004, 05:02:41 PM »

I agree with Steve that a 5/8 wave whip mounted in the center of the vehicle roof, and a 70w rig on FM is going to give you about the best performance on 2 meters independent of repeater infrastructure.

A higher gain antenna, such as a 2m hamstick or Hustler CG-144 collinear 2x5/8 "may" not result in significant practical improvement over the 5/8 wave's 3dB, because its radiation pattern is lower on the horizon and will frequently shift above and below the horizon as the antenna whips in the wind, and its narrower "doughnut" doesn't get out and around terrain and building obstructions as well as the "fatter" pattern of a 1/2 or 5/8 wave with a good ground plane.

However the flexible upper phased element of a 2x5/8 collinear does have some horizontal components to itse radiation pattern at Interstate highway speeds, which can help SSB performance, if you have an all-mode rig. I would definely consider a 2m all-mode if you can find one at least 25 watts.  I have an older Kenwood TM255A, which has one of the most sensitive receivers ever put in an all-mode mobile, a fully-featured modern rig with 100 memory presets and 40w barefoot output. Read the Eham.net reviews of this one!

I used to run one of these in an '87 Cadillac having an M-squared horizontal loop on a 39" trunk mounted mast, so that the plane of the loop was about 1/4 wave above the vehicle roof, paired with a Larsen NMO150, having the two antennas switchable by an A/B switch on the transmission hump.  This was almost heaven,  routine SSB contacts to 100 miles and FM simplex to 30 to 50 miles or so until I sold the car for a 2000 Jeep Cherokee and set it up for HF mobile where I spend most of my time on 40 and 60 meters, but I also have a 2m rig and a 5/8 wave!
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WB2WIK
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« Reply #9 on: February 04, 2004, 06:55:44 PM »

There's no good reason to use a helical whip on VHF or UHF, and there are a few good reasons *not* to.  The advantage of a helically wound whip is to reduce its physical length while maintaining the desired electrical length; however this is always done with some sacrifice in performance, because a half-wave 2m whip that's 12" tall is still a 12" whip, and isn't nearly as efficient a radiator (or receptor) as a 39" whip.  

Another reason it's not as efficient is that the material used to wind the helix, even a loose and widely wound one, has some resistance that is likely (almost always) higher than the resistance found in a straight, rigid conductor.  The Larsen Kulrod, for example, is a silver-plated, copperclad stainless-steel material.  Stainless, to make it strong and forgiving (and springy!), copperclad to create a surface that will accept silver plating, and silver plating because silver is the world's best conductor, and since all the RF current is handled only by this layer, silver is the best possible material and results in the highest Q and lowest loss.

Still another reason to avoid this approach is that unless you wind the helix on an air form, in which case it will collapse with nothing to support it, any other dielectric material used will cause the antenna to be even *shorter* and even *less* efficient!  Also, most dielectrics will have some absorptive loss at VHF, unless expensive materials like Teflon are used...and Teflon's not very flexible.

A typical "helically loaded" antenna for VHF is the conventional "rubber duckie," which is 4~6" long and about 20 dB down from a full-sized whip in performance.  I can't think of a single reason to want to wind a VHF helically loaded whip.

Bunching up the windings near the tip of the antenna doesn't make it radiate any differently; the advantage of end-loading, rather than base-loading, is purely from reduced I^2R losses because there's less current at the tip than there is at the feedpoint.  

Engineers have been trying to re-invent this wheel for about fifty years now, and nobody's done it, yet.

WB2WIK/6

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KC2MMI
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« Reply #10 on: February 05, 2004, 01:14:47 AM »

Steve, I appreciate what you are saying and I'm getting an education from it. What I'm seeing as I run the numbers I have, is that a 0.28 wave antenna will have a 52 ohm native impedance, what we call quarter wave.

Applying this to 2 meter (approx. 79") this makes a 1/4 wave antenna 22" long while a 5/8 wave would be 49" long. The quarter wave would make my car about 7' tall, which clears about everything. The 5/8 wave would bring it over 9' tall, which starts to hit things.

I know, the extra elevation also helps, if the antenna stays on.<G>

But from the little I find in my old books, I should be able to wind a helical antenna which is 2-1/2 feet long or so (still well under 8' tall) and electrically is a 5/8 wave antenna (for the +3db gain over 1/4 wave due to better radiation pattern) AND, the only reference I have says that if the helical is wound properly, it becomes the distributed inductor which can make an impedance match, the same way that a base coil would.

The advantage (or not) would be in comparing the loss in the base coil, however small that is, to any loss in the helical itself. Which I guess comes back to the question of how much loss is there from the Larsen high-Q base coil? Compared to, what other loss would the helical have? I know silver is good but the copper wire I'd be using should be close. And actually, silver wire isn't THAT expensive, it is also possible to find and use if that would make a real difference. This is still less than six feet of thin wire.<G>

So...any references on how to wind the helical so that it forms a distributed coil which replaces the base coil? Or what the Larsen base coil loss actually is? The other factors?

I don't expect to discover anything new, but I know that commercial products often are purpose built--and sometimes the purpose just doesn't match what can be done by hand.

And more important, is that I'm learning--not just buying a box full of stuff and using it blind.
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KC5AOS
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« Reply #11 on: February 08, 2004, 01:52:59 AM »

A few points. Don't worry to much about the magmount thing vs. puting a hole in your roof. Capacitive coupling gets better with higher frequencies. If you want to minimize the losses due to the capacitive coupling, purchase the magmount with the biggest base (rule of thumb)and this will increase the capacitance.

Also, you mentioned being in parking garrages often. You will probably want to go with a shorter dual band antenna to minimize contact with structure. Even with spring flexable whips it is not a good idea to have them hitting on structure. And since you stated that you wanted to txmit from within the garage, folding is out of the question also. I think the average clearance is around 9ft, so subtract the height of your mounting point and 6 inches for clearance and that should give you a good starting point.  
       

--------------------------------------------------------------------------------
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WB2WIK
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« Reply #12 on: February 09, 2004, 06:51:42 PM »

Most parking garages I have to enter around here, including those at L.A. Airport, don't have anything near 9' of clearance -- it's more like 6-1/2 feet, and my manufacturer-installed car radio whip antenna, which is a motorized, telescoping type, cannot make it past the garage entrance without breaking off (as I've found out, much to my chagrin, $150 or so later).

So, my rule of thumb about parking garages is: Either use a very flexible whip that can bend over double without any damage, or take the whip off altogether prior to entering the garage.  I don't have the time or inclination to constantly remove and install whips, so I choose the "very flexible" route.  Although a Larsen 5/8-wave mag-mount *is* extremely flexible and can easily double over without damage, the mag mount base (and probably *any* single-magnet base, made by anyone) isn't quite strong enough to hold it to the roof in this process.  So, if I expect the whip to double over, it must be securely installed via a through-hole mount.

The "loss" in the Larsen NMO-150 loading coil is very likely too small to be measurable because it's wound of very thick silver-plated wire and only has a few turns on a large diameter.  You wouldn't want to use wire like that for a continuously-wound, helically loaded whip because it's too thick and not terribly flexible.

As for radiation efficiency, using small-gauge wire with small surface area will do more to destroy radiation efficiency than almost anything else you can do.  Skin depth at 146 MHz would be measured in millionths of an inch: The way to improve efficiency is to add surface area, not to decrease it.

Of course, for general-purpose "strong signal" FM-repeater work, nobody notices a couple of dB one way or the other unless they're really looking for it.  Weak-signal VHF work is all done using horizontal polarization, so discussions about "whips" are inappropriate.

Surely, ham radio is the place to experiment!

WB2WIK/6





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