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Author Topic: SMALL 60M MOBILE ANTENNA???  (Read 7958 times)
N4VNV
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« on: January 22, 2011, 06:43:17 AM »

I am looking for a 60M Mono-band bottom load mobile antenna that looks like a Larsen with the NMO mount. AND a short whip on top.(about 4 feet) All I have found are HUGE multi-band antennas with whips 6 to 9 feet long. And I just want one to cover 60M only. I am NOT INTERESTED in multi-band antennas like the TarHeel or whatever. And NO to the Hustler also. Unless they have a NMO mount with a coil and short whip. If you know of one please reply with who sells it. I already have the "HamStick" type mobile antenna for 60M and it works fine, but it's way to tall for my use. I want to have one I can use with my existing mounts on my luggage rack that does not stick up over 5 feet high.
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WX7G
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« Reply #1 on: January 22, 2011, 07:07:12 AM »

The Hustler RM-60 resonator can be used without a mast. With this you have the 4' mobile antenna you want. You can use an NMO to 3/8-24 adapter to attach it to your NMO mount.

HRO stocks both items. The adapter part number is TRAM 1295 with a price of $10. The RM-60 is $31.

Lastly you will need a 1.5" length of 3/8-24 to attach the RM-60 to the Adapter. HRO does not stock this but DX Engineering does. Part number DXE-.5MTHRF-2, $6. Or buy a 3/8-24 bolt from the Hardware store and cut off the head.

If the RM-60 is too tall the RM-80 will work with the whip cut shorter.


« Last Edit: January 22, 2011, 08:37:59 AM by WX7G » Logged
AD4U
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« Reply #2 on: January 23, 2011, 05:21:06 AM »

I understand what you are asking...a 60 meter antenna that is about the size of a NMO mounted Larsen two meter antenna.

Just remember that on 60 meters......small = bad performance.  Something like what you described would be on the order of 1% or 2% efficient, but it will work to some degree.

Even a rather large mobile antenna on 60 meters will be very inefficient.  In this case size (and good design) really does matter.

Dick  AD4U
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WX7G
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« Reply #3 on: January 23, 2011, 07:48:43 AM »

This roof mounted 4' mobile "antenna" isn't so small. The antenna length consists (roughly) of the vertical length of the vehicle plus the 4' "antenna" making it about 9'. That can beat a bumper mounted screwdriver antenna where the vehicle vertical length is (roughly) subtracted from the screwdriver length. A 10' bumper mounted screwdriver can end up being a 5' antenna.

A NEC simulation of the RM-60 mounted on the roof of my 5' tall vehicle shows a radiation efficiency of 4%. This is with ground loss adding 10 ohms to the system loss resistance and an inductor Q of 100.

« Last Edit: January 23, 2011, 09:33:55 AM by WX7G » Logged
K0BG
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« Reply #4 on: January 23, 2011, 12:39:09 PM »

Numerical engines are good tools, but they aren't panaceas.

A four foot antenna, at 5.4 MHz, will have a radiation resistance of just under.18 ohms. With 10 ohms of ground loss (it's probably more), how do you figure it will be 4% efficient?
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WX7G
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« Reply #5 on: January 23, 2011, 02:58:35 PM »

The "antenna" is not 4 ft. The "antenna" consists of the vehicle and the 4' mobile base loaded whip.

While the 5 MHz radiation resistance of a 4' base loaded vertical is 0.3 ohms, the radiation resistance of a 5' vehicle with a 4' base loaded whip mounted on top is 1.3 ohms.
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K0BG
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« Reply #6 on: January 23, 2011, 03:07:47 PM »

You must be using one of your patented NEC programs.

So, we're saying that part of the radiation is from the antenna element, and some of it from the vehicle itself. I'll buy that to the extend that the body is acting as the ground plane. However, if we're looking at it as a dipole, which it isn't, then we have two ways to look at it. Either we're working the antenna against the body, or the body against the antenna. So, we have to ask ourselves, which one has more loss?
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KC8IUR
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« Reply #7 on: January 23, 2011, 03:10:31 PM »

How are you ensuring the vehicle's body resonates at your frequency? (I don't think it works that way, man).
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AD5X
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« Reply #8 on: January 23, 2011, 03:24:19 PM »

Dave - are you saying that the system looks sort of like an off-center fed vertical dipole?  If so, that is an interesting idea.  That both raises the radiation resistance, and does make the car part of the radiating element.  I need to think about this a bit more...  Interesting, and kind of makes sense.

Phil - AD5X
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K0BG
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« Reply #9 on: January 23, 2011, 06:27:58 PM »

The issue is, the body of the vehicle does radiate, because it is an imperfect ground. But trying to compare it with a ground-mounted vertical is fraught problems. The other issue missed here is, the losses encountered are a result of standing waves between the body of the vehicle in question, and the surface under it. You just can't ignore the issue.
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WX7G
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« Reply #10 on: January 24, 2011, 05:06:30 AM »

HF mobile antenna: Vertical, ground plane, or dipole?

Let's take look at the 60 meter antenna system here. It consists of a 4' base loaded whip mounted on top of a 5' tall vehicle.

Let's define a vertical as a monopole that requires earth ground for normal operation. Modeled over perfect ground this antenna system is resonant at 5.30 MHz. Modeled in free space (no earth ground) it' resonant at 5.33 MHz. This antenna does not require earth ground for normal operation, therefore it is not a vertical.

Now let's define a ground plane as a monopole having a non-radiating counterpoise. Modeled with a non-radiating counterpoise the 4' whip has an input impedance of 0.3 ohms. Modeled on the vehicle the input impedance is 1.3 ohms. The vehicle is radiating; in fact it accounts for over half of the radiation Therefore this is not a monopole.

That leaves us with vertical dipole. This antenna system is a vertical dipole mounted close to lossy earth. Charge flow in the lossy earth produces ground loss. For this particular antenna loading coil loss accounts for 3/4 of system loss while ground loss accounts for 1/4.

Now that we can clearly view an HF mobile antenna system as a vertical dipole we can optimize it as such. A short, loaded dipole works better the longer it is. The radiation resistance is proportional to the length. So, we want to make the antenna as long as possible measured from the bottom of the vehicle to the tip of the whip. We would not fold a dipole back on itself and we should not do it to this dipole. Bumper mounting the whip folds our dipole back on itself.  

We cannot control how close to ground our dipole is nor can be control the shape of the bottom half of our dipole. So, ground loss is a fixed quantity. We can control the antenna effective length (and radiation resistance) by how the whip is mounted, how long the antenna system is, and by controlling the current distribution in the whip (top loading structure). And can control loading coil loss.
« Last Edit: January 24, 2011, 07:45:18 AM by WX7G » Logged
K0BG
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« Reply #11 on: January 24, 2011, 08:04:47 AM »

Ground loss IS NOT a fixed quantity! It is a varying quantity which cannot be accurately measured.
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WX7G
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« Reply #12 on: January 24, 2011, 08:34:32 AM »

When parked in one location ground is a fixed quanity. What I'm saying is ground is not a design parameter we have control over.

In the case of this particular antenna (with a Hustler loading coil) you can park it over salt water, you can park it over desert ground, or you can drive off a cliff and have no ground until you hit the ground. The difference in radiation efficiency between these three senarios is about 1 dB. So, ground quality is a second order effect on radiation efficiency. Now change the coil Q from 100 to 500 and the quality of the local ground will have a first order effect on radiation efficiency. But again, we still have no control over the ground we happen to be driving over.
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K0BG
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« Reply #13 on: January 24, 2011, 10:06:19 AM »

Well, isn't that the whole point?

I will agree that there is some radiation from the body of the vehicle, because it is a lossy ground plane. But by that fact alone, you cannot include that as part of the radiation resistance of the antenna system.
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N3OX
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« Reply #14 on: January 24, 2011, 11:14:49 AM »

But by that fact alone, you cannot include that as part of the radiation resistance of the antenna system.

Why not?  All radiation contributes to the radiation resistance of the antenna system.

How much it contributes is a matter for specific prediction.

I'd like to see an EZNEC wireframe model of a lossless antenna on a lossless car in free space compared with the same out of software that can handle actual metallic surfaces, etc.

But the basic mechanism that WX7G's proposing is the same that causes the feed impedance of a ground plane antenna to go up as you slope the radials down.  There's a substantial vertical component of the current on sloping radials, and that raises the radiation resistance by reinforcing the radiation from the vertical "radiator"

The horizontal components of the currents mostly cancel in a ground plane, and the radiation from the radials in a perfectly planar ground plane is a very weak many-lobed pattern and contributes very little to the antenna's radiation resistance.  But the vertical component of the current in the radials doesn't cancel, it reinforces, and this adds to the radiation resistance because there's less overall radiation cancellation from a similar current distribution.

If you go the other way and slope the radials UP on a ground plane, the radiation resistance drops.  If you go far enough you've basically built a "non-radiating" transmission line stub....

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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