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Author Topic: Tuner and Whips  (Read 922 times)
N4ZYV
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Posts: 90




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« on: September 18, 2003, 10:20:45 PM »

Thinking that more metal in the air is better, has anyone tried using two 102" whips and a tuner(s)?

I know nothing about and have never used a screwdriver or the whip/tuner combos. Was just thinking.

Many here have far more knowledge about antennas, being able to calculate radiation resistance etc. Has anyone done this in-depth analysis of a mobile antenna system using a tuner and two whips? Or, is there something simple and fundamental that I am missing? The idea being not for directivity, just somwhat more effecient tuning or radiation.
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KC8AXJ
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« Reply #1 on: September 18, 2003, 11:36:56 PM »

"The idea being not for directivity, just somewhat more efficient tuning
or  radiation. "

A Phased Array is really only for "directivity".

"more Efficient radiation"  If gain is what your getting at here, yes you would have more, but only in a direction the phased radiation propagates.

Problem is these 1/4-wave elements need to be precisely 1/4 wavelength apart so on a vehicle you have limits to the band you want to make this thing for, without sticking into the other guys lane hi hi!  
Tuners (transmatches) won’t help with the spacing deal you have to cope with, which changes with frequency somewhat.

AM radio stations use this trick but they have a lot of real estate and a set frequency.

BASICLY:
I believe by referring to 102” whips your thinking 10 Meter band. These two elements, having to be about 8 feet apart, would then need to be feed off a “T” with each having a  “branch” of coax of a different, special length (per a fraction of a wavelength
 at the giving freq.) so that one element will work in phase with it’s twin,
 without canceling each other. “Kinda” like a 2 element beam, but BOTH are driven elements.

73
Steve.
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N4ZYV
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Posts: 90




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« Reply #2 on: September 19, 2003, 12:18:17 AM »

Not why I'm thinking of this. The problem with mobile antennas is the lack of length for the radiating element. So, instead of going longer, how about using two whips.

I'm not trying for directivity or gain. Just better effeciency at the lower freqs.
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WA4PTZ
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« Reply #3 on: September 19, 2003, 06:12:36 AM »

Stick to one whip.....believe me, 2 are not going
to impress anyone or improve your signal.
73 - Tim
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N4ZYV
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Posts: 90




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« Reply #4 on: September 19, 2003, 07:45:28 AM »

Impressing anyone was not the idea.

Forget it.
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K0BG
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« Reply #5 on: September 19, 2003, 09:40:29 AM »

The effective gain is rather low even on 10 meters and requires more than just a a chunk of coax or two, and in my opinion not worth the effort. By the way, it doesn't make any difference whether you use an auto coupler or not.

You are certainly correct, mobile antennas are rather short and inefficient. However you can get much better results by proper mounting of the antenna and by bonding the various parts of the vehicle together.

Alan, KØBG
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KC8AXJ
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« Reply #6 on: September 19, 2003, 10:34:21 AM »

Rande,
I think I'm tracking with what your thinking here.

Problem is ELECRICAL length of the radiating element and if has one, it's ground element or radials is what RF works with. With radials, more can be better as per the statement below.

True that in a ground plane (like a vehicle becomes with mobile antennas) MORE is better, especially with HF, as far as metallic square inches go. The radiator has a LOT to work with then to get a somewhat omni pattern, as the radiating element can “see” (or mirror) itself all the way around.

But in your thinking with the radiating element, more is not necessarily better.
Any time you take away from the length of that ONE element, you cheat on how much of the RF cycle can exist in that ONE element at a given moment in time, therefore reducing it effectiveness in crating a field around itself.

¼ wave elements are good, ½ wave is better in radiating efficiency, but adding elements to the CENTER wire of your coax doesn’t add to the “system”.

Another angle to the dangle as they say:

Think of what you have with these two elements you whan to use.

TWO 1/4 wave elements…….. Oh ! Sounds like a DIPOLE. Only thing is with a dipole, One element is FEED, the other is connected to the braid or ground of the coax feedline.
The element connected to ground is what the RF cycle(at any given moment in time) in the radiating element “works to” as it is opposite in phase of the cycle both electrically and magnetically. It (the “ground element”) “attracts” the RF, setting up the building and collapse of the magnetic and electric fields which are what is the PUSH force to make the wave propagate out away from the antenna. Opposites attract.

If you have interest in antennas ( I think they are the most amazing thing in radio) get some books and you will understand more as to their working principles. They can be very complex but basically they are easy to understand if you don’t get to deep into the  theory, and stick with basic electrical principles.

I am in NO way a expert or engineer so what I say here is just as I comprehend it.
I am NOT trying to be arrogant like I know it all.
Just trying to answer the question folks !

Good question, that’s how we ALL learn, and I haven’t got all the answers yet, though my wife says that I think I know everything. That’s why I stay with her I guess!

The purists can beat me up now.

73
DE KC8AXJ

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N4ZYV
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Posts: 90




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« Reply #7 on: September 24, 2003, 12:10:11 AM »

Ok, so a "T" is not the way to go. Electrical length is what I need to get. So then, if one of the whips is fed in the normal way, then another coax is taken to the other side of the truck to the other whip, wouldn't the radio see that like an antenna fed part way up the length? Like a wire that has the two 102" lengths plus the length of coax between them and that length of wire is fed 102" from one end? Problem is, the center section of the antenna would be surrounded by the braid of the coax run between the two whips. But wouldn't this create more elctrical length thus allowing the tuner to more easily match to antenna at lower frequencies? And with twice as much metal in the air, would'nt it radiate better?

If that is not wierd enough, how about two 102" whips, fed at the bottom of one and the two tops connected by welding a rod between the two tops. The top flat piece and the other whip would add to the length and but the other whip would have no coax attached to it. The mount would act as an insulator. The antenna would look like a tall, thin, square roll bar on the truck.
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KC8AXJ
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« Reply #8 on: September 24, 2003, 09:40:15 AM »

I'll need to digest that for a while and get back to you.  [8-/

You're a Vikings fan aren't you ?

Steve
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KB9YNB
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« Reply #9 on: September 24, 2003, 11:59:39 AM »

There is only one way for you to know what works and what doesn't.

Build it and see what happens.  Use low power levels.

On your other question tho... if you connected the tops of two 102 inch whips, you'd basically have a vertical loop antenna... ALMOST.  Because normally a loop antenna is just a long wire bent back on itself until it ALMOST becomes a loop.  In this case, you would actually have a complete loop.  I don't have any idea what that would do to your radiation pattern.

Build it and see what happens.  Use low power levels.

Good luck!
KB9YNB
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AA4PB
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« Reply #10 on: September 24, 2003, 03:31:22 PM »

Think about it this way. If you take two 10 ohm resistors and connect them in parallel you get 5 ohms, not 20 ohms. To get 20 ohms you have to put them in series, end to end.

If you feed two whips in parallel all you are doing is splitting the power, sending half to each whip. The net radiated power is still the same and you gain nothing.

As you make a whip physically shorter than 1/4-wavelength, its radiation resistance (ability to radiate signal) decreases. The radiation resistance is determined primarily by its physical length. The only way to increase the radiated signal is to decrease other losses (loading coils, ground, etc) or to increase the radiation resistance by increasing the length of the whip closer to 1/4-wavelength.
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N4ZYV
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Posts: 90




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« Reply #11 on: September 24, 2003, 05:52:37 PM »

Ok, so I feed one whip normally and tie the tops together, leaving the other whip unfed at its base. Now I have an 18' whip if I just bend tops together and weld them up. That increases the electrical length. Now I wonder about the voltages and current on the antenna and will the "top" of the antenna (which is now actually the bottom of the other whip) cause any trouble being that close to the body of the truck?
 
/\ like so, only the tops actually welded together.

I know there is current and voltage on the antenna that varies along its length. These can both get pretty strong as I understand it. So, having the "top" of the antenna bent over and attached to the other, unfed, mount puts it close to the body of the truck. Is this likely to cause trouble?

The idea of just going ahead and doing it to see how it works will just have to wait a couple of months as I need to spend my money on my old Mustang and my truck first (I don't yet have a mobile HF rig). Thought I would spark some conversation here to get a better grip on what I'm trying to do.

I could also build a roof rack for the fiberglass bed cap and feed that. How about a recangular loop antenna, 6' on the long sides and about 4' on the short side. Not a very big loop. I think the two whips tied together, either directly to one another or via another piece of stainless rod between them would give me the extra electrical length and allow the tuner an easier time of matching and also (hopefully) get a bit more signal out.

I'm kind of surprised that this hasn't been done before. Either it has and didn't work so good, or I just think wierd.
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AA4PB
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« Reply #12 on: September 24, 2003, 10:17:27 PM »

Sorry to be so negative but I think you will have problems with the capacitance between the bottom of the second whip and the vehicle. The end of the whip, being unterminated, will be a very high impedance.

You will have the same sort of problem with the roof rack loop on HF. You are mounting the loop directly over, and very close to, a large metal structure (the roof). The capacitance will be a substantial problem. You might get away with it on 10M but then a 1/4-wave whip is not a big problem on 10M and it probably works better.

As Alan said a while back, working on reducing grounding losses (bonding body parts together) will probably produce much better results than anything you can do to the whip from a practical standpoint. Mobile HF antennas and efficiency are at the opposite ends of the spectrum.
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N4ZYV
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Posts: 90




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« Reply #13 on: September 24, 2003, 11:31:52 PM »

you guys are so negative. Wink

Actually the roof rack would be mounted on the fiberglass bed cap not the metal roof of the cab.

Ok then, capacitance on the far end of the bent-over whip because it is too close to the metal body of the truck. How much capacitance do you think there would be with about 1' foot of that end near the body? What effect would this capacitance have? Would I have to move the end very far away to reduce the capacitance to a manageable level? Could I just use a piece of plastic rod or pipe to move that end of the whip up to clear the body and fix this capacitance problem.

You see, I have read several threads about the short length of the mobile whip but I don't remember seeing any serious discussion on methods to increase its length. Any discussion I did see, revolved around making the whip taller and thereby hitting overpasses and low-flying aircraft. Why can you only make the whip taller? Why not just make it longer?

Its always said that a whip/tuner combo will not radiate well at lower frequencies because of the extreme difference in what is needed at low frequency to make a 1/4 wavelength and what can be dome with a 102" whip. Well then, lets make that silly whip longer.

What about these small loop antennas that have a tuner built into them? Iso-loop I believe is what one is called. How is that different from my fiberglass bed cap mounted roof rack? I know for certain that attianing the efficiency of a full size 1/4 wave vertical can't be done. But can we improve on the 102" whip/tuner combo?

In another thread in this same forum, someone is looking for a taller whip. Could this do the same thing without going higher?
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KB9YNB
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« Reply #14 on: September 26, 2003, 11:35:32 AM »

I think if you setup your antennas like you described in the /\ style, you'll have a longer whip, but it will work much like the military and law enforcement whips that are super long, mounted on a large ball mount in the rear of the vehicle, and then the top is tied down to the front of the vehicle.  All of your propogation will be local, as your radiated signal travels mostly straight up, is reflected off the layers of the atmosphere, and comes back.  They call it NVIS (Near Vertical Incedent .... Somethingorother, probably Scatter) I think.

So, you'll have the advantage of a longer whip, but the disadvantage of your signal being radiated mostly straight up.

build it and see what happens.

KB9YNB
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