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Author Topic: Newb question on antenna tuning  (Read 3380 times)
W1STU
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Posts: 16




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« on: August 09, 2012, 07:46:30 AM »


I could not find much googling this but:

Can someone please explain the relation (if any) between antenna tuners and antennas that require physical manipulation for different bands?

Would an auto tuner eliminate the need to manipulate a wire on a portable antenna (the buddi-stick for instance) or is that just a function of antenna design. [I suspect it would not]


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N4NYY
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« Reply #1 on: August 09, 2012, 08:11:39 AM »

If you have an auto tuner and some type of multi-band antenna, be it a vertical or dipole, the tuner allows you for adjustment for the specific band you are transmitting on. Especially if you have a multiples like 40M and 15M.

However, if you have a Buddipole tuned for like 12M, and then you want to tune it for 17M or 20M, you would have to adjust you antenna closest to the band that you want to transmit on, then tune the auto tuner. An auto tuner is not a magic "put up any size pole and the tuner automatically makes it works on any band".

There are random wire auto tuners out their for that purpose, and am not too familiar with them. But I think you would need several ground rods or numerous ground radials for these random wire tuners.

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W5DXP
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« Reply #2 on: August 09, 2012, 08:12:06 AM »

A tuner cannot change the feedline losses and/or antenna losses which, under certain conditions, can be unacceptable. For instance, an autotuner plus CB whip is 14dB (about 3 S-units) down from a good 75m bugcatcher which itself, is 10dB (about 2 S-units) down from a good dipole. In other words, an autotuner plus CB whip used on 75m is about one half of one percent efficient compared to a 1/2WL dipole.
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W1STU
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« Reply #3 on: August 09, 2012, 08:23:26 AM »

Ok thanks, that answers my question and that was what I figured.

Maybe adjusting an antenna is not as cumbersome as I am thinking it would be once I get the hand of it.

Based on what W5DXP said maybe a mini buddi-pole would be a better option than a vertical like the stick.

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N4NYY
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« Reply #4 on: August 09, 2012, 08:28:32 AM »

Buddipole is a compromise antenna. Its purpose is portability for portable use, possible homeowners restriction dwellings, and so forth. It usually comes in a duffel bag. If this is you home base antenna you you are not subject to restrictions, you would be better off with a dipole, inverted L, or trapped vertical.
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W1STU
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« Reply #5 on: August 09, 2012, 08:45:32 AM »

My restrictions are that I live in a 1st floor apartment with no private outdoor space, balcony, or ledge and most importantly windows that don't open, so I'm looking for a "take to the park" or go hiking type of setup.
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N4NYY
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« Reply #6 on: August 09, 2012, 09:06:20 AM »

My restrictions are that I live in a 1st floor apartment with no private outdoor space, balcony, or ledge and most importantly windows that don't open, so I'm looking for a "take to the park" or go hiking type of setup.

Got it.
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WB6BYU
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« Reply #7 on: August 09, 2012, 09:50:40 AM »

Quote from: W1STU
My restrictions are that I live in a 1st floor apartment with no private outdoor space, balcony, or ledge and most importantly windows that don't open, so I'm looking for a "take to the park" or go hiking type of setup.


I have found wire dipoles to be quite effective for outdoor portable operation, whether
in a park, backpacking, etc.  Smaller, lighter, cheaper, more efficient, and wider bandwidth.
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W5DXP
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« Reply #8 on: August 09, 2012, 11:37:00 AM »

I have found wire dipoles to be quite effective for outdoor portable operation, ...

When I lived in AZ and went camping a lot, I had a portable antenna kit that consisted of two Sunny Day 20' telescoping fishing poles, antenna and radial wires, RG-8x and 300 ohm ladder-line. I could create a dipole to be hoisted on a pole or into a tree. I could also create a phased vertical system.
« Last Edit: August 09, 2012, 11:40:32 AM by W5DXP » Logged
W1STU
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« Reply #9 on: August 09, 2012, 01:32:50 PM »

I like cheap, I could try out a wire dipole, shouldn't be too hard to string up in trees.
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STAYVERTICAL
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« Reply #10 on: August 09, 2012, 03:12:00 PM »

This is not the cheap option but it is very flexible.

First, some background:

At any frequency, a particular piece of wire will present an impedance at its feedpoint.
Impedance represents resistive and reactive components.
The resistive part represents the sum of the radiation resistance ("loss" due to radio waves going into space), and losses due to other factors.

The efficiency of an antenna is equal to: radiation_resistance/(radiation_resistance + other_losses).
So you can see that if other_losses are zero the efficiency would be 1 (or 100 percent when multiplied by 100).
So minimising other_losses is good.

The reactive component of the impedance is simply because of phase shifts and does not take away power.
But the reactive component will mean that stopping reflected energy from the feedpoint is difficult.
So steps are taken to remove a particular reactance, by adding the "opposite" type.
If an antenna is electrically shorter for a particular frequency, it generally shows a capacitive reactance.
So we add an inductive reactance to compensate - this is why coils are placed on short whips.

Buddipoles, and loaded verticals simply have the coils for this purpose.
But, since the coil has current flowing in it, and it modifies the current distribution, where you place it has an effect on the antenna's performance.
If you place it right at the end, where little current is flowing, it has little effect - while at the base of a vertical it has a large effect.
Many times, placing it near the middle is a good compromise, which is why you often see coils at these locations.

Capacity hats are another device frequently used which cause the current to stay higher towards the end of the radiating element.
These are generally a very effective and low loss method of improving performance of a compromise antenna, at the cost of increased physical complexity.

So you can see the problem of environment restricted operation antenna's are threefold:

1. Matching to 50 ohm feedline.
2. Portability and mounting flexibility.
3. Maximising efficiency.


Here are my suggestions:

1. If possible, without breaking the bank, get a small portable automatic antenna tuner e.g. LDG Z100+ (operates for a year from 10 AAA cells).
Although most of these tuners are made to tune coax in the shack - with a SO239 to Banana plug adaptor they can be made to work like a remote ATU.
This solves the matching problem.

2. Unless you know you will always have trees or other structures to use as antenna supports, a self supporting antenna is the most flexible option.
The simplest option is a vertical with a number of wires thrown on the ground as radials.
The vertical can be a fishing pole with a wire attached to the tip, and can achieve some good height for very little weight.

Alternatively, you could use the fishing pole as one end of a sloping wire support.
The other end would come to your operating position, where you could end feed it with simple homebrew tuner (tapped tuned circuit).
A ground stake, or counterpoise finishes the installation.
This option does not need a feedline hanging from the antenna and is capable of very good results.

3. A fishing pole can achieve 5 metres which is a quarter wave on 20m and with a number of ground laying radials be reasonably efficient.
If the radials could be elevated around a metre, then two would provide a very decent antenna.
However, if you are on public land, having trip wires is not a good idea so that is why I suggest some ground laying wires, suitably held down.

The end fed sloping wire antenna in a portable situation is an efficient antenna and easy to set up as well.
In addition, you can make your own inexpensive end fed tuner (look on the net), and have an efficient and simple antenna.

Good luck and happy portable operation.

73 - Rob
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W5FYI
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« Reply #11 on: August 10, 2012, 10:15:06 AM »

Here's the deal; if you have an antenna that exhibits a perfect match (ie. 50Ω ±oj) to the feedline and transmitter, practically all of the power generated by the transmitter will be radiated.  For a less-than-ideal antenna, less than ideal power will be radiated.

Antenna tuners will help minimize losses in the transmission line, and will deliver more power to the antenna, but cannot turn a bad antenna into a good one.

In your case, I'd set the antenna where it has the most wire in the radiating element, then tune the tuner to lowest feedline losses. A bad antenna will never be as good as an efficient one, but bad antennas can be made to work where better antennas are impossible.

GL.
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VA7CPC
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« Reply #12 on: August 11, 2012, 11:58:06 AM »

Quote
3. A fishing pole can achieve 5 metres which is a quarter wave on 20m and with a number of ground laying radials be reasonably efficient.

Two ways to get longer poles:

www.jackite.com      for 31' windsock poles, very lightweight, easy to carry (I have one for portable operation)

http://www.spiderbeam.com/home/index.php        for 39' fiberglass poles, heavier than the JacKite poles

They'll cost more than raising a dipole using trees, but they won't break most people's bank account.

A 20m (31' long) vertical dipole will fit on the JacKite pole, and gives a righteous antenna.
 
         Charles
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KG6MZS
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Posts: 476




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« Reply #13 on: August 12, 2012, 08:56:44 AM »

Here's the deal; if you have an antenna that exhibits a perfect match (ie. 50Ω ±oj) to the feedline and transmitter, practically all of the power generated by the transmitter will be radiated.

Is this really true?  Couldn't an antenna present 50 ohms at the feed point and act like a dummy load?
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AC5UP
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« Reply #14 on: August 12, 2012, 09:21:39 AM »

Couldn't an antenna present 50 ohms at the feed point and act like a dummy load?

If the feedline is a truly wretched length of coax that is water contaminated and oxidized enough to absorb virtually all of the power applied, you betcha'... Power goes in, nothing comes back, SWR meter says it's a 1:1 match. Yippee!

Which is why a new antenna install that reads absolute perfection at the radio end of the coax is probably not a cause for celebration. Perfection is an unnatural state.   Wink
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