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Author Topic: Why use a tuner  (Read 18894 times)
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #25 on: June 04, 2008, 12:49:17 PM »


Some setups just aren't well suited to open-wire feeding, however. If the shack sits at or near the end of the ant, you're bound to get coupling between the ant and feedline. Such would be the case for me if I were to run a balanced line all the way to the shack, which is part of the reason I have the tuner sitting at the base of the antenna.

--Thom
Killer Agony One Zipper Got Caught


Good point, Tom... never thought of that.

Yes, when the shack is on the corner of the lot, running coax on the ground (or buried) would solve the problem of keeping the feedline square 90 degrees and running it away out of the antenna radiation field.


You said, "It's just one out of a hundred effective methods to get on the air."     - Yep agreed.    Every operation is a custom fit. The smartest hams ( I'm not included) sat down and planned out their QTHs on a piece chart paper.  Let's see... I operate mainly local on 3850 so that means a coax dipole at 60'... I also work Europe DX 75M, so that means a dipole at 130'...    I operate 160M both fone and CW, so better put up an openwire fed dipole to cover the whole 160M band and maybe catch 80M cw too.... etc.  If I mount this antenna here, then I can install the other one at right angles so there is no interaction...  

But in reality, most of us put antennas up and take them down more or less on sudden inspiration  and that's OK too... Grin

T



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K3ZS
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« Reply #26 on: June 04, 2008, 01:02:50 PM »

I use a 135 ft. center fed doublet fed with ladder line (14 gauge window line actually).   I use a T tuner with an external balun rated at 10KW.   It is the best multiband antenna I have used, for several reasons.    It is easier to maintain one dipole rather than one for each band.   The antenna is about 180 ft from the house and the feedline losses are almost nonexistant, that is why the SWR on the feedline doesn't matter.   You can use the whole 80 meter band without SWR loss.    It has bidirectional gain on 40, and even higher gain on the higher frequency bands. 
A few days ago I was making contacts on 10M teaming with a fellow about 50 miles away.    We were both using about the same power.   He was using some sort of quad.    We both worked many skip stations in all directions.   On most of the contacts, we both received the stations about the same strength and were given about the same signal reports.   One station in California could barely hear me, and I could barely hear him although he was S9 both ways with the other station nearby.   I guess he was in one of those nulls that shows up when the antenna gets a few wavelengths long.
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K1JJ
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« Reply #27 on: June 04, 2008, 01:40:55 PM »

 On most of the contacts, we both received the stations about the same strength and were given about the same signal reports.   One station in California could barely hear me, and I could barely hear him although he was S9 both ways with the other station nearby.   I guess he was in one of those nulls that shows up when the antenna gets a few wavelengths long.

Hi Bob,

Sounds like that openwire dipole is perfect for your situation.

I was curious about the nulls you mentioned. Also about the gain on higher bands, so I modeled your 135' dipole  at a height of 60'.

We can use 80M as the reference, since it is a 1/2 wave dipole on that band at 1/4 wave high.


Models:

80M :  6.48 dbi gain, take off angle peak at 70 degrees.  Broad textbook Figure 8 pattern  (Reference)

40:   Your best band:  8.22dbi, take off 32 degrees,  very sharp figure 8.  Compared to a standard dipole, your signal will fall off sharply when not broadside due to the sharp lobe. (the price of gain)

20M:  -1.45 dbi gain, take off 16 degrees. This -5db of  negative gain is due to the cloverleaf pattern spreading out the energy in four major directions. Four deep nulls.

10M:  5.91dbi gain,  8 degrees vert take off.  This is less gain than a 1/2 wave dipole cuz of the octapus pattern spreading the RF in a circle. Eight Deep nulls as you experienced.  The energy is now starting to radiate off the ends of the antenna with sharp nulls broadside.

Your vertical take off angles are very good for DX from 40-10M, however, the horizontal patterns are tough to work with on 20-10M.  ie, the antennas no longer radiates off the broadside like on 80-40M.


All in all, it's a decent compromise and low maintainence antenna as you said -  though the modeling shows the advantage of horizontal pattern control and stable gain that separate 1/2 wave coax dipoles would give in comparison.  No free luches.

73,

T
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K3ZS
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« Reply #28 on: June 04, 2008, 03:50:38 PM »

Thanks for the model info.    The antenna actually has a slight V shape in the horizontal plane, because of the feedline pulling it out of line.   It does seem to work good to the south on the higher bands, the wire runs N-S.
I tried modeling it using the trial version of EZNEC but I could not get enough segments.   I think I would like to try an extended double zepp for 10M.   This should work OK down to around 20M.    I was thinking of making it a vertical EDZ and feeding it also with ladder line.    I suppose technically it wouldn't be a balanced feed if used vertically.   I like wire antennas, as they are cheap and easy to maintain and install.
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K3ZS
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« Reply #29 on: June 04, 2008, 03:54:38 PM »

Just another thought.    I always assumed that a dipole is theoretically 2.16 dbi, when I modeled the antenna I figured that anything over 2 dbi was gain over the dipole.    I should mess with some modeling software to become more familiar with it.
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WA1GFZ
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« Reply #30 on: June 04, 2008, 04:57:59 PM »

A fan antenna with any feedline is easier to tune and wider bandwidth.
I ran a 2 wire rhombic as a kit because my flash box had problems with a singlr wire unterminated.
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K1JJ
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« Reply #31 on: June 04, 2008, 05:48:19 PM »

Just another thought.    I always assumed that a dipole is theoretically 2.16 dbi, when I modeled the antenna I figured that anything over 2 dbi was gain over the dipole.    I should mess with some modeling software to become more familiar with it.


You're correct, Bob.   But most specs add in ground reflection gain too.

When comparing antennas relative to one another, the difference between the two is what counts.  The absolute numbers can be misleading sometimes to suit a manufacturer's agenda, etc.   What you are seeing in the modeling is about 6db added in for perfect mirror ground reflection compared to an isotropic. (vertical in free space, globe 3d pattern)  It's something like the error when measuring IMD using ham or military standards... Grin

A full-size 3el Yagi is really only about 5-5.5 db over a dipole at equal heights above ground.  But manufaturers rarely quote it that way.


Here's a quick read about antenna gain:  http://www.w4pvw.us/Discussion%20on%20Antenna%20Gain.htm

When ground and height are factored in, then the gain readings get more complex because of ground reflections adding and cancelling.  BTW, vertical takeoff angle is the most important factor for antenna performance. For example, what good is an extra 2-3db horizontal gain when the vertical angle fading can be 20db+?  Putting the antenna at the optimum height for a given path makes a huge difference, whether it be for local or DX. The difference between a 1/4 wave high dipole and 1/2 wave dipole can often be 15-20db when comparing them with local or DX signals.

As you know, "gain" is simply the focused energy in a particular direction, like a flashlight at the expense of other directions. With 5db forward gain over a dipole (3 el Yagi) you pay the price big time by being 20db weaker off the back... but that's what you want.  ie, to add only 5db to the front requires a 15-20db hit off the back.   That's the problem with using long dipoles on the higher bands... there's nulls everywhere and the small gain you get isn't worth it in a few narrow directions.

I sometimes think the best antenna system compromise might be (using a single 60' tower or higher) an inverted vee for 160-40M fed with openwire and a rotary log periodic for 20-10M at the top.    Add a coax fed 1/2 wave 40M inv vee at 30' (right angles to the 160-40M ) for broad local coverage to complement the 40M clover leaf pattern of the 160-40M ant)

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
K3ZS
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« Reply #32 on: June 05, 2008, 09:46:50 AM »

Thanks Tom for the real scoop on HF antennas.    I spent a lot of time in my past career doing antenna pattern and gain measurements, but at microwave frequencies from towers.   HF, ground and the ionosphere adds a whole dimension to reality.
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