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Author Topic: Fan Dipole Tuning Question  (Read 10268 times)
VA7CPC
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Posts: 2414




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« Reply #15 on: December 18, 2013, 06:08:04 PM »

Quote
. . . This works great when you are measuring the feedpoint impedance right at the
antenna, but not necessarily when you are measuring through a length of
coax, as is often the case when the antenna is up in the air out of reach. In that
case, finding the frequency of lowest SWR is often a better approach. . . .

Interesting -- I'd never thought of that.  It's using the coax as a fixed-setting tuner.

But, from the original post:

Quote
. . .   I have a 20-12-10m fan dipole that I am installing in my attic (HOA restrictions). . . 

So the OP can measure resonance at the feedpoint.

.       Charles

PS -- as an exercise, I should borrow an antenna analyzer from my Club, and check my own attic-mounted fan dipole.  But I'm afraid of what I'll find -- my attic _does_ have wires and stuff in it!
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G3TXQ
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Posts: 1532




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« Reply #16 on: December 19, 2013, 02:47:46 AM »

On a simple HF wire antenna such as a half-wave dipole, minimum SWR and resonance measured at the feedpoint will be very close to the same frequency. If you look at the frequency/impedance characteristic on a Smith chart, you'll see the closest approach to the centre occurs where the curve crosses the resistance axis.

Not that it really matters, because adjusting for minimum SWR is nearly always what we want to achieve.

Steve G3TXQ
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KB4QAA
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Posts: 2487




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« Reply #17 on: December 19, 2013, 09:08:06 AM »

-Don't get distracted by red herrings.  In Most cases,  frequency of lowest swr is very close to frequency of resonance.  The difference is seldom significant when dealing with sufficiently broad-banded antennas like dipoles.

-The insulated wire, and temporary wrap over can be another confounding factor.  Use insulated or strip the working ends, or let it hang down, whatever.  Pick a method and stay with it.  Once you are satisfied with the pruning, leave it alone. There is no reason the wire must be stripped and soldered.  Just secure it mechanically so it is secure, e.g. wire ties, tape, dental floss... Smiley

-Make no mistake, attic antennas are greatly affected by the house and all the stuff in it.

-This is one dipole you absolutely should use a quality 1:1 current balun!

-A nearly flat swr across a band is a clue of possible problems or misleading/misinterpreted readings (you must understand the factors).  Get up in the attic with about 10' of coax and make your readings there.  Expand your readings outside the band until you start to see some sort of obvious dip/bowl shape!!!  Plot out your readings for a visual presentation, even if roughly on a cocktail napkin.

-No dipole free of confounding influences (or really inefficient design, ala Bazooka) will ever give you a flat swr across the 10m band.  It is just too wide.  Same for 40m-160m.  The narrower bands, can usually be covered with a single antenna, but you should still be able to see a definite curve if you expand your measurement frequencies.  This is your cross check!

-Fan dipoles can be very touchy to tune.  Wider spacing helps (most people probably use about 6 inches, but actually "fanning" them out helps more.  They interact.  I suggest starting with the longest element working to shortest, since the longest element will be less affected by than the shorter elements.  Several iterations will be required.

-I don't trust the "flat 2:1 swr" readings until you take wider readings.  Once you do confirm you are pretty well centered at the lowest swr within the band, realize that
*Given the antenna design, the location and height, you may not do any better than 2:1.  This isn't bad.  You need a tuner.  Even with the best pruning it is not always possible or reasonable to expect a perfect 1:1 swr on any given antenna!

Keep pluggin, there is lots of learning going on!  Bill
« Last Edit: December 19, 2013, 09:32:25 AM by KB4QAA » Logged
W1JKA
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Posts: 1821




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« Reply #18 on: December 19, 2013, 10:18:59 AM »

A question, I'm a little confused as to the TYPE of Fan dipole the OP is talking about. I can only picture his description as 3 sets of different band radiator lengths separated by 6 in. (one over the other) evenly the full length in a Vee configuration with the 3 radiators at a common point on each side of the center insulator. I'm wondering if this is just a question of what we each think a Fan dipole is? I built my  generic (so I thought) Fan dipole with the recommended 3.5 ft. spacing at the far ends and 3 in. vertical spacing of each bands radiator on each side of the center insulator which made for very easy tuning. As I understand it a true fan dipole is nothing but 3 or 4 different band dipoles each fanned out from a central point in a different quadrant from each other and not one over the other.
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WB6BYU
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Posts: 13567




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« Reply #19 on: December 19, 2013, 10:42:29 AM »

Quote from: W1JKA

.... As I understand it a true fan dipole is nothing but 3 or 4 different band dipoles each fanned out from a central point in a different quadrant from each other and not one over the other.



We certainly do have some confusion of terminology.

As far as I'm concerned, a true fan dipole is a 2-dimensional version of a conical dipole,
composed either of triangles cut from sheet metal or a fan of wires of about the same length to
replace a flat sheet.  (Similar to the bow-tie dipoles used for VHF TV.)   You find the term used that
way in Kraus and some of the other older engineering books, either as a dipole or a monopole.
Like this commercial version.

They aren't common in ham usage, but a fan with 5 wires each 40' long on each side of a 4 : 1 balun
you can get an SWR of about 3 : 1 or better continuously from 7 to 30 MHz - within the range of
most built-in autotuners.  This may give better results than trying to use parallel dipoles that are
so close in frequency:  20 / 15 / 10m isn't bad, but 20 / 17 / 15 / 12 / 10 can be problematic.


Clearly whoever applied the term to multiple dipoles on a common feedpoint hadn't read enough of
the old antenna literature!

For multiple dipoles, they can either be run parallel or the ends fanned out - my experience is
that the further the ends of the wires are separated, the less tuning interaction there is
among the various bands.  Parallel wires seem to be preferred when the number of tie-off
points is limited, but I've had good luck letting the shorter wires hang down below the
longer ones by adjusting the string that ties the end insulators to the end of the longest
element.

So, yes, there is a lot of room for confusion in the terms we use...
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KB4QAA
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Posts: 2487




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« Reply #20 on: December 19, 2013, 12:00:45 PM »

I'm always in favor of accuracy in antenna naming! 
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W1JKA
Member

Posts: 1821




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« Reply #21 on: December 19, 2013, 02:19:38 PM »

Re: WB6BYU reply #19

Interesting and informative info as always. Your posted pic of a fan dipole looks just like my summer hammock after Lap Band surgery. This accuracy in antenna naming that QAA alludes to makes me wonder how many more mutant varieties of the original designed G5RV, Windom, Fan dipole etc. will be dreamed up in the coming years.
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WB6BYU
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Posts: 13567




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« Reply #22 on: December 19, 2013, 04:54:27 PM »

We already have plenty of variants:  the original "Windom" used a single wire feed, for
example, unlike modern OCFD types that have adopted the same name.


Not only are original names applied to different antennas, but there seems to be an
unfortunate trend (for 40+ years) of inventing fanciful names for antenna designs
published in magazines or (nowadays) on the web, even if there really isn't any
significant difference between them and standard types.  This leads to too
many
names in use, which tends to disguise the commonality among the many
types of antennas.  For example, building a dipole or doublet out of fence wire
might justify a name such as the "barbwire dipole", which is pretty self-explanatory,
but the "Super Moo Zapper II" serves only to confuse newcomers.

That's why I try to stick with simple names and/or descriptions:  I'll say I'm using
a dipole, and if it seems somehow important to the conversation I can remark on
the type of wire, the matching network, whether it uses traps or parallel wires
to operate multiple bands, or whatever else about the design might be significant.
But first and foremost it is a dipole, and the person I'm talking to doesn't have to
search through disreputable web pages to discover that.


Sorry, I'll get off my soapbox...
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KD0VWH
Member

Posts: 9




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« Reply #23 on: December 19, 2013, 07:49:10 PM »

Thanks for all the feedback.  This is great.  I was using fan dipole and multiband dipole interchangeably and I shouldn't have.  I am building a multiband dipole similar to the following

http://www.hamuniverse.com/multidipole.html

I took out the strain relief as suggested earlier and that helped widen the band a bit on 10 m, but not much.  On the 20 and 12 m I widened the frequency range to see the bowl and I have been able to tune such that I am centered in the bowl.  I am essentially flat across both bands with slight 0.2 SWR towards the ends.

I was talking to another local ham and he suggested that I move either the 12 or the 10 band perpendicular to each other and see if that helps.  I was also reading other articles and come to find out a 700 kHz wide spike with the top end being around 4 isn't all that bad on 10 m because 10 is such a wide band, so maybe I am not all the bad.

This has been a great learning experience for sure and I appreciate all of the help.
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W1JKA
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Posts: 1821




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« Reply #24 on: December 20, 2013, 02:16:21 AM »

Re: KD0VWH

Glad you found your posted site, that's the antenna I built with the SRI technique at the center insulator. Even though I used 3 in. spacing rather than SRI's 5 1/2 in. it was still easy tune on my 40/30/20m build  and no balun at the feed point .GL with yours
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KD0VWH
Member

Posts: 9




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« Reply #25 on: December 20, 2013, 05:29:34 PM »

I finally had success!  It took moving the 10 m legs so that they are rotated about 45 degrees away from the 20 and 12 m legs.  I am still not flat across the band like the 20 and 12, but I am less than 4 across the band and have a nice range where I am at 2.  Once my tuner shows up I will be able to get everything where I want.  This has definitely been a learning experience for my first antenna and I want to thank everyone for their help!!!
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NO9E
Member

Posts: 439




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« Reply #26 on: December 22, 2013, 05:43:14 PM »

Seems that sleeve antenna is easier to do than the fan dipole.  Only the longest element fed and the other elements close.

I made a 20/15 sleeve antenna as in QST Oct 2011. This is from ladder line with only 20m element fed. I have sewn wire in the middle of the ladderline (not connected anywhere) and have SWR at resonance < 1.5 on 10/15/20.

Ignacy, NO9E   
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