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Author Topic: Inv V fan dipole interaction at max end seperation?  (Read 1947 times)
G7MRV
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« on: August 24, 2012, 10:41:44 AM »

Im working on a three band inverted V fan dipole, for 40m, 20m and 17m. I have the 40 and 20m dipoles working ok, but for some reason cant get the 17m one to resonance.

The feedpoint angle is 45deg, feedpoint height 8m. The three dipoles are arranged around the mast equally spaced, so the ends have the absolute maximum seperation possible in this arrangement


What level of interaction  should i expect? Im having trouble working out why the 17m dipole wont come to resonance. The best i can get for the swr is 1.8:1 at 18.135MHz. At no point could i get the reactance to close to zero, tuning the analyzer either side of the band the readings just went up! so it looks like this is the best it gets?
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WB6BYU
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« Reply #1 on: August 24, 2012, 11:27:50 AM »

Quote from: G7MRV

The feedpoint angle is 45deg, feedpoint height 8m.



Is this the angle between the wires and the mast, or between the wires on either side of the
feedpoint?  In the latter case, the angle is MUCH TOO SMALL, and the wires need to be flatter
(closer to a straight line.)  This will cause the radiation resistance to drop very low, and 1.8 : 1
isn't unreasonable in such a case.

While having the wires 45 degrees to the mast (total included angle of 90 degrees if the wires
are in the same plane) does lower the radiation resistance closer to 50 ohms, I prefer to use a
larger angle where possible.



Quote


...Im having trouble working out why the 17m dipole wont come to resonance. The best i can get for the swr is 1.8:1 at 18.135MHz. At no point could i get the reactance to close to zero, tuning the analyzer either side of the band the readings just went up! so it looks like this is the best it gets?



At what frequency is the SWR minimum?  That is the frequency that you want centered in
the operating bandwidth, even if the SWR is not as good as you would like.  If the point of
lowest SWR is in the band, then the dipole will be resonant there, or close enough not to
matter.

How long is the coax between the antenna and the SWR analyzer?  If it is longer than about
1 foot, then tuning for X = 0 on the SWR analyzer will probably NOT give you an antenna that
is resonant in the band unless you correct the analyzer reading for the exact length and
type of the feedline.  That's because, while the SWR remains constant along a cable (except
for a slight decrease due to loss), when the SWR is not perfect the values of resistance and
reactance DO vary with length.

You can see this impact with VK1OD's transmission line calculator, available online here:

http://vk1od.net/calc/tl/tllc.php

Assume that the load impedance is 28 + j0 ohms, representing a resonant load impedance with
an SWR of 1.8 : 1.  (Just enter it as"Zload" and "28" in the calculator.)  Fed through 10' of RG-213
coax on 18 MHz, the impedance at the rig end is 83 - j17.  Clearly the X term is not zero, even
though the antenna was resonant to start with.

However, you can use the same calculator to find the feedpoint impedance based on your
analyzer reading:  select "Zin" and enter the measurement (like "83-j17" with no spaces)
and it will calculate the actual impedance at the antenna feedpoint.
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W9GB
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« Reply #2 on: August 24, 2012, 11:50:50 AM »

Martin,

Dale, WB6BYU has covered the first steps you should take for your current fan dipole installation.
Ideally, an Inverted-V angle is greater than 90 degrees, but less than 180 (straight).

IF you still have issues, after opening up (~120 degree) your 17 meter inverted-V, then you may be dealing with interactions with the other 2 elements (20 meter and 15 meters -- as a 3/2-wavelength).

What is this fan dipole's performance on 15 meters (3/2-wavelength of 40 meters)??
« Last Edit: August 24, 2012, 11:53:46 AM by W9GB » Logged
G7MRV
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« Reply #3 on: August 24, 2012, 12:02:15 PM »

The angle is between the element and mast (ie the angle to the horizontal). The two elements are 180 degrees apart (or as close as i can make them) per dipole, the angle between the elements of each seperate dipole i guess is 60 degrees (6 elements equally spaced around a circle). The dipole lengths were taken from an online calculator that automatically reduced the length by 5% for the 45deg angles, with just a little added to allow for adjustments.

Sadly, since this is on a fibreglass telescopic mast, theres no way i can use the analyzer at the feedpoint, and theres a 10m run of RG-58 between. However, i would have thought that if i was close with the initial measurements, ie the dipole was close to resonance where needed, the effect of the coax would be minimal? Perhaps its that effect im seeing?

If i can reasonably rule out interactions between the dipoles in this configuration, then i can happily lay the blame on the coax.

Im going to recheck the lengths on this one, in case i have done something silly with the cutting. I can also test the 17m dipole on its own. This is something i can only do with a single dipole - i have a narrow garden! With a single set of stays i can put up a dipole, but to put up all three i have to go some distance to an open field!
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WB6BYU
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« Reply #4 on: August 24, 2012, 12:57:52 PM »

Quote from: G7MRV

...the angle between the elements of each seperate dipole i guess is 60 degrees (6 elements equally spaced around a circle).



There normally would be relatively little interaction with the elements that far apart.



Quote

The dipole lengths were taken from an online calculator that automatically reduced the length by 5% for the 45deg angles, with just a little added to allow for adjustments.



That is one possible problem:  there are many factors that affect the resonant frequency of
a given length antenna:  the wire diameter, insulation type, wire angle, ground characteristics,
how the ends are attached to the insulators, etc.  That's why most of us cut the antenna wire
long to start with, then trim it to get the SWR dip centered in the band.


Quote

Sadly, since this is on a fibreglass telescopic mast, theres no way i can use the analyzer at the feedpoint, and theres a 10m run of RG-58 between. However, i would have thought that if i was close with the initial measurements, ie the dipole was close to resonance where needed, the effect of the coax would be minimal? Perhaps its that effect im seeing?



The effect of the coax depends on what measurements you are taking.  The frequency at
which the SWR is lowest stays the same when you add coax, so that is a perfectly reliable
measurement that can be done through a long length.  (Well, not perfectly reliable if
you don't have an effective balun, as the outside of the coax is part of the antenna, and
changing that changes the antenna.)

The measured antenna impedance varies in a predictable manner, making circles around
the Smith Chart.  The range of impedances is smaller the closer the match, but the frequency
at which X = 0 varies in a very complex manner (as it depends on the variation of the
antenna feedpoint over frequency.)  Tuning an antenna for X = 0 at the end of the feedline
is NOT a reliable method of adjusting the antenna, unless you correct for the feedline length.


Quote

If i can reasonably rule out interactions between the dipoles in this configuration, then i can happily lay the blame on the coax.



I don't think either is to blame.  Well, I suppose it is possible that you got some 75 ohm
coax by mistake, but so far I haven't seen any indications that there is a problem with
the coax, and I would expect minimal interaction among the wires with 60 degree spacing.



Quote

Im going to recheck the lengths on this one, in case i have done something silly with the cutting. I can also test the 17m dipole on its own.



The first approach is to sweep the SWR analyzer through the bands and record the frequency
of minimum SWR for each band - it might not be inside the band edges.  Use that frequency
to determine whether to shorten or lengthen the wires.  When you get all the minimum SWR
points where you want them, the antenna is resonant (or close enough). 

That doesn't mean that the SWR will be low - those are two different factors.  The resonance
is set by adjusting the wire length.  The SWR at resonance is determined by the feedpoint
resistance:  making the wires flatter will increase this, reducing the angle will lower it.  Once
you get the antenna resonant in the band, you can measure the impedance at the end of the
coax and use the VK1OD calculator as described previously to determine the actual resistance
at the feedpoint:  it might be 28 ohms or 90 ohms, both of which would give an SWR of 1.8 : 1.
Knowing that, you can try adjusting the vertical angle of the wires to reduce the SWR.


But the first step is to plot the SWR vs. frequency across each band of interest: you want
the frequencies of minimum SWR to be inside (or close to) the bands of operation.  That is
the most reliable way to adjust your antennas - and it is very common that they need some
adjustment once installed, rather than just relying on traditional cutting formulas (which are
only good as an estimate anyway.)
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G7MRV
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« Reply #5 on: September 04, 2012, 09:26:44 AM »

Well, there was no further time available to test it anymore, so it had to be used as was. I took an ATU just in case (wasnt planning on that, hence the fan dipole idea).

On site, and with the extra 30m of RG-213 to take the feed to the 'shack', the 17m element showed a usable swr of just under 1.5:1. Whether that was down to extra losses in the longer feeder, or just the vagaries of the site and the wire positions/tensions, i guess i'll never know.

However, i have learnt a few lessons from the event and from the antenna. I'll share them here in case they have any small use to anyone -

1. Putting on a 100w Special Event station on the same weekend as Field Day, and being limited to three bands, two of which are in use by contesters, severely limits your capabilities!


2. The 45degree angle used in this design meant the antenna was not very efficient. A shallower angle would be better, but we had to work within the space alloted, and had gone for a 8m radius (it turned out we could have used much more space, but we didnt know that)


3. Although a fibreglass fishing pole mast works well, the number of guy lines and antenna wires/support lines meant this design is not trmendously practical! Much tangling of string resulted!

It was a fun experiment, but i think in future events we'll go back to the old standby of a doublet, and beg or borrow more ground space from the organisers! At least then we'll be able to get clear of the contesters and hit more WARC bands!

Thanks guys for all the advice
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W0FM
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« Reply #6 on: September 04, 2012, 11:15:31 AM »

I tried to add 17M to my Alpha-Delta DX-EE (40, 20, 15 & 10M) parallel dipole and could never get 17M to coexist with the other three elements.  Adding the 17M element threw the entire array out of whack.  The antenna slopes a bit in an inverted "V" but not as steep an angle as yours (closer to 120 degrees than 95).  I ended up building a second parallel dipole for 30-17-12M with its own feed, and now both parallel (fan) dipoles are happily matched.

I deducted (with no technical certainy or documentation) that the single feed point of the DX-EE just did not like mixing with a 17M element, no matter how I tried adjusting the lengths.

Good luck with the project.

73 de Terry, WØFM
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G4AON
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« Reply #7 on: September 05, 2012, 01:14:53 AM »

Im working on a three band inverted V fan dipole, for 40m, 20m and 17m. I have the 40 and 20m dipoles working ok, but for some reason cant get the 17m one to resonance

There is no free lunch in parallel wire antennas, sooner or later there will be a clash of frequencies where you can't get a good match. You might try adding traps for 14 MHz to your 20m antenna with the end wires matched to 40m, giving a 20/40 trap dipole with a 17m dipole in parallel. I have not tried that particular combination, but at first glance it's worth a try and costs little.

I have an 80/40 trap dipole with a 20m dipole in parallel. The 20m dipole is spaced approx 6" from the larger dipole using 1/4" fibreglass tubes held in place with self amalgamating tape. The ends of the 20m dipole have an insulator and short length of cord to the main dipole. There are 5 spacers on each side of the dipole. 1/4" fibreglass tubing can be obtained from kite suppliers, including some on eBay.

SWR on all three bands is excellent.

You can easily home brew traps if you have a GDO, 7.5 KV 25 pF "door knob" capacitors and plastic waste water pipe work well. There are further details on trap building at my vertical antenna page (www.astromag.co.uk/vertical/).

73 Dave
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