[Antennaware] HF Colinear Dipoles

Terry Conboy n6ry at arrl.net
Fri Aug 30 13:41:22 EDT 2019

I've attached copies of the AutoEZ.weq files for the 2 element col(l)inear
antenna with Karin's parameters.  I added the 2.8 foot lengths of RG-400 in
the baluns (great attention to detail, Karin).  The end spacing of the
elements is 4 feet.  Ground type is Real/High Accuracy with "Sandy" (0.002,
10) soil.

One version is the in-phase configuration and the other is out-of-phase.  I
let AutoEZ optimize the element lengths and main feeder lengths to get
minimum SWR at 14.175.  With the #18 wire up 25 feet, the elements are
33.73 feet long and the Bury Flex lines are 24.66 feet long.  This gives an
SWR of 1.08 at mid-band and 1.15 at the band edges for the in-phase
configuration (no matching network used).  I was amazed that the 2:1 SWR
bandwidth is about 1.9 MHz and the pattern stays quite clean over that
range, too.

For the out-of-phase configuration, the common point impedance drops to
30.4 ohms (nearly non-reactive), so I let AutoEZ design a lowpass L network
with a series 0.27 uH inductor and shunt 180 pF cap to get the mid-band Z
to 50 ohms.  SWR at the band edges is under 1.4.  The 2:1 bandwidth is 800
kHz and again the pattern stays quite clean over this range.

Alternatively, the main feed lines can be 53 feet each if the 24.66 lines
don't reach mechanically or they wind up running too closely parallel to
the radiating elements for comfort.

AutoEZ can also optimize the gain by varying the spacing between the ends
of the elements, but doing so allows some fairly large side lobes to
develop.  With 4 foot spacing, the broadside gain is 7.05 dBi at 30 degree
elevation and side nulls are at -30 dB.  With 37 foot spacing, the gain is
8.24 dBi broadside, but with sidelobes only 8.3 dB down at +/-60 degrees.
The approximately one wavelength feedlines are needed for this, of course.

Keep clam,
Terry N6RY

On Thu, Aug 29, 2019 at 5:41 PM Gedas <w8bya at mchsi.com> wrote:

> Hi Terry. I was wondering if you could share your AutoEZ file with me? I
> am just starting to play with it and this would be neat. TU !
> Gedas, W8BYA
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them speak.
> On 8/29/2019 6:54 PM, Terry Conboy wrote:
> Hi Karin,
> I looked at your model, and it looks pretty good.  One change I would make would be to use the more accurate Real/High Accuracy ground type instead of Real/MININEC ground.  The gain and impedance differences between them aren't too great at 25 foot AGL on 20m, but it’s almost always a good idea to use that ground with the NEC2 engine unless you have antennas that need a hard ground connection.  The calculation is a little slower, but this model is simple enough that the calculation time is very short either way.
> I’m curious about the two types of feed line, one with VF=0.695 and other other with VF=0.82.  It appears that your feed line just reaches from the element feed points to the common point just above the ground.  You’d have a bit more slack if the whole feed used the 0.82 VF line (or you could use a full wavelength line instead of the 1/2 wavelength line).  Not that it’s a bit deal, but line loss for LMR-400 at 14 MHz is 0.47 dB/100 ft and RG-213 is 0.65 dB/100 ft, but maybe you have some Andrew Heliax or equivalent.
> Since you can adjust the length of the elements to give a 2:1 SWR on the line and then adjust the line length slightly away from 1/2 wavelength to give 100 ohms resistive at the common point, you should be able to get 1:1 exactly at the junction.  These lengths will vary if you change the height of the elements or their spacing, of course.
> Is the function of the L-C network to get the common point Z back to 50 ohms when the elements are out-of-phase?  Perhaps I missed that detail.  I’m jealous that you have a supply of vacuum relays and capacitors at your disposal!
> BTW, I created a version of this model in AutoEZ, so it can optimize the element and feeder lengths for SWR and/or gain.  It saves a lot of manual adjustment for lazy people like me.
> Good luck dodging the high winds.
> 73, Terry N6RY
> On 2019 Aug 29, at 11:53 AM, Karin Johnson <karinann at tampabay.rr.com> <karinann at tampabay.rr.com> wrote:
> Hi Terry:
> I didn't go much beyond the simple LC matching that just jumped out at me
> when I saw the plot from the attached EZNEC file.  I can't absolutely say
> that I can get this pattern from a real world build, but it looks
> encouraging.
> I've done remote relay switching before, in fact my 80/40 vertical does just
> that using two vacuum relays.  What's interesting about a remote matching
> system, other than the need to supply switchable DC at the antenna, is that
> with the addition of a 150 pF capacitor to ground just ahead of the 0.25 uH
> inductor you can match the antenna to very close to 50 for the feedline.  It
> requires a bit of messing around with the length of the two dipoles but it
> can be done, at least as far as EZNEC is concerned.  I've got an HP8753C VNA
> so when I get around to putting the wires up I can at least see what the
> feed point impedance of each dipole is and then design any matching network
> around the measured values.
> Of course all of this will have to wait for cooler weather and the lack of
> Hurricanes here in Florida.  Luckily I'm on the west coast so it shouldn't
> be too bad.  Went through three of them in 2004, with barely a small amount
> of broken tree limbs in the yard.
> Cheers for now,
> Karin
> -----Original Message-----
> From: Terry Conboy [mailto:terry.conboy at gmail.com <terry.conboy at gmail.com>] On Behalf Of Terry Conboy
> Sent: Thursday, August 29, 2019 12:56 PM
> To: Karin Johnson
> Cc: antennaware at contesting.com; weinfurt at ohio.edu
> Subject: Re: HF Colinear Dipoles
> Karin,
> It's hard to beat horizontal antennas on 20m and up due to the ground
> reflection gain.  Same for 40m if you can get a decent height (> .5 wl).
> As for matching the switched 2-element collinear, after I sent out those
> models, it occurred to me that you feed them quite simply with 3/4 wl 75 ohm
> lines from each antenna, then in the shack, use two 1/4 wl 75 ohm lines
> connected in line with the two feeders.  For in-phase, feed at the junction
> of the two 1/4 wl lines (total 1 wl to each antenna) or for out-of-phase,
> feed at the junction of one 3/4 wl feeder and a 1/4 wl line (so 3/4 wl to
> one element and 5/4 wl total to the other).  Due to the mutual impedances,
> the elements are ~100 ohms in-phase and ~50 ohms out-of-phase, so this
> should give you 50 ohms at the common point for both patterns and switchable
> 0 - 180 phase with just a SPDT switch or relay.
> There shouldn't be a need for L & C matching, unless you are really picky
> about SWR.  Of course, if you use wider spacing between the elements, the
> element drive impedances will move closer to 75 ohms and things get more
> complex.
> A bigger question that has always bugged me: why are there two L's in
> collinear?
> 73, Terry N6RY
> On 2019 Aug 28, at 9:50 AM, Karin Johnson <karinann at tampabay.rr.com> <karinann at tampabay.rr.com>
> wrote:
> Hello Terry and Greg:
> I appreciate the responses and the models.  I also received an email from
> Gedas who pointed me in the right direction.  First off let me say I'm not
> a novice to this task.  Although most of my efforts on antenna design in
> the professional space have been in the 2 GHz and up arena.  I've actually
> got some patents on some of the designs.  With regards to that task HFSS,
> a
> very expensive modeling software is a joy to use.  Now with respect to HF
> antennas.  What got in the way for me was the matching task and the
> influence of one dipole on the other.
> Although the Low Band DXing book addresses verticals the basic concept of
> mutual coupling still exists.  You can see this effect if you model two
> collinear dipoles, look at the source data, then remove one of the dipoles
> and look at the source data.  The source impedance at the generator will
> be
> different.  Gedas sent me an article he wrote some time ago and this
> set off the light bulb in my brain.  From there I've been able to make
> some
> progress.  The main reason for me doing all of this is curiosity.  Right
> now
> I have a 40 meter dipole in place and have recently put up a switched
> 80/40
> meter vertical.  I find that I am almost always using the vertical now and
> have toyed with the idea of a gain antenna for 20 meters to take the place
> of the 40 meter dipole.  I also have another 20 meter dipole that will
> remain in place but will be oriented 90 degrees to the collinear array if
> I
> decide to build it.  So for now this is mainly an academic exercise.  With
> regard to some of the simulations I have done with EZNEC I've found a
> really
> simple way to match both the in phase and out of phase arrangement of the
> two dipoles in collinear orientation.  It does require some vacuum relays
> at
> the junction point of the two feed lines but this is very doable.  I also
> need a 150 pF vacuum cap, and a small 0.25uH inductor.  I know vacuum cap
> probably is overkill but I tend to over design things.
> Regards,
> Karin   K3UU
> <GudColinear_20m.EZ>
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