[Antennaware] HF Colinear Dipoles

Terry Conboy n6ry at arrl.net
Thu Aug 29 18:54:01 EDT 2019

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> 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] 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>
> 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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