[Antennaware] HF Colinear Dipoles

Karin Johnson karinann at tampabay.rr.com
Thu Aug 29 20:28:12 EDT 2019

Hi Terry:
All set here for the hurricane. Hopefully it will only be a 1 or 2 by the
time it gets here.
With regard to the different velocity factors.  The VF=0.695 is for the 
RG-400 used in the DX engineering 1:1 current balun.  I use them at the
Feed point of the dipoles to keep any common mode currents off the feed
lines.  The VF=0.82 is Bury Flex cable and I only show a length of about 25
feet to go from the Balun to the junction point of the two coax cables from
each dipole.  I plan to build the matching network in a small enclosure to
include a 180 degree reversing transformer from the Low Band DXin book.
Plus the switching relays and the inductor and capacitor.  From that
junction box there will be a coax run of about 85 feet to the operating
position. The length of the Bury flex cable will get me to about the middle
of the two dipole array elements.  About 18 feet direct from the feed point
of each dipole to the collinear middle point.  The extra 8 feet of coax will
permit me to locate the junction box at a diagonal from the feed points.
All of this going on the peak of the roof on my home as I live in a deed
restricted community and this will be virtually invisible from the street as
the roof line will hide the boxes.  Yes the LC components will get the total
array feed point back to about 50 ohms so I can feed this array at the
common point with 50 cable.  I have not included the LC components in the
model I sent to you.  I have a very old copy of XLZIZL from Dan that I've
completely redone for looking at the effects of coax and matching components
Makes life easier for me.


-----Original Message-----
From: Terry Conboy [mailto:terry.conboy at gmail.com] On Behalf Of Terry Conboy
Sent: Thursday, August 29, 2019 6:54 PM
To: Karin Johnson
Cc: antennaware at contesting.com; weinfurt at ohio.edu
Subject: Re: HF Colinear Dipoles

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

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>
> 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
> 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
> with the addition of a 150 pF capacitor to ground just ahead of the 0.25
> inductor you can match the antenna to very close to 50 for the feedline.
> 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
> 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
> 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
> 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
> 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
> 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
> 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
> 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
>> 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
>> 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
>> 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
>> have toyed with the idea of a gain antenna for 20 meters to take the
>> 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.
>> 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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