Topband: Several BOG questions

[Guy Olinger K2AV]

Hi Mike et al,

BOGs have a well deserved reputation for being cantankerous, with great
results for K4ABCD over here and awful results for K3WXYZ over there. There
is a part of the explanation for that variation that really isn't fixable.
But better outcomes are possible with a more detailed explanation of how a
BOG works, and understanding its significant differences with a standard
beverage RX antenna.

First off, a normal beverage will have a velocity factor (VF) of ~ 95%,
maybe a little more if it is high. That means that the VF can be ignored,
and all the well-documented regular beverage understandings and procedures
apply. However...

When you put the wire on/in the ground, notched below the grass, etc,
**measured** VFs in my area have ranged anywhere between 45% and 85%. This
means a lot we understand about a beverage antenna does not apply. We
measured VF by finding the resonance of a 151 foot (46 meter) dipole on
ground (DOG), laid in the same place and manner as an intended BOG.

The most common VF's were in the high 50% range. The values did not seem
related to any local factor easily exposed. There were times when rotating
the DOG around the center 90 degrees made a large change in VF, even though
the owner of the property insisted there was no piping, cable or drainage
fields beneath and nothing overhead. Presumably exact composition of dirt,
rocks, and varying water table or water presence, were responsible. This in
turn varies the response to the specifics from site to site.

Modeling a BOG in NEC4 exposes the issue: the incident incoming RF is
traveling at almost speed of light, and the RF already collected at a given
point has traveled on the wire at only half that. A physical quarter wave
of wire on ground appears roughly like a slowed down half wave. This causes
RF energy on the wire from the far end to be nearly out of phase with
incident RF near the feedpoint. BOG lengths beyond 250 feet can actually
start to reverse their pattern at higher frequencies. This makes a BOG a
one band antenna for front-to-back.

220 feet terminated by a pair of those 450 ohm beverage termination
resistors in parallel usually seems to do "passable" for a quick
slam-it-down-and-go-operate-in-a-contest five minutes later. But neither
length or termination is predictable to anything like the same degree as a
normal beverage.

A BOG is lossy. Frequently a BOG will work better with a
properly-feedline-isolated remote preamp.

A decently-centered BOG with a modicum of F/B can be had by first
installing it as a DOG. If you live in a primarily arid area then do this
exercise when it is dry. If wet or damp is common in the 160 "season", do
it in a wet time, so the ground is NOT dry.

Operating later, if things have gone dry, if you installed it in the wet
you can always go "water the BOG" before an important operating event.
W0UCE referred to this ritual as "water the boggies".

To create a location specific BOG, using 1.825 MHz as center:

Starting with a 250 feet length of PE or teflon insulated wire, lay the
wire out in the exact line and manner as you intend for the BOG. Bend the
end tips of the wire up an inch in the air so they don't short to the
ground. Break the center of the wire and temporarily attach an SO239. You
have created a DOG which you will now prune to vagaries specific to your
location and placement.

Use your RF analyzer at the center to find the lowest frequency resonance,
which will be well down in the AM broadcast band somewhere. The R where X=0
will usually be anything from 100 to 250 ohms. This is not critical.

***Trim equal lengths from the ends*** to move the resonance (X = 0) up to
1140 kHz (one point one four zero MHz). R is immaterial. Don't get in a
hurry, the effect of cutting off a foot (6 inches both ends) is often not
linear because the instant VF at a point on the wire most often is NOT
uniform across the wire. Only when you have trimmed the DOG to 1140 kHz in
this manner, reconnect and re-insulate the center, and only then place feed
transformer, termination and run feedline. A BOG done in this fashion will
have a predictable pattern (for a BOG) which makes best use of the fast vs.
slow RF velocities involved.

A ground of two equal length 40, 50 foot bare wires notched 2, 3, 4 inches
into the ground at right angles to the main wire will do as well as needed.
50 feet either way seems to be enough, though this is a very fuzzy length,
and will never be precise. Terminate the far end with 225 ohms (two 450's
in parallel) to ground. Less than 40 feet may start to lose signal transfer
to the feed transformer increasing an R in series with the primary winding.
But do what you can

In poor ground, a short ground rod by itself might be an adequate
termination. This provides a termination by providing a very resistive
ground, quite unsatisfactory for other applications. You need SOME
termination R, but the value is not critical. Any precise F/B with variable
R listening to a back side signal will **vary significantly** in the
weather with how damp your ground is, how far down in the lawn or leaves
your wire is TODAY vs. when it was set up, etc, etc. It's just not worth
it. You can set it right now, but an hour with sunshine, or an hour with
settling dew, will UN-set it for you. Soaking rain could throw it off the
cliff. Luckily the pattern does not depend so much on termination as the
VF.

For far end termination, use 225 ohms made from commonly available 450 ohm
resistors made for the application, or try poorly grounding it. Note that
this poor ground does not work so good at the feed end, where you really
need the wires.

The regular *beverage* procedure (looking at SWR through the feed
transformer to the far end to set termination value) does not work well on
a BOG. This is quite obvious in NEC4 analysis of the issues. The best
solution involves a devil's balance of the reflected power *and* the
incident VF *and* the along-the-wire VFs. It will not be found with SWR,
quite contrary to a standard beverage.

The lack of cutting BOG as DOG to 1140 kHz, thereby not incorporating local
conditions and VF, is probably most responsible for poor initial
performance. In a close second for screwing up results, varying wetness and
varying "buried-ness" are most responsible for changes for the worse across
time.

The NEC4 modeled BOG incoming pattern at best does not improve over a
well-done K9AY. But if you get it to work, it's "invisibility factor" could
be priceless. Further, the BOG seems excellent at rejecting very low angle
vertically polarized local noise sources off its sides to a degree I've not
seen with a K9AY.

W0UCE, out in the woods at an earlier QTH, utilized a pair of phased
side-by-side 220 foot BOGS spaced 200 feet for NE and a similar pair for
NW. I operated with those and found them excellent RX antennas, far better
than listening on any of his TX antennas. But more than once we had to go
out with a hose and soak them in very dry spells. Performance did suffer
with leaf buildup and when we finally figured that out, we had to pull the
wires up to get them on top of the leaves. The BOGs DID need a preamp, my
opinion, a conclusion not shared by all those who operated out there.

73 and good luck with yours,

Guy K2AV


On Mon, Aug 6, 2018 at 1:06 PM, Mikek <amdx at knology.net> wrote:

>  I have installed a 253ft Bog,
>  I used WD-1A a pair and just twisted the two wires together.
>
> Any reason I should separate the pair and just use a single wire?
>
> Can the WD-1A be used for a Bi-directional BOG, or is the spacing too
> close?
>
> I have installed a variable termination on the North end of the BOG and a
> matching transformer
> on the South end, for receiving from the north. I'm in the Florida
> panhandle about 1 mile from the gulf.
>
>  I'm a little disappointed that the Variable termination does not have
> much effect between 200 ohms and 1300 ohms.
>  I use AM Band stations as a guide to the directionality of the BOG and it
> does seem to have good directionality,
> just no noticeable difference while adjusting the termination. It is also
> quiet when compared to a 150ft long wire.
>
> In an effort to make the antenna seem 'apparently' longer I have loaded
> the BOG with 9 ferrite binocular cores.
> Using info from W8JI's page,
>
>> https://www.w8ji.com/slinky_and_loaded_beverages.htm
>>
> I originally measured my Velocity Factor as .78 and by installing the
> cores I was shooting for a VF of .55
>
>  I made a single pass through a single hole in the binocular core.
> A single pass through a single hole in the binocular core is about 8.9uh.
> My calculation said I needed
> 11.8uh, but I installed all the cores I had and ended up with a VF of .67,
> lower but... when I get more cores
> I'll install them and test VF again.
>
>  Your thoughts?
>
>
> I have measured my ground rod resistance using the method on page 24 of
> the this pdf.
>
> www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf
>>
>
>  I got 117 ohms on both ends, I bought some calcium chloride and put 2.5
> lbs into and
> around my 3 copper pipe rods. I pre-drilled the rods every 3" to allow the
> solution to
> distribute more evenly around the pipes. (5ft rods/pipes) I did this at
> one end.
> A day later I retested the ground rod resistance, and it was 73 ohms, some
> improvement,
> and another couple days I remeasured and it was down to 60 ohms.
>  A week later I added more Calcium chloride and I'm at 34 ohms now.
>  I just finished retesting ground resistance, North end 34 ohms, South end
> 23 ohms.
>
> Is the method in the pdf article a reasonable way to measure ground
> resistance?
>  I used 60 hz, because it's available. My last test used ~ 25V and
> supplied 600 mA.
>  I have no other AC source at a different frequency that would apply that
> much power.
>
> I bought a 74AC14 TDR and used it to measure the characteristic impedance
> and VF.
>
> I measured the Characteristic Impedance, which I then use to calculate the
> matching transformer.
> Does the ground resistance show as part of the Characteristic Impedance?
> ie. If my ground was 117 ohms and I treated it and it is now 23 ohms, will
> my Characteristic Impedance,
> impedance drop by 94 ohms?
>
> I measured my Characteristic Impedance as 358 ohms before Binocular core
> installation,
> and measured 501 ohms after Binocular core installation.
>
>  OK guys, this is my first post, there are a few questions, I hope to get
> some help on this project.
> btw, I did make a video showing how it compares to my long wire antenna
> using AM band stations.
>
>> https://www.youtube.com/watch?v=QuMwGDK6IhQ&t=51s
>>
>                     Thanks, Mike Knowlton
> _________________
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