An alternate procedure to the cut and prune to 1140 kHz method, if one does
not have an analyzer that goes down there, is to lay down a measured 151 ft
(46m) as a DOG. This will usually measure somewhere in or around the 160
meter band. Remember to install the DOG in the same manner as you intend
for the final installation. Determine the resonance. Then the bog length
is:
220 feet (67m) times 151' (46m) DOG resonance in MHz divided by 1.750 MHz
The drawback of this method is that there may be something different about
the dirt NOT underneath the 151 feet and take the BOG off spec. The method
will work fine if the dirt and conditions are essentially the same
everywhere under the longer wire.
73, Guy
On Mon, Oct 17, 2011 at 9:39 AM, Guy Olinger K2AV <olinger@bellsouth.net>wrote:
> Based on typical figures for velocity factor measured at various sites
> around Raleigh, and the manner in which a BOG works, 500 feet is too long
> for one to work *well* and depending on your particular dirt, may have a
> "blob" pattern with no front-to-back or may even have reversed at that
> length. We have measured VF's between 45% and 80%. This sometimes varies
> by position on the same property. It cannot be read off a table.
>
> The way to set a BOG, is to place 300+ feet down installing it AS IT WILL
> BE USED WHEN DONE. Notch it into the dirt or sod BEFORE measurement if that
> is how you want it to be when you are done. If you are going to leave it
> laying on top of the grass all winter, then measure it that way. Velocity
> factor varies wildly with small changes in actual height above the dirt.
>
> Temporarily break it in the middle, leaving the ends up in the air a little
> so they don't short out to ground (creating a dipole on ground) and prune
> the "DOG" to resonance at 1140 kHz by adjusting the ends equally. This will
> be a broad dip but adequate, close enough. Reconnect and insulate the
> center, and terminate the end with 250-350 ohms.
>
> Note that some analyzers won't go down in the BC band. It will be worth
> your time to borrow one that does for this project, because when you are
> done, it has been fitted to YOUR dirt, which is the only dirt that counts.
>
> Take all the usual precautions at the feed regarding blocking common mode,
> etc. Not following the common mode isolation protocol will cost you a lot
> more with a BOG because the signals are ~20 dB less with a BOG than a
> beverage.
>
> BOGs vary hugely from normal beverage behavior because the signal on wire
> is traveling only one-half to two thirds speed of light. Being next to the
> ground slows it down. This means that at longer BOG lengths, near the feed,
> the accumulated incoming signal on the wire is *out of phase* with the
> incoming signal from the sky *in the desired direction*.
>
> A good working BOG length has this nasty characteristic accounted for and
> means that 500 feet is not a good design length for, even if you can hear
> stuff. You would hear more with better front to back using shorter. Also
> for the same reason, a BOG is a very poor multiband antenna, even if you can
> hear stuff. A BOG cut for 80 meters will hear better, with front to back,
> much better than on a 160 BOG.
>
> Pro's of a BOG are mainly that it needs to be shorter, and seems to reject
> nearby man-made noise much better than anything above ground.
>
> There are variants of BOGs, such as loops, vees, that have useful noise
> rejection properties and are much smaller. A 160 loop on ground is only 70
> foot square and receives in two directions perpendicular to the broadside at
> the feedpoint, e.g. fed on the east or west side, will hear north and south.
>
>
> 73, Guy.
>
>
> On Sun, Oct 16, 2011 at 4:47 PM, Art <k6xt@k6xt.com> wrote:
>
>> Topbanders
>> I just ran out a 500 ft BOG. Now its unterminated. I'd appreciate any
>> suggestions for a terminating resistor value. Or, is 500 ft a good
>> length to leave it unterminated?
>>
>> --
>> 73 Art K6XT
>>
>> _______________________________________________
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>
>
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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