Hi Rys,
The terminationresistance of a BOG (including the loss in the
ground rod and/or radials) is typically about 250 ohms. The value is
non-critical, especially in the case of a BOG where most of the f/b
ratio derives from the lossy ground in very close proximity to the
antenna and not from the termination resistor.
A 4:1 transformer works well for 75 ohm coax
A 6:1 transformer may a better choice for 50 ohm coax,
depending on the optimum termination resistance.
In either case, the termination resistor attached to the unused coax
(approximately 50 or 75 ohms) can be optimized to improve the
front-to-back ratio.
73
Frank
W3LPL
----- Original Message -----
From: "Ryszard Tymkiewicz" <rtym@ippt.pan.pl>
To: donovanf@starpower.net
Sent: Wednesday, January 10, 2018 9:55:34 PM
Subject: Re: Topband: Supporting Ladder line
Hi Frank...
Yes I used to have reversible beverages even 15 years ago and of course I
used these terminated resistors...but I was not sure about BOGs and RG-174..
now its clear..almost..why 6:1 transformer and not 4:1? (the end of your
e-mail)
..THANKS a lot
Rys
SP5EWY
W dniu 2018-01-10 o 22:31, donovanf@starpower.net pisze:
Hi Rys,
Simply change the turns ratios of the reflection transformer and
matching transformer to correspond to the approximate termination
impedance and feed impedance. 4:1 transformers work well for a
reversible BOG and a 9:1 transformers works well for an elevated
Beverage.
For a reversible Beverage, its very important that the unused coax
be terminated in approximately a 75 ohm resistor. If you use a single
coax feed, the switch box at the feedpoint must use a resistor to
terminate the unused feed transformer. Its possible to optimize the
termination resistor value to improve the front-to-back ratio, but the
improvement is small especially in the case of a BOG.
RG-174 has approximately 1 dB loss per 100 feet on 160 meters which
is quite acceptable for a 220 foot BOG. You should use 6:1 transformers.
73
Frank
W3LPL
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|