Topband: Bowtie Flags

[Mark Connelly]

As Nick undoubtedly knows, Bowtie antennas have been in use by medium wave broadcast band DXers for some time now.

One I tested here in 2010 is shown in this sketch:
http://www.qsl.net/wa1ion/pictures1/bowtie_s_yarmouth.gif

Some use a dual feedline scheme so the pattern can be reversed.  This would be implemented by 16:1 transformers at each end if 50-75 ohm coaxial cable is used or 9:1 transformers if using 90-150 ohm balanced feed (CAT-5, speaker wire, two conductor zip / lamp cord, twisted pair etc.) as recommended by Dallas Lankford and others.  In the balanced feedline case, a 1.8:1 or 2:1 transformer (e.g. #73 binocular core with 4 turns : 3 turns or 7 turns : 5 turns) is used in-shack to get to unbalanced coaxial en route to the receiver / transceiver, RPA-1 or similar amp, or phasing unit.

Dual feedline split flag is another variant:
http://www.qsl.net/wa1ion/pictures1/dual_feed_split_flag.gif

Dual feedline antennas have a 4-pole 2-throw switch at the shack end so either end can be switched to the receiver path and the opposite end to a 250 or 500 ohm pot that facilitates in-shack null termination adjustment.

Some people go with a single orientation antenna and put a preamp such as the Wellbrook FLG100LN ( http://www.wellbrook.uk.com/FLG100LN-1 ) right at the forward side.  This tends to overwhelm any feedline pick-up (common-mode or otherwise) although common-mode choking is still advisable near the antenna when longer runs (over 60m / 200 ft.) of feedline are involved.

In the case of a single orientation set-up, you then get to figure out what termination end scheme is best.  These basically boil down to the following options:
(1) Fixed resistor in the 680-1200 ohm range either modelled / guessed-at or determined by use of a potentiometer if it can somehow be adjusted midway up the side of the antenna while monitoring a target station to null.
(2) "Vactrol" termination: a photoresistor adjusted by a DC voltage applied on a control line from the shack: see page 9 of http://www.durenberger.com/documents/PRESBT2016.pdf
(3) Potentiometer adjusted by a motor drive: see http://www.bamlog.com/remotepotbox.htm

My experience with the Bowtie I installed in 2010 was that it was about 10-12 dB less sensitive than a Flag or Kaz Delta occupying the same rectangular "box" of air space.

Nulling off the back could be wider than that of a Flag but the bandwidth at which the null stayed optimum at a particular terminating resistance was not as wide as with the Flag (or its SuperLoop base-fed variation: http://www.bamlog.com/superloop.htm ).

Seeing that the MW broadcast band 530-1710 kHz has a greater max/min frequency range ratio than the 1800-2000 kHz of 160m, issues that might be problematic for broadcast DXers could matter a lot less on 160.

Some of the perceived problems of low gain and null bandwidth could easily have been cured by a combination of rigorous common-mode choking as done on the Waller Flag ( http://www.kkn.net/dayton2011/N4ISWallerFlag.pdf ) and high gain at-antenna amplification.
 
Mark Connelly, WA1ION
South Yarmouth, MA

<<
Nick,?

?
Although I did not measure it, the front to back, and front to side is
better, overall more quiet.
?
It is receiving toward the Caribbean.?
Europe, and USA stations, are lower in signal strength.
?
Can often work?Caribbean and South American stations through a small
pile up when they are not working split.
?
In my case it was a worth while change.
?
The original plan was to be able to drive my car under it.
?
73
Bruce-K1FZ
?

On Mon, 12 Dec 2016 16:35:14 +0000, Nick Hall-Patch wrote:

What specific improvements did you note when the Delta was raised Bruce?

Thanks,

Nick
VE7DXR
>>