On 2 dic, 13:24, "Geoffrey S. Mendelson" wrote:
I made a 300 ohm twinlead folded dipole that is resonant at around 15mHz.
It was cut because I wanted to try some 20m operation with an indoor antenna
and that fit exactly the space.
The ham I wanted to reach was km away, across a valley. I figured with a
tuner and a few watts, I could reach him.
I never used it for that, he moved before we ever got together.
I put the antenna up outside to use for an SWL antenna. It is fed with
about 6 feet of twinlead and then 75 ohm coax. It worked best at 15mHz,
and slowly degraded as frequencies got lower. It was useable at 7mHz for
reception.
At MW frequencies it's basicly a noise pickup and no signals could be
received.
I made a balun from 9 trifilar turns of wire that I had (.5mm plastic
insulated). The instructions I was following said to use 10 turns of
#14 enamel on a wider core, but 9 were all that fit on the core I had.
The core is 8mmx80mm with a permeability of 400.
When I hooked it up to the antenna, I found that the resonant frequency
shifted to 14mHz. My guess is that the feedline now is part of the antenna.
However at 7Mhz it is totally dead. So dead that a signal that reads S5 on
my receiver using a 2m JPole hung from the near end of the folded dipole
as an antenna does not move the s-meter at all. The null extends from
around 6950 kHz up to around 7300.
The question is why and what can I do to improve it?
Is it because there is null in the system because it is resonant at 14mHz and
it is unresonant at 1/2 of that? Is it because the balun acts as a 7mHz trap?
I now have some cores that are 8x140 and 10x200. If I make a balun using the
same wire (enamled wire is not available to me) with a different number of
turns, will it behave differently? For example, if I make a balun with a few
more turns, will the null move down?
My guestimate is that I can get 15 turns on a 140mm rod, and 22 slightly larger
ones on a 200mm rod. *
How far down can I go? Will moving it down affect upper frequency response?
Can I move it down far enough that the balun will do something useful
at AM broadcast frequencies?
Some of the 10mm rods were broken into pieces in shipping. Does that affect
their performance? Can I just use the pieces as they are?
Thanks in advance,
Geoff.
--
Geoffrey S. Mendelson N3OWJ/4X1GM
Those who cannot remember the past are condemned to misquote it.
Hello Geoffrey.
I think the problem is in the folded dipole. It works as long as it is
in resonance (so that current in all wires is in phase). At half the
frequency, it will behave more like a halve wave loop with very small
loop area (as the wires are in parallel). This is not an effective
arrangement. Maybe you can try to cut the short (so it becomes an
open dipole with non-straight halves).
When you live in a quiet environment, a 4:1 or 9:1 balun will increase
both signal and noise for frequencies below resonance (so that antenna
noise exceeds your receiver's noise).
Regarding the feedline between antenna and balun. If the balun does
function well, the feedline will not contribute to the radiation
pattern (current are opposite), but it will provide impedance
transformation, so it becomes part of the tuning (hence resonant
frequencies).
From your balun description, I can't figure out what you have in your
hands Maybe you have link to a description you followed. I prefer
baluns that provide common mode suppression so that when there is some
asymmetry in the system, the coaxial feeder doesn't become part of the
radiating/receiving antenna structure. A guanella type balun wound on
two magnetically separate cores/rods does provide common mode
suppression.
Regarding ferrite rods (for a 4:1 guanella balun for example), if the
broken surfaces still mate closely, just use some liquid adhesive and
join them. As most of the field lines go to air, effective
permeability doesn't change practically.
For making a really wide band balun, you need the right ferrite
material. For wide band applications I use, for example, fairrite
material 43 [ur=800] (material 31 [ur=1500] would be better). You need
a material that has high permeability at low frequency and also
reasonable permeability at high frequency. Many power ferrites have
good permeability at low frequency, but it drops rapidly at increasing
frequency.
If your choice is limited, try to get ferrite material indicated for
HF (1.. 100 MHz) EMI suppression. As it is for reception only, I would
not go for rods, but toroids. Toroids provide higher inductance at low
frequency because of the closed magnetic path (so you need less
turns).
Regarding MW broadcast reception. I get best results with vertical
polarized antennas, so your dipole may not be good for this.
Best regards,
Wim
PA3DJS
www.tetech.nl