antenna 303wa-2 apex schematic
- Thread starter netosilva
- Start date
Test Case 1: a 1.8 m (6 ft.) vertical whip
Preferred broadband match = 100:1 transformer
On the face of it, one would think that this is much too small an antenna to deliver any useful
signal, without amplification, at frequencies much lower than 5 MHz. The usual approach is to
use a high to low impedance buffer amplifier (such as a TI / Burr-Brown BUF634) to couple a
short “E-field probe” such as this to a 50-ohm input receiver. And this is exactly what you
should do … most of the time.
A drawback is that amplifiers can cause intermodulation distortion (IMD) products at strong
signal locations. A strong signal on 6100 kHz and another strong signal on 7200 kHz can react
in the amplifier to produce undesired second order distortion signals at the difference frequency
of 1100 kHz and at the sum frequency of 13300 kHz. Third order products can occur at the
lower frequency minus the spacing (6100-1100=5000) and at the higher frequency plus the
spacing (7200+1100=8300). Urban areas cause the greatest amplifier stress. Dallas Lankford, in
a project done for the AMRAD group, has designed a very high intercept point amplifier, but it
requires a 28 volt DC supply and an expensive transistor.
Another problem is that, to maximize signal-to-noise ratio, such short whips are placed outdoors,
often on a high mast or roof, exposed to the weather. The amplifier, co-located with the antenna,
is more likely to fail in this setting than if indoors. If the whip could be efficiently and passively
coupled to the 50-ohm feedline, a durable urban-capable antenna can be set up outside. With
proper weatherproofing of the transformer box and feedline connection, a very low maintenance
antenna can be had. Amplification, if still needed, can be applied indoors at the operating
position. Most likely, in the city, you’d be using high-Q regenerative preamplification rather
than something that’s broadband. In less RF-intense areas, you could use a W7IUV or other 50-
ohm in / out broadband amplifier at the receiving position.
Three matching transformers were tried. Best results were with a 100:1 autotransformer
consisting of 70 turns total on an FT140-J core. This winding occupied about two-thirds of the
core. One end of the 70-turn winding went to the whip and the other end to ground. The low
impedance output was from a tap 7 turns from the ground end and 63 turns from the antenna end.
Nearly as good results were obtained with a 64:1 transformer consisting of 64 turns opposite 8
turns on an FT114-J core. Readings across the 380 to 2000 kHz range were within 2 dB of those
obtained with the 100:1 version. In third place was the Mini-Circuits T36-1-X65 transformer.
Its output level trailed the homebrew 64:1 and 100:1 models by about 3 dB around 1600 kHz and
by at least 6 dB down below 800 kHz.
Preferred broadband match = 100:1 transformer
On the face of it, one would think that this is much too small an antenna to deliver any useful
signal, without amplification, at frequencies much lower than 5 MHz. The usual approach is to
use a high to low impedance buffer amplifier (such as a TI / Burr-Brown BUF634) to couple a
short “E-field probe” such as this to a 50-ohm input receiver. And this is exactly what you
should do … most of the time.
A drawback is that amplifiers can cause intermodulation distortion (IMD) products at strong
signal locations. A strong signal on 6100 kHz and another strong signal on 7200 kHz can react
in the amplifier to produce undesired second order distortion signals at the difference frequency
of 1100 kHz and at the sum frequency of 13300 kHz. Third order products can occur at the
lower frequency minus the spacing (6100-1100=5000) and at the higher frequency plus the
spacing (7200+1100=8300). Urban areas cause the greatest amplifier stress. Dallas Lankford, in
a project done for the AMRAD group, has designed a very high intercept point amplifier, but it
requires a 28 volt DC supply and an expensive transistor.
Another problem is that, to maximize signal-to-noise ratio, such short whips are placed outdoors,
often on a high mast or roof, exposed to the weather. The amplifier, co-located with the antenna,
is more likely to fail in this setting than if indoors. If the whip could be efficiently and passively
coupled to the 50-ohm feedline, a durable urban-capable antenna can be set up outside. With
proper weatherproofing of the transformer box and feedline connection, a very low maintenance
antenna can be had. Amplification, if still needed, can be applied indoors at the operating
position. Most likely, in the city, you’d be using high-Q regenerative preamplification rather
than something that’s broadband. In less RF-intense areas, you could use a W7IUV or other 50-
ohm in / out broadband amplifier at the receiving position.
Three matching transformers were tried. Best results were with a 100:1 autotransformer
consisting of 70 turns total on an FT140-J core. This winding occupied about two-thirds of the
core. One end of the 70-turn winding went to the whip and the other end to ground. The low
impedance output was from a tap 7 turns from the ground end and 63 turns from the antenna end.
Nearly as good results were obtained with a 64:1 transformer consisting of 64 turns opposite 8
turns on an FT114-J core. Readings across the 380 to 2000 kHz range were within 2 dB of those
obtained with the 100:1 version. In third place was the Mini-Circuits T36-1-X65 transformer.
Its output level trailed the homebrew 64:1 and 100:1 models by about 3 dB around 1600 kHz and
by at least 6 dB down below 800 kHz.
Nice work. I recall reading a USN test where they determined a 6ft vertical rx antenna did very well on mf/hf circuits when matched properly. That said, they were using an entire ocean underneath a conductive warship as groundplane for that 6ft piece of wire.
When I was a wee lad I had a 11m vertical up about 1wl and it did fantastic on general hf rx, after I grounded it properly it worked spectactularly from vlf up. I suppose it had something to do with being set up over some of the most conductive soil in the US per fcc soil conductivity maps.
Also the grounding I employed was the other part of the "dipole", kinda like the warship/ocean combination was likely the main contributor to the success of the 6ft piece of wire as an rx antenna in the USN testing.
I knew little to nothing about matching back when I was a wee lad, now I understand the need to match transmission lead to an antenna to maximise transfer of power from one to the other. The game is efficiency.
When I was a wee lad I had a 11m vertical up about 1wl and it did fantastic on general hf rx, after I grounded it properly it worked spectactularly from vlf up. I suppose it had something to do with being set up over some of the most conductive soil in the US per fcc soil conductivity maps.
Also the grounding I employed was the other part of the "dipole", kinda like the warship/ocean combination was likely the main contributor to the success of the 6ft piece of wire as an rx antenna in the USN testing.
I knew little to nothing about matching back when I was a wee lad, now I understand the need to match transmission lead to an antenna to maximise transfer of power from one to the other. The game is efficiency.
I built an AMRAD active whip using the pricy transistor and it works really well. If the transistor and circuit boards are still available I would highly recommend it.
discovery the secret how the balun works (coupling coil), I made a balun and connected it to my 1.60m antenna, it worked perfect from 100 KHz to 30 MHz. the secret is in the balun inside the antenna.
see the balun, primary (connected to the antenna and ground) 4 groups of 8 turns, secondary 4 turns (connected to the antenna capacitor 470nf and ground), every 8 turns of the primary 1 turn of the secondary. totaling 32 primary turns and 4 secondary turns. in the photo below balun of the antenna AOR SA7000, same size of the ApexRadio 303WA-2, 1.80m.
see the balun, primary (connected to the antenna and ground) 4 groups of 8 turns, secondary 4 turns (connected to the antenna capacitor 470nf and ground), every 8 turns of the primary 1 turn of the secondary. totaling 32 primary turns and 4 secondary turns. in the photo below balun of the antenna AOR SA7000, same size of the ApexRadio 303WA-2, 1.80m.
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