Receiving homemade Vertical HF antenna

[nevesjerry]

Hello to all Members
I have a question regarding constructing a Vertical antenna that would cover from 500 kHz to 30 MHz.
I do have an RF Systems passive antenna which is about 6 feet tall fed with coax cable and it gets fantastic
reception on AM band as well has SW. Obviously this antenna is fed with a coaxial cable with a PL259.
I always wonder what’s inside this unit, and I know for sure it has a wire elliptically wond inside. Other than that I have no idea. My question is when you feed a vertical antenna with coax cable would the wire inside the antenna connect to center of coax feed line, where does the braid of feed line attach to inside the vertical?
Is there a magic transformer inside the antenna? Perhaps a balun?? The commercial RF Systems vertical has no radials or a ground connection!
I am puzzled. Any help appreciated.
Best regards to all, Jerry

 

[prcguy]

If its a passive antenna then there is probably a magic transformer inside. More specifically a transformer that matches the 50 ohm coax to a very high impedance of several thousand ohms.

A very, very short whip antenna for VLF/HF in the couple of feet length will be really high impedance and possibly 2K to 5K ohms but fairly constant over VLF and the lower HF range as it will be really darn small in wavelengths until you get above 10Mhz or so. The VLF/HF section of the AOR SA7000 is one example. This type of antenna is sometimes refereed to as an E field or Voltage probe antenna.

The transformer could be an actual transformer with primary and secondary where one side of the secondary will get grounded to the shield of the coax and the other side goes to the whip, or it could be a tapped auto transformer that will have a single secondary that will only attach to the whip. Either way, going from a 3 or 6ft whip attached directly to coax vs having the magic transformer in between can make a 10dB or more difference in the signal level that reaches your receiver.

Some of the characteristics of the transformer will be the permeability of the core, or how it modifies the inductance of the windings, and the ratio of the primary and secondary which determines the impedance step up or step down ratio. With the right ferrite permeability you would probably have two to four turns on the primary that connects to the coax and 10 to 30 or ?? windings on the secondary. The typical transformer impedance ratio will be on the order of 50:1 or greater for matching a 3 to 6ft whip to 50 ohms.

You would test the transformer by making two of them and connecting them back to back and test for insertion loss across your desired frequency range and VSWR and also test each one individually for VSWR with the predicted value of non inductive resistor across the secondary, depending on the turns ratio of course.

Hello to all Members
I have a question regarding constructing a Vertical antenna that would cover from 500 kHz to 30 MHz.
I do have an RF Systems passive antenna which is about 6 feet tall fed with coax cable and it gets fantastic
reception on AM band as well has SW. Obviously this antenna is fed with a coaxial cable with a PL259.
I always wonder what’s inside this unit, and I know for sure it has a wire elliptically wond inside. Other than that I have no idea. My question is when you feed a vertical antenna with coax cable would the wire inside the antenna connect to center of coax feed line, where does the braid of feed line attach to inside the vertical?
Is there a magic transformer inside the antenna? Perhaps a balun?? The commercial RF Systems vertical has no radials or a ground connection!
I am puzzled. Any help appreciated.
Best regards to all, Jerry

 

[majoco]

Something like this, perhaps....

http://www.philipstorr.id.au/radio/technical/Squid pole.pdf

...maybe with a 9:1 or greater transformer at the bottom instead of a tuner for reception.