Hi AMSAT's,
Because I can not set up my rotor/elevation system at the new QTH, I try to get some omni antenna's to be active on the OSCAR's. I made a 2m version and 70cm version from some calculation website (www.jcoppens.com) on the internet. Unfortunately both were too low resonant in frequency. Cutting the lengths shorter, (to get a higher resonance frequency) didn't work well, as I got too short elements at a certain time and the mechanical construction didn't allow more shorter elements.
So I started all over again, this time taking the well known Rothamel antenna book as example. There is an GPS example which I re-calculated to a 70cm version and combined it with an infinite balun that is also described one page further. For the elements (one element as infinite balun) I used Aircell-7 coaxial cable. When I measured this antenna it surprisingly had also a too low resonance frequency, some 60MHz lower then the expected 435MHz... Another thing I noticed is that the return loss is around 10dB best case for this kind of helix antenna (that's at 375MHz resonance). It gets better when I touch the elements, but that should not be the way it should work...
Does anyone has experience with quadrifilar helix antenna's on 2m and 70cm for TX ?
When using the infinite balun principle, should it be a number of 1/4 wavelength ? or should it fit the antenna element length ? Because the quadrifilar element length (=infinite balun length) was shorter than 3/4 lambda length. Due to this fact I actually expected a higher resonance frequency, but it went out the opposite way.
FYI: The supporting of the elements is all done with PVC tubes, to avoid any metal influence.
Kind regards,
73 de PE1RAH, William Leijenaar ---
Hi William, PE1RAH
In a separate email I have sent to you copy of my article "Experimental Investigation of Quadrifilar Helix Antennas for 2400 MHz that was published into the AMSAT Journal May/June 2004 particularly covering the infinite balun theory and applications if you need a End-Fire or Back-Fire radiation from the Antenna.
Also I added an article written by Walter Maxell W2DU one the well know guru about the Helix Antennas.
Finally I have added an artiche that I have written for Radio Rivista about a QFA for 70 cm but sorry it is in italian but you can read the drawings.
Best 73" de
i8CVS Domenico
----- Original Message ----- From: "William Leijenaar" pe1rah@yahoo.com To: amsat-bb@amsat.org Sent: Saturday, January 21, 2012 11:19 PM Subject: [amsat-bb] Quadrifilar Helix antenna question
Hi AMSAT's,
Because I can not set up my rotor/elevation system at the new QTH, I try to get some omni antenna's to be active on the OSCAR's. I made a 2m version and 70cm version from some calculation website (www.jcoppens.com) on the internet. Unfortunately both were too low resonant in frequency. Cutting the lengths shorter, (to get a higher resonance frequency) didn't work well, as I got too short elements at a certain time and the mechanical construction didn't allow more shorter elements.
So I started all over again, this time taking the well known Rothamel antenna book as example. There is an GPS example which I re-calculated to a 70cm version and combined it with an infinite balun that is also described one page further. For the elements (one element as infinite balun) I used Aircell-7 coaxial cable. When I measured this antenna it surprisingly had also a too low resonance frequency, some 60MHz lower then the expected 435MHz... Another thing I noticed is that the return loss is around 10dB best case for this kind of helix antenna (that's at 375MHz resonance). It gets better when I touch the elements, but that should not be the way it should work...
Does anyone has experience with quadrifilar helix antenna's on 2m and 70cm for TX ?
When using the infinite balun principle, should it be a number of 1/4 wavelength ? or should it fit the antenna element length ? Because the quadrifilar element length (=infinite balun length) was shorter than 3/4 lambda length. Due to this fact I actually expected a higher resonance frequency, but it went out the opposite way.
FYI: The supporting of the elements is all done with PVC tubes, to avoid any metal influence.
Kind regards,
73 de PE1RAH, William Leijenaar --- _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Gentelmen: Thank you for your input. I've measured 2.4:1 at it's intended design frequency, using a half wave stub. The antenna is essentially two loops (one capacitive and one inductive) in parallel, giving an intended 50 Ohm resistive impedance at resonance. I'm measuring 25 Ohms and a bit inductive. Do have more data if anyone is interested. (smith chart, return loss) Tomorrow I'll build both versions, lhcp and rhcp. Tonight's live demo on so50 shows promise, though I'll need two and have to switch polarity. 73 de Norm n3ykf
Hi Norm, N3YFK
Are you sure that your Antenna Analyzer Times T-100 is reading a correct VSWR and R+/-jX impedance at 435 MHz ? My MFJ-269 connected to a professional 50 ohm termination goog up to 18 GHz reads VSWR = 1.2 :1 but unfortunately the MFJ-269 do not read R+/-jX impedance above 144 MHz
Do not forget that to get RHCP the helix must be winded lefthand and on reverse to get LHCP the helix must be winded righthand.
In addition,depending on how you connect the four terminals of the loops into the infinite balun you can get or End-Fire radiation or Back-Fire Radiation.
I am interested on your experiments using your data to put it into the Smith Chart and check
BTW on 70 cm using half-turn quadrifilar (two half-turn bifilar loops). with dimensions that yield the correct phase relationship with a loop wire diameter of 0.0088 wavelenght as follows and suggested by Walter Maxell W2BU
and using:
Smaller loop: D = 0.156 wavelenght lenP° = 0.238 wavelenght Perimeter= 1.016 wavelenght
Larger loop: D = 0.173 wavelenght lenP° = 0.260 wavelenght Perimeter = 1.120 wavelenght
I got a return loss of -20 dB about a VSWR of 1.22 : 1 at 435 MHz as measured on my HP8555A Spectrum Analyzer usind a well calibrated HP11692D directional coupler.
73" de
i8CVS Domenico
----- Original Message ----- From: normn3ykf@stny.rr.com To: "AMSAT-BB" amsat-bb@amsat.org; "i8cvs" domenico.i8cvs@tin.it; "William Leijenaar" pe1rah@yahoo.com Sent: Monday, January 23, 2012 3:03 AM Subject: Re: [amsat-bb] Re: Quadrifilar Helix antenna question
Gentelmen: Thank you for your input. I've measured 2.4:1 at it's intended design
frequency, using a half wave stub.
The antenna is essentially two loops (one capacitive and one inductive) in
parallel, giving an intended 50 Ohm resistive impedance at resonance. I'm measuring 25 Ohms and a bit inductive. Do have more data if anyone is interested. (smith chart, return loss)
Tomorrow I'll build both versions, lhcp and rhcp. Tonight's live demo on
so50 shows promise, though I'll need two and have to switch polarity.
73 de Norm n3ykf
participants (3)
-
i8cvs -
normn3ykf＠stny.rr.com -
William Leijenaar