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Resonating and Matching To A 476 MHz Yagi Antenna
Hi,
Without purchasing expensive equipment I have a UHF hand held transciever and I have disconnected the flexible 1/4 wave antenna and connected some coax (this assumes the output impedence of the handheld into the flexible antena was 50-70 ohms). I have designed and built a 10 element Yagi. To optimise the matching to the driven element I used a gamma match, the reality is I don't know if the driven element is at resonant frequency let alone correctly adjusting the gamma match. I was just reading another posting on rec.radio.amateur.antenna, where the author suggested using 300 ohm TV ribbon as it is less lossy than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will remove the need for a gamma match at the antenna end, but at the transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will be required. Would a normal TV set balun suffice for 0.5 watt of transmit power? To resonate the driven element I have contemplated using a UHF Gate Dip Oscillator (GDO), has anyone succeed in doing this with minimal test equipment such as a GDO? If the folded driven element resonates at 476 MHz I will accept that it's impedence is 300 ohms. All this may seem very disjoint but it is why this hobby has never ceased to fascinate me. Thanks in advance for any useful suggestions. Ian |
Resonating and Matching To A 476 MHz Yagi Antenna
Hi Ian
Sounds like you are having fun! - I would suggest that using a GDO for meaningful antenna work at UHF would incur quite a few inaccuracies. The big problem is how to couple it to the element itself without detuning it by its presence. You will hear some howls of protest but I personally wouldn't bother to try making it a resonant length. It's really not that important for radiation efficiency. The gamma match introduces a huge reactance that also has to be allowed for. - Using Ocarc's loss calculator 300r ribbon runs about 2.5dB for 100ft/30m. 450r open window ribbon runs about 0.7dB for the same distance! (http://www.ocarc.ca/coax.htm) RG213 (approx 1/2") coax is around 5dB. Keep in mind that ribbon bends and proximity to objects will cause problems that coax wont. - Converting coax to a balanced feed at the antenna can be very easily done with a 4:1 coax balun. This is basically a extra loop of 1/2 wave length coax built like http://www.n-lemma.com/calcs/dipole/balun.htm You will still need to match at the antenna in some way. Maybe replace the gamma with a hairpin, use a delta match or maybe increase the number of driven elements folded dipole style to get the Z up to 200 ohms. A yagi feedpoint Z is going to be maybe 10-20r. One extra driven element (simple folded dipole) will multiply that by 4 and a third element another 4 times. That will be close enough to 200 ohms to work fine. You can also play with the extra element diameters to vary the final feed Z. - I personally wouldnt use a UHF TV balun without testing first. The problem is quality of manufacture! Hope you find this useful. Only some of my ideas! Cheers Bob W5/VK2YQA wrote: To optimise the matching to the driven element I used a gamma match, the reality is I don't know if the driven element is at resonant frequency let alone correctly adjusting the gamma match. I was just reading another posting on rec.radio.amateur.antenna, where the author suggested using 300 ohm TV ribbon as it is less lossy than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will remove the need for a gamma match at the antenna end, but at the transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will be required. Would a normal TV set balun suffice for 0.5 watt of transmit power? To resonate the driven element I have contemplated using a UHF Gate Dip Oscillator (GDO), has anyone succeed in doing this with minimal test equipment such as a GDO? If the folded driven element resonates at 476 MHz I will accept that it's impedence is 300 ohms. |
Resonating and Matching To A 476 MHz Yagi Antenna
On Jan 25, 11:25 pm, Bob Bob wrote:
Hi Ian Sounds like you are having fun! - I would suggest that using a GDO for meaningful antenna work at UHF would incur quite a few inaccuracies. The big problem is how to couple it to the element itself without detuning it by its presence. You will hear some howls of protest but I personally wouldn't bother to try making it a resonant length. It's really not that important for radiation efficiency. The gamma match introduces a huge reactance that also has to be allowed for. - Using Ocarc's loss calculator 300r ribbon runs about 2.5dB for 100ft/30m. 450r open window ribbon runs about 0.7dB for the same distance! (http://www.ocarc.ca/coax.htm) RG213 (approx 1/2") coax is around 5dB. Keep in mind that ribbon bends and proximity to objects will cause problems that coax wont. - Converting coax to a balanced feed at the antenna can be very easily done with a 4:1 coax balun. This is basically a extra loop of 1/2 wave length coax built likehttp://www.n-lemma.com/calcs/dipole/balun.htmYou will still need to match at the antenna in some way. Maybe replace the gamma with a hairpin, use a delta match or maybe increase the number of driven elements folded dipole style to get the Z up to 200 ohms. A yagi feedpoint Z is going to be maybe 10-20r. One extra driven element (simple folded dipole) will multiply that by 4 and a third element another 4 times. That will be close enough to 200 ohms to work fine. You can also play with the extra element diameters to vary the final feed Z. - I personally wouldnt use a UHF TV balun without testing first. The problem is quality of manufacture! Hope you find this useful. Only some of my ideas! Cheers Bob W5/VK2YQA wrote: To optimise the matching to the driven element I used a gamma match, the reality is I don't know if the driven element is at resonant frequency let alone correctly adjusting the gamma match. I was just reading another posting on rec.radio.amateur.antenna, where the author suggested using 300 ohm TV ribbon as it is less lossy than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will remove the need for a gamma match at the antenna end, but at the transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will be required. Would a normal TV set balun suffice for 0.5 watt of transmit power? To resonate the driven element I have contemplated using a UHF Gate Dip Oscillator (GDO), has anyone succeed in doing this with minimal test equipment such as a GDO? If the folded driven element resonates at 476 MHz I will accept that it's impedence is 300 ohms.- Hide quoted text - - Show quoted text - Bob, Thank you for esponding to my questions, and apologis for my delay in responding. GDOs have their limitation as the coupling between the oscillator and resonant circuit causes the frequency to be pulled - a GDO with a frequency counter might be a very useful instrument. My main question about the use of a GDO is how high in frequency can a practical GDO get to in frequency and still oscillate, couple to a similar frequency resonant system and be able to observe a "dip". Your advice on coupling and matching to the antenna are excellent help, what I didn't understand was the impedence value at the folded driven element. Probably something from long ago, but I believed the impedence of a folded element was 300 ohms??? Could you please explain what you meant by :- "or maybe increase the number of driven elements folded dipole style to get the Z up to 200 ohms. A yagi feedpoint Z is going to be maybe 10-20r. One extra driven element (simple folded dipole) will multiply that by 4 and a third element another 4 times." In physical term I don't know what this means. An article that caught my attention recently that I will share with you is a cheap method for measuring complex impedance, it is at:- http://web.ukonline.co.uk/g3ldo/aegextra.htm Once more thanks for your help. Regards, Ian |
Resonating and Matching To A 476 MHz Yagi Antenna
wrote:
My main question about the use of a GDO is how high in frequency can a practical GDO get to in frequency and still oscillate, couple to a similar frequency resonant system and be able to observe a "dip". My cheap Knightkit GDO meter goes up to 300 MHz with a single wire loop for the coil. -- 73, Cecil http://www.w5dxp.com |
Resonating and Matching To A 476 MHz Yagi Antenna
"Cecil Moore" wrote in message . net... wrote: My main question about the use of a GDO is how high in frequency can a practical GDO get to in frequency and still oscillate, couple to a similar frequency resonant system and be able to observe a "dip". My cheap Knightkit GDO meter goes up to 300 MHz with a single wire loop for the coil. -- 73, Cecil http://www.w5dxp.com And, my Measurements Corp Grid Dip Meter goes up to 420 MHz. Jerry |
Resonating and Matching To A 476 MHz Yagi Antenna
wrote in message oups.com... On Jan 25, 11:25 pm, Bob Bob wrote: Hi Ian Sounds like you are having fun! - I would suggest that using a GDO for meaningful antenna work at UHF would incur quite a few inaccuracies. The big problem is how to couple it to the element itself without detuning it by its presence. You will hear some howls of protest but I personally wouldn't bother to try making it a resonant length. It's really not that important for radiation efficiency. The gamma match introduces a huge reactance that also has to be allowed for. - Using Ocarc's loss calculator 300r ribbon runs about 2.5dB for 100ft/30m. 450r open window ribbon runs about 0.7dB for the same distance! (http://www.ocarc.ca/coax.htm) RG213 (approx 1/2") coax is around 5dB. Keep in mind that ribbon bends and proximity to objects will cause problems that coax wont. - Converting coax to a balanced feed at the antenna can be very easily done with a 4:1 coax balun. This is basically a extra loop of 1/2 wave length coax built likehttp://www.n-lemma.com/calcs/dipole/balun.htmYou will still need to match at the antenna in some way. Maybe replace the gamma with a hairpin, use a delta match or maybe increase the number of driven elements folded dipole style to get the Z up to 200 ohms. A yagi feedpoint Z is going to be maybe 10-20r. One extra driven element (simple folded dipole) will multiply that by 4 and a third element another 4 times. That will be close enough to 200 ohms to work fine. You can also play with the extra element diameters to vary the final feed Z. - I personally wouldnt use a UHF TV balun without testing first. The problem is quality of manufacture! Hope you find this useful. Only some of my ideas! Cheers Bob W5/VK2YQA wrote: To optimise the matching to the driven element I used a gamma match, the reality is I don't know if the driven element is at resonant frequency let alone correctly adjusting the gamma match. I was just reading another posting on rec.radio.amateur.antenna, where the author suggested using 300 ohm TV ribbon as it is less lossy than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will remove the need for a gamma match at the antenna end, but at the transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will be required. Would a normal TV set balun suffice for 0.5 watt of transmit power? To resonate the driven element I have contemplated using a UHF Gate Dip Oscillator (GDO), has anyone succeed in doing this with minimal test equipment such as a GDO? If the folded driven element resonates at 476 MHz I will accept that it's impedence is 300 ohms.- Hide quoted text - - Show quoted text - Bob, Thank you for esponding to my questions, and apologis for my delay in responding. GDOs have their limitation as the coupling between the oscillator and resonant circuit causes the frequency to be pulled - a GDO with a frequency counter might be a very useful instrument. My main question about the use of a GDO is how high in frequency can a practical GDO get to in frequency and still oscillate, couple to a similar frequency resonant system and be able to observe a "dip". Your advice on coupling and matching to the antenna are excellent help, what I didn't understand was the impedence value at the folded driven element. Probably something from long ago, but I believed the impedence of a folded element was 300 ohms??? Could you please explain what you meant by :- "or maybe increase the number of driven elements folded dipole style to get the Z up to 200 ohms. A yagi feedpoint Z is going to be maybe 10-20r. One extra driven element (simple folded dipole) will multiply that by 4 and a third element another 4 times." In physical term I don't know what this means. An article that caught my attention recently that I will share with you is a cheap method for measuring complex impedance, it is at:- http://web.ukonline.co.uk/g3ldo/aegextra.htm Once more thanks for your help. Regards, Ian Hi Ian If you get serious about building a device for measuring the complex impedance of circuits at VHF / UHF, I have some recent experience I'd share with you. The problem with "Home Made" seems to be very difficult to get acurate data with when the load impedance is close to matching the Zo of the "Impedance Meter". For measuring complex impedances that present a VSWR of 2:1 or greater, a home made slotted line works quite well. Jerry |
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