435Mhz Yagi antenna .. Help on Impedance

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aq_mishu

Joined: 16 Jul 2007
Posts: 120

21 Apr 2009 12:27 yagi antenna impedance

Guys,

Here is the Antenna specification...

Code:

│ ┌─────────────────────────────┤
│ OPERATING FREQUENCY........... {MHz} 435 │ │
│ REFLECTOR LENGTH.............. { m } .345678 │ FORWARD GAIN = 11.34 dBi │
│ FED ELEMENT LENGTH............ { m } .322834 │ │
│ REFLECTOR SPACING............. { m } .097670 │ F to B RATIO = 35.08 dB │
│ #of DIRECTORS 5 Rfl EL DIAM {mm } 5 │ │
│ D 1 SP { m } .078809 D 1 LEN { m } .318990 │ INPUT IMPEDANCE = │
│ D 2 SP { m } .097462 D 2 LEN { m } .291958 │ 48.7 +j 0.0 Ohms │
│ D 3 SP { m } .120921 D 3 LEN { m } .298453 │ │
│ D 4 SP { m } .152478 D 4 LEN { m } .284187 │ ARRAY LENGTH = 0.74 m │
│ D 5 SP { m } .191322 D 5 LEN { m } .264914 │ │

Now It is folded dipole... since the input impedance is a little lower than 48.7, then for a 50 Ohm line, what should i do??? advice me please... also in relfector, can i use a grid reflector instead of one single rod??? i mean will it improve the performance??

Please?

vfone

Joined: 10 Oct 2001
Posts: 2328
Helped: 327

21 Apr 2009 13:15 435mhz anten

I think 48.7 ohms is fine, and you can let like this. Anyway, your long coaxial cable (if is not the best quality) will introduce more impedance offset that you get now.

A grid reflector will improve the Front/Back ratio of the antenna compared with a simple reflector. But the elements length and spacing of the antenna shall be recalculated, because will be slightly different.

paxem

Joined: 27 Apr 2009
Posts: 2

27 Apr 2009 8:48 9 elements yagi 435 mhz

hi aq_mishu
have you used balun to this antenna ?

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aq_mishu

Joined: 16 Jul 2007
Posts: 120

09 May 2009 8:21 yagi 435 mhz

Hi guys... sorry for late reply... here it is i am now working with since material is available for that spec.

Code:

│ ┌─────────────────────────────┤
│ OPERATING FREQUENCY........... {MHz} 435 │ │
│ REFLECTOR LENGTH.............. { m } .346 │ FORWARD GAIN = 11.42 dBi │
│ FED ELEMENT LENGTH............ { m } .33 │ │
│ REFLECTOR SPACING............. { m } .087000 │ F to B RATIO = 65.92 dB │
│ #of DIRECTORS 7 Rfl EL DIAM {mm } 6 │ │
│ D 1 SP { m } .070000 D 1 LEN { m } .313 │ INPUT IMPEDANCE = │
│ D 2 SP { m } .081000 D 2 LEN { m } .296 │ 88.1 +j 31.3 Ohms │
│ D 3 SP { m } .096000 D 3 LEN { m } .29 │ │
│ D 4 SP { m } .112 D 4 LEN { m } .282 │ ARRAY LENGTH = 0.91 m │
│ D 5 SP { m } .13 D 5 LEN { m } .258 │ │
│ D 6 SP { m } .155 D 6 LEN { m } .255 └─ F4: Optimize ───── F6: BW ─┤
│ D 7 SP { m } .18 D 7 LEN { m } .236 ┌─ F3: Plot ───── F5: Mode M ─┤

Bend Dia: 50mm (Center to center). Folded dipole. Material 6mm Al rod.

Since 88.1, so I am planning to use a 4:1 balun that is used traditionally and the length is for the frequency exactly... So should it be work??

Also if it works fine, then i may add a grid. Actually i will require to Tx and Rx from a far (very far) distance with mid power. thus i need more gain right?? with a reasonable f/B???

FvM

Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany

09 May 2009 9:54 yagi grid reflector antenna

Quote:

Since 88.1, so I am planning to use a 4:1 balun

Completely wrong, as already mentioned in the contrubtions to your other thread.

aq_mishu

Joined: 16 Jul 2007
Posts: 120

11 May 2009 22:43 folded dipole antenna measure impedance

According to the thread

http://www.edaboard.com/viewtopic.php?t=349409&highlight=

and FVM's ans, i have no way to trim the driven element since it already has been folded to the length. mounted. now all i can do is an impedance measurement (practical) and then to use matching/balun. now please advice me on this... i mean how to measure the actual impedance? (i have no way to manual cut/trim almost any of the elements at this point...)

Mishu~

FvM

Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany

11 May 2009 23:07 feed point impedance yagi

Consult a Ham website regarding antenna matching and simple homemade impedance measurement techniques.

markken

Joined: 14 May 2009
Posts: 5
Helped: 1
Location: New Zealand

14 May 2009 9:35 yagi feed impedance

Hi. This is very easy do put right. Foldered dipole is very broad and will be about 300 ohms. At 435 MHz cut a length of 50 OHM coax to 277mm with each end braid cut back say 50mm. so total cut length will be 377mm. Take the feed cable and prepare the ends the same. Now connect all the braids together and cut them short as possible. this will leave you with 3x centre conductors, which you will join the feed one to one of the others. The two remaining ends (single cable and double cable go to the termination ends of the dipole. I found this... look at the very last diagram on this page http://web.telia.com/~u85920178/antennas/balun0.htm . Make this and it should work fine. I have been making them for about 20 years and they always work well.
A bird watt meter is having if you can get you hands on one. Cheers Mark. ZL2WHO.

Added after 3 minutes:

On the distance, im using wideband modulation 50KHz at 410MHz. the base power is about 3 watts and that into a 10element yagi gets me over 20KLMs with good signal to noise ratio. Mark.

Added after 12 minutes:

Im not proofing my text before sending very well.
A bird watt meter is worth having if you can get you hands on one. Cheers Mark. ZL2WHO. Im not familiar with you ant design program, but if its any good this balun will work fine. How to work out the length of the balun.. Take a figure of 300 and devide it by the freq. Take the answer and multiply it by 0.5 take that answer and multiply it by 0.66. this is the cut length where the shield/braid stops. so you need add the tail lengths as discussed. Hope this hasnt confused the hell out of you. Very Happy

mark.

aq_mishu

Joined: 16 Jul 2007
Posts: 120

21 May 2009 13:00 435Mhz Yagi antenna .. Help on Impedance

guys... sorry for late reply... i did not got the chance to sit for net...

anyway, first, reading the replies, then letting you know...

Thanks a LOT for all the replies...

Added after 9 minutes:

well, markken, Thanks... i already did that... made a 4:1 for the folded dipole part... dont know how it is working since another guy will use it from tomorrow... and well, if antenna feed point is 200ohms, and feed line is 50ohms, then it dafinitely will work fantastic. but say feed point became 88.4 and feed line is 50. then how to match?? that's my main challange...

VSWR

Joined: 07 Feb 2002
Posts: 767
Helped: 72

21 May 2009 14:10 Re: 435Mhz Yagi antenna .. Help on Impedance

What is the impedance at 434 and 436 MHz? Can you upload the screen shots for these as well?

Code:

brumi

Joined: 17 Sep 2006
Posts: 119
Helped: 13
Location: Uruguay

23 May 2009 3:58 Re: 435Mhz Yagi antenna .. Help on Impedance

Don't worry about yagi calculator ''specs" .
A folded dipole in a yagi antenna is nearly 200 ohms and as Markken said you need a 4:1 balun, 1/2 wave coaxial cable will match it to 50 ohms.(1/2 wave lenght *cable velocity factor).Is very easy and works very good.
To be sure your impedance is really 50 ohms there is not other way that test it with a ROE meter.
The easiest way to modify folded dipole impedance is variyng the distance between it and the nearest director to the folded dipole.

aq_mishu

Joined: 16 Jul 2007
Posts: 120

29 May 2009 22:11 Re: 435Mhz Yagi antenna .. Help on Impedance

guys, thanx a lot for all of your advice. at this moment, the antenna is working with the guy i made it for. and he is satisfied. because today i got his signal loud and clear and everybody appreciated it.

now instead of folded dipole, if i use only dipole... i mean without folding, then it will become 50ohm at feed point right??

well, all i need to know now is if i just change the folded dipole to a regular dipole, then do i need the 4:1 balun?? i think not. instead, i need 1:1 balun. so what that will be???

Mishu~

vfone

Joined: 10 Oct 2001
Posts: 2328
Helped: 327

30 May 2009 9:31 Re: 435Mhz Yagi antenna .. Help on Impedance

In free-space a thin dipole at resonance presents an input impedance of
approximately 73 ohms.
You can feed it with a 50 ohms coaxial cable if you use a balun as in the picture below (using two λ/4 of 75 ohms coaxial cables in parallel)

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