Ham Radio Forum

hi

Has anybody made "cheap Yagi" with half folded dipoles

http://www.fredspinner.com/W0FMS/CheapYagi/vjbcy.html

What will be the impedance of the driven element alone in this case?? (eg: simple dipole 75ohm, folded dipole 300ohm)

Thanks

trupti

Fed as shown on that web page, except in the exact center of the dipole, instead of slightly off-center as shown, the dipole feed impedance would be 288 ohms if the dipole was in free-space without the parasitic elements. However, both nearby parasitic elements and feeding the dipole slightly off-center would lower the feed impedance of the Yagi. If those combined effects were just right, the result could be a good direct match to 52 or 72 ohm coax cable.

It may seem intuitively that a half folded dipole like that should have some free-space feed impedance other than 288 ohms, because that is the feed impedance of a folded dipole in free space. The reason is due to the fact that the feed line connects between the center of the dipole and the end of the half dipole instead of the between the ends of two half dipoles in the case of a folded dipole.

A half dipole constructed like that is a good design to use with coax feed line, because the feed connection is inherently unbalanced, eliminating the need for a balun.

Matt

After making the post above, I realized that I should have added the caveat that a balun is not needed only if the coax shield connects to the exact center of the dipole. A balun should be used if it doesn't.

Matt

thanks a lot for the reply

i made a 15 element half folded dipole for 916 MHz and connected 75ohm coax a bit off centre. i modeled the yagi using quick yagi software which gives input impadence as 69+j0.0 for feed element length of 15.1cm. the software shows gain ~12dbi and f/b ratio of around 50db

i am connecting the antenna to a reciever of 75ohm input impadence and trying to catch signals from a 916MHz transmitter of 100mw power. i want a range of about 1km (the transmitter website says that such range is possible if high gain yagi is put at reciever end. the orignal range is 250-300mts with whip antenna)at present i can attain a max range of 500mts. i am suspecting impedance mismatch problem so will be connecting a 300 t0 75 ohm balun at feed point and see if there's any improvement. plz help me!!

thanks

trupti

I haven't modeled a Yagi similar to the one you are using, but I would expect the feed impedance to be closer to 75 than to 300 ohms. Let us know what happens.

Matt

What kind of antenna are you using on the other end of the circuit? Is it possible to improve it? If the antenna on the other end has little or no gain it will be easier to improve the total path gain by improving it than by improving the performance of the 15 element Yagi.

To matt:
Is it OK to use ready-made 300ohm to 75ohm balun even though the antenna impedance is near 75 ohms or should i use toroid 1:1 balun?

To silas:
The other end (ATV transmitter) also has a whip antenna, but i can't change it for two reasons:
1.the transmitter is too small,costly and it will be very difficult to remove its antenna on connect a new one

2.the transmitter will be on a small airplane and will be sending back videos to ground as it cruises in circles so it has it has to be omni-directional

also the following article gives more reasons for not fiddling with the transmitter:

[url]http://northcountryradio.com/Articles/range.htm[/url]

trupti

Connecting a 4:1 balun improved the performance a lot. Almost getting double the range. Just a 100-200mts short of 1km when tested in an open corridor. Maybe will get 1km in open space.