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70cm folded dipole

L

Leland

Member
May 26, 2013
11
0
11
Hello all,
I'm rather new to all of this and was hoping for some help.
I would like to start on a 70 cm folded dipole that could also be used on 2 meters. First is this possile. 432.5 mhz and 144.188 mhz on the same element. How do I go about this.
Thanks
 

Might be a lot easier if you built a 2M antenna. IT should work just fine on 70cm, being an odd number of half wavelengths long. (Why a "folded" dipole?)
 
Don't know, just want to get started on a project. If I did a dipole (not a folded dipole), would it be easier to match? Also, Do I want the feed point to start a new wave or does it matter?
 
Leland said:
... Do I want the feed point to start a new wave or does it matter?
Click to expand...


well, for MONO band antenas, I do favor "tuned feedlines", however, 2 meters is 4 MHz wide. I would pick the part of the band you intend to operate on and aim for that freq range.

for some projects, go here and look around Antenna Projects

or perhaps a ground plane antenna built on a chassis connector would be fun

welcome to da forum
 
camaro1 said:
why does this guys j-poles have the center conductor going to the short pipe while all the calculators I have seen have the center conductor going to the long pipe of the j-pole?


2 Meter Amateur Radio J-Pole Antenna | KB9VBR J-Pole Antennas
Click to expand...


He likes to be different?

Since RF is AC it makes no difference really which way the coax connects to a J-pole. Same idea as a dipole. Which wire gets connected to the center lead? I know Doc would say the pink wire. :D
 
Captain Kilowatt said:
He likes to be different?

Since RF is AC it makes no difference really which way the coax connects to a J-pole. Same idea as a dipole. Which wire gets connected to the center lead? I know Doc would say the pink wire. :D
Click to expand...


jeez,... ya mean I was wasting my time when I put a red rubber cap on one of my ground planes radials and then used a rotor to turn it?:headbang
 
I do like the jpole idea however, I wanted a horizontal antenna for the weak signal portion of the bands. What if I lay the jpole on it's side?
 
Captain Kilowatt said:
Maybe I just can't see the forest for the tress but what on earth does this mean?
Click to expand...
Like I said, I'm new to all of this so please, bare with me.
I have 9' of lmr400 sitting here and I worked some math like this and please tell me if I did something wrong. Aiming for a center frequency of 432.5mhz
300/432.5=.6936 meters x velocity factor of .85=.5895meters x 3.2808 to get into feet=1.9343 looks like 1' 11 3/16"

To get the antenna up as high as I can with what I have, would be 4.5 waves on the feed line. Could I feed a dipole at a half wave and still get it to perform well. Or should I feed it at the beginning of the fourth wave. School me please!
 
Leland said:
Like I said, I'm new to all of this so please, bare with me.
I have 9' of lmr400 sitting here and I worked some math like this and please tell me if I did something wrong. Aiming for a center frequency of 432.5mhz
300/432.5=.6936 meters x velocity factor of .85=.5895meters x 3.2808 to get into feet=1.9343 looks like 1' 11 3/16"

To get the antenna up as high as I can with what I have, would be 4.5 waves on the feed line. Could I feed a dipole at a half wave and still get it to perform well. Or should I feed it at the beginning of the fourth wave. School me please!
Click to expand...


OK now I understand what you were meaning. Sorry for the confusion but a lot of folks refer to "the first wave" or something and leave off the word "length". then it should read wavelength which has a whole different meaning than simply "wave". Forgive me for being precise. It's the old broadcast engineer in my rearing it's ugly head again after almost 8 years out the field. What you are asking about is if you should run a certain length of coax between the transmitter and the antenna such as cutting it off to make 4 wavelengths of cable since the cable you have is not long enough to make it out to 5 full wavelengths.

ANSWER: No. Install the antenna at the end of the cable and run the rest back to the transmitter. There are very few times when you need to make sure the cable is a precise length and this is not one of them. On 70cm you want to keep cable losses to a minimum and that requires using the best cable you can in addition to the shortest length you can get by with. It gets tricky up on VHF and UHF when trying to deal with multiples of wavelengths anyway because a small error gets multiplied a great many times in the end. A 1/2 inch of error at 1/2 wavelength will add up to an extra 1/4 wavelength on 2m if the total cable is about 65 feet long effectively negating any precision one was trying to obtain in the first place. this is why I like using an analyzer on the total length of cable and trimming the final length if I have too.
 
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Captain Kilowatt said:
OK now I understand what you were meaning. Sorry for the confusion but a lot of folks refer to "the first wave" or something and leave off the word "length". then it should read wavelength which has a whole different meaning than simply "wave". Forgive me for being precise. It's the old broadcast engineer in my rearing it's ugly head again after almost 8 years out the field. What you are asking about is if you should run a certain length of coax between the transmitter and the antenna such as cutting it off to make 4 wavelengths of cable since the cable you have is not long enough to make it out to 5 full wavelengths.

ANSWER: No. Install the antenna at the end of the cable and run the rest back to the transmitter. There are very few times when you need to make sure the cable is a precise length and this is not one of them. On 70cm you want to keep cable losses to a minimum and that requires using the best cable you can in addition to the shortest length you can get by with. It gets tricky up on VHF and UHF when trying to deal with multiples of wavelengths anyway because a small error gets multiplied a great many times in the end. A 1/2 inch of error at 1/2 wavelength will add up to an extra 1/4 wavelength on 2m if the total cable is about 65 feet long effectively negating any precision one was trying to obtain in the first place. this is why I like using an analyzer on the total length of cable and tuning trimming the final length if I have too.
Click to expand...

AlgebraWouldSaveOurLives - YouTube s :whistle:
 
Leland said:
Like I said, I'm new.... Could I feed a dipole at a half wave and still get it to perform well. Or should I feed it at the beginning of the fourth wave. School me please!
Click to expand...


to answer that part of the question,...... yes, feeding an antenna @ elect 1/2wl mults is the same as full wl's.

in other words, looking at the sine wave, 0 deg, 180 deg, and 360 deg are on the same axis,
 

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