FOLDED DIPOLE

[AE1CT]

ON A 75 METER FOLDED DIPOLE  WHATS THE MININUM LENGTH 450 OHM LADDER LINE THAT I CAN USE TO MAKE IT WORK EFICIENTLY, AND CAN I USE RG-8U ON THE END COMMING INTO THE SHACK.

[wa3dsp]

There are certain lengths of open wire feed line to be avoided especially if you intend to use the antenna multiband. The (older?) ARRL antenna book has a chart which shows the lengths to be avoided. If you don't have access I can look it up. What length do you have in mind?

You say shortest. My question would be do you intend to ONLY use this antenna on 80/75 meters? If so why not run coax directly to the antenna and use a 4:1 voltage balun center insulator like a BN1 at the antenna. You could also put the balun just outside the house with a good ground at that point. They make nice waterproof baluns like that.

If you do intend to use it multiband then the openwire line would be a better choice BUT you cannot transition to coax without a balun either just outside or inside the house. If you want to bring the openwire line into the house and use a balanced tuner you can use two lengths of coax, shields connected together and grounded, with the two center conductors connected to the openwire line. This should be a short section but allows you to get it through a window or sidewall easily.

I know a lot of people do it but I personally would not bring openwire line into the house without some kind of effective lightning protection installed outside. This could be as simple as manually disconnecting it entirely and grounding outside when not in use BUT Murphy says the one time you forget to disconnect it is the time it will get hit.

[W2JBL]

even if you use a balun, by transitioning from open wire to coax you lose all the advantages offerd by open wire lin. don't do it. bring the open wire line all the way to the tuner. you can even just close the window on it if you can't think of a way to get it inside.

[n1uvi]

GARY,
ON 75 / 80 METERS
FEED LINE LENGTHS TO USE ARE
       34 TO 40 FEET
      90  TO 102 FEET
     160 TO 172 FEET

AVOID  66 FEET
         135 FEET
         190 FEET

BUT WHAT DO I KNOW
LOOK AT ME IM JUST A DOG  

GOOD LUCK

[wa3dsp]

It is not true at all that transitioning to coax negates the openwire benefit. I have done this many times and so have others. If the majority of the transmission line length is openwire and you only use coax for a very short distance there is little additional loss. By short distance I mean 10 feet or less which is plenty to get from an outside balun to an inside tuner.

Yes there could be problems doing this running high power. The balun and the coax need to be able to take the mismatch. There are high power baluns and large Teflon coax that would certainly be able to handle it. For 100-200 watts most baluns would work. I happen to prefer an air balun like the old Heathkit but unless you find an old one or build your own there isn't anything out there. 

At my vacation property I use a 320 foot rectangular loop fed with openwire to a Heath balun in a waterproof container. There is a multiple ground system around the house and the balun box is about 1 foot from the ground and grounded to that ground bus. There is about 8 feet of RG213 to the inside where I use a standard unbalanced tuner. As an added precaution I have an MFJ ferrite balun in the box in series with the coax output. It works very well on all bands. I have never had any RF problems

You do what you have to do. Not everyone has the capability or desire to run openwire line into a residence.

[AE1CT]

MY SHACK IS ABOUT 20FT FROM THE CLOSEST WINDOW SO I DONT REALLY WANT TO RUN OPEN WIRE INTO MY SHACK. IF I CAN PUT A 4:1 BALUN AT THE FEED POINT OF THE FOLDED DIPOLE AND RUN COAX THATS GOING TO BE MUCH EASIER AND SAFER FOR ME. WILL THIS SETUP WORK USING A TUNER?  OR AM I STUCK USING OPEN WIRE FEED LINE. THE FOLDED DIPOLE IS FOR 75 METERS ONLY SINCE I ALREADY HAVE A 40 METER DIPOLE UP AND THATS FED WITH COAX.

[Steve - K4HX]

If you can run coax all the way from the feed point, why mess with a folded dipole?

[wa3dsp]

A folded dipole is a wider band antenna than a straight dipole. If you are only going to use it on 75/80 then using the 4:1 center balun and coax to the shack is fine. Tuned for the center of the band it should work good across the entire band. Any rig automatic tuner should tune it as well as any manual tuner. You probably will find that no tuner is required at all.

I remember you talking about a shortened folded dipole in an earlier message. Is this the case or is it the full 126+ foot length for 75? 

[AE1CT]

I ONLY HAVE ROOM FOR NOTHING LONGER THAN 75FT, SO IM BUILDING A FOLDED DIPOLE FROM THE PLANS THAT I GOT FROM KB3AHE. THE FOLDED DIPOLE IS ONLY 60FT END TO END AND IS FED WITH LADDER LINE, BUT I DONT WANT TO USE LADDER LINE. NOW THAT IVE GOTTEN ALOT OF INFO THE ANTENNA SHOULD WORK FOR ME ON 75 METERS USING A 4:1 BALUN AND FED WITH COAX..

[Steve - K4HX]

You will have a HUGE SWR (well over 100:1) on the coax with the KB3AHE antenna. That's why it MUST be fed with open-wire or ladder line. Not sure where you got the idea you could place a 4:1 balun at the feedpoint and use coax.

I sure we covered all this in a previous thread.

http://amfone.net/Amforum/index.php?topic=33641.0

[wa3dsp]

And YES, Steve is totally correct anything less than a resonant 1/2 wave dipole or folded dipole (126+ feet) will NOT work with coax feed. You have a number of choices -

1. Use a different antenna
2. Feed it with open wire and a balanced tuner
3. Feed it with open wire and use the SHORT double coax to bring it through the wall to a tuner.
4. Use a waterproofed remote tuner and bring coax inside.

Question - Why are you limited to 75 feet? Can't you come up with some way to squeeze in the extra 50 feet? It could be vertical or at an angle from the main wire. You just don't want it to be less than about a 100 degree angle. If you could get a resonant antenna on 75 it would sove a lot of problems. Just use a center balun and coax to the shack.

[KK4YY]

You might try the Off-Center-Loaded Dipole. Jerry Hall, K1TD describes this in Sep 1974 QST that is also in the Antenna Handbooks. I found a webpage by Chris Arthur, VK3CAE for his version of this that is only 66 feet long and he includes construction details here:
http://www.qsl.net/vk3jeg/loadpole.html

Don

[n1uvi]

A simple folded dipole 60 feet end to end
would be for around 40 meters wouldn't it?
and have an impedance of around 300 ohms???
something tells me the design your going to use
wants to have a specific length of ladder line from the feed point
as an integral part of the antenna



is this it?
feed it with ladder line to a 4:1 balun at the closest point of entrance, coax from there
to a good tuner
I would make it as long as space will allow or at least a little longer
this is a folded dipole not a folded dipole
or do I have that backwards??

[w1vtp]

Just use the Cobra Ultra light and be done. You can make one yourself if thats your thing.  Run that to the tuner of your choice or a 4 to 1 balun on the outside wall of the house. Then short coax to the shack.  Its 73 ft long. I am sure you can make that work on your property

http://www.k1jek.com/

c

Agree with Clark on this.  I gave the exact recommendation on another thread.  Just because it's a folded dipole doesn't mean you can run 1/2 the half wave dimension.  You will take a serious hit in your signal. 

Read the comment on the aforementioned site carefully.  It should work for you.  You likely will need a tuner also.

Al

PS: Font size adjusted for your sight limitation

[n1uvi]

electrically its the same
physically the Cobra Ultralight is easier to deal with

[AE1CT]

WELL I MADE A 75 METER FOLDED DIPOLE USING A 4:1 BALUN AND FEEDING IT WITH RG8 COAX AND IT WORKS. WITHOUT USING MY TUNER I HAVE A SWR OF LESS THAN 2:1 BETWEEN 3825 AND 3900. THANKS TO ALL OF YOU WHO GAVE ME SOME INSIGHT ON WHAT WILL AND WILL NOT WORK AND WHAT  I SHOULD DO.


ENOUGH OF THIS TOPIC NOW, LETS ALL MOVE ON.