How to Trim an Off-Center-Fed Dipole (

[@garydejulio484]

Thanks!

[@kenslimmer5251]

Great Video, one thing, the formula using 468 is not due to velocity factor, its shorter due to end effect. " End effect, capacitive loading of the unterminated wire ends against ground

[@cuban9splat]

I have been experimenting with OCF dipoles for about 15 years now, and I must say, Dave you are spot on with your analysis and explanations. My current OCF (35'/90') is strung between some very tall trees. I live in the forest with lots of very tall Oak, Pine and Hickory trees. There is a difference in tuning between summertime when the leaves are full and winter when the leaves are on the ground. I think it's the leaves that cause the change between seasons. It's not a significant change but I can tell it changes. I use an old Drake MN 2700 Matching Network with calibrations on the knobs. Great stuff, Dave. Keep up the good work! 73 de K7RMJ Frank

[@ironton655]

The off center fed dipole that is fed 1/3 off center is a 200 ohm antenna, not 400 ohms! It uses a 4:1 balun to convert the 200 ohms to 50 ohms and it's not a "BAL-BAL" ! A balun that transforms 400 ohms to 50 ohms would be an 8:1 balun - 400/50 =8! A 4:1 is 200 ohms to 50 ohms - 200/50 = 4! The unit you are speaking of is called a "Hybrid Balun" which contains a 4:1 balun or the impedance transformation, followed by a 1:1 current balun which impedes the flow of common mode current on the outside of the coax shield.

[@kentk9aee898]

Had a POTA QSO with Mark, K3PPP yesterday. Hi Mark. 73 K9AEE

[@davidsradioroom9678]

Good explanation!

[@timmack2415]

I have an ocfd, 90' one side, 180' on the other side. I have a DX engineering, four to one current balance. The SWR is just about flat on 160, 80, acceptable on 40, 20, part of 10 and part of 6 meters. It is 65-70 feet at the apex and about 15' at the ends. For some odd reason, despite the quite acceptable SWR, the antenna does not perform well. My G5 RV beats the pants off it every time. I'm glad not sure why. It is the antenna that I wanted to love, but don't.

[@n9amiwavelengthradio]

Id doesn't perform well because you went with a 160 ocf.. Switch to a 80 meter version and it will perfrom. 160 ocfs do not work well. Try it you will find out.

[@timmack2415]

@@n9amiwavelengthradio That interesting. I have a 'double' G5RV that is fantastic! The 160 meter OCF dipole is just awful. The only issue with trimming it down for 80, is that I used 160 M for probably 75% of the time. Because of the way that my property is, and where my house is situated, the OFC was a perfect fit. But I don't rarely use it, so I guess I won't be missing much.

[@Bikemen61]

Also when trimming any OCF antenna is that you might optimize let’s say 40 meters in this case but the other band will also be affected. There’s always a compromise when using multiband antennas.

[@timdbl7804]

Hi, a way to almost? eliminate the "everything effects everything" dilemma: I have used capacitive loading on a Windom/OCF to good effect, to get good match simultaneously on all bands. E.g. If resonance is low on 80 metres, but OK on the other bands, stick a capacitor in the long leg at a point which is current maximum on ~3.7 MHz. This will increase the f.res on 80m, but should have little effect on all (/most of?) the other bands where this point will be a voltage maximum. I needed to do some empirical modelling (I used EZ-NEC) to get the exact position and capacitance value optimum, but in practice it worked very well. (I suppose the same principle applies if using inductive loading, but I think most often the issue is the lower band(s) being low in frequency owing to ground-proximity, so I suggest that capacitive-loading is most appropriate in this case). EZ-NEC will also tell you what voltage-rating is needed for your capacitor! Then I add at least 50%! Hope that helps, 73 all, Tim

[@BartVanAllen]

well - unless you use a wide range tuner, then no need to shorten anything. I use an 80m OCF and it works everywhere (never tried 160), but is not easy to get a match on 60. And I use chokes at both ends to keep stray rf out of the shack.

[@Bikemen61]

Agreed but whenever you have to tune anything grater than 4:1 with a tuner located in the shack the real SWR (power coming back) between the tuner and the antenna will be dissipated as heat by the coaxial cable. Making the antenna extremely inefficient.

[@AC3HT]

Thank you Mr. Casler for the good video.📺 73 de AC3HT

[@kc6nfv746]

My ocf antenna 132 feet one side 88 other is 44 3.8 swr 3.1 on 3.5 swr 1.7 I want be in the 3.8 how I trim the antenna

[@davecasler]

No simple wire antenna will cover all of 80 meters. If you trim the antenna, it will affect all the bands. 73, Dave, KEØOG

[@ironton655]

The off center fed dipole that is fed 1/3 off center is a 200 ohm antenna, not 400 ohms! It uses a 4:1 balun to convert the 200 ohms to 50 ohms and it's not a "BAL-BAL" ! A balun that transforms 400 ohms to 50 ohms would be an 8:1 balun - 400/50 =8! A 4:1 is 200 ohms to 50 ohms - 200/50 = 4! The unit you are speaking of is called a "Hybrid Balun" which contains a 4:1 balun or the impedance transformation, followed by a 1:1 current balun which impedes the flow of common mode current on the outside of the coax shield.

[@Bruce-bo4hq]

I didn't understand how 50:400 would be a 4:1 bal-bal

[@peteranoia4570]

I thought it would be ~200ohm at feed point

[@subramanianr7206]

Usually the feed point impedance (at the usual heights hams can manage) of an OCF would be around 200-400 ohms as he said in the video. When the impedance transformation is 200 ohms it's 4:1. Dave, mistakenly mentioned in the video. As the feed point height becomes half lambda (for the lowest frequency band) or more the impedance becomes higher near to 600 ohms and we need to transpose with a 6:1 BalUn (though not common in the commercial designs) The 4:1 BalUn must be followed by an 1:1 CMC (common mode current) choke to avoid RF in the shack issues. De VU2RZA

[@Brenda-jf2pe]

Balun designs has a high quality 4:1 with built in 1:1 for an ocf dipole! It’s better than mfj but you need to buy wire and dog bone insulator’s! Highly recommended by ehamnet! JohnBoy UtAh (KJ7TBR)

[@VE9ASN]

Is that their "hybrid" balun, I've had my eye on one. 73

[@Brenda-jf2pe]

Yes , the hybrid will not be affected by being close to the roof or over the top of a house by at least 20 feet!

[@Brenda-jf2pe]

Cord you mean wire!!

[@richardbaker1308]

Thanks!

[@davecasler]

Thank you for your financial support of this channel! It is greatly appreciated! 73, Dave, KE0OG