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Author Topic: 88ft (Cebik) Doublet Feeder question  (Read 3088 times)
G7MRV
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« on: November 30, 2007, 08:39:15 AM »

Hi,

I use a Cebik style 88ft doublet, fed with 300ohm ladder line, via either a 1:1 or 4:1 coaxial balun from a MFJ 498(?) tuner

I have a small RFI problem, in that operation on 80m or 40m wipes out my broadband router, about 15ft from the tuner, i think because the ladder line is routed along the wall close to the shack rather than directly away (the only route i can use) and i cant move the router or phone lines.

Ive wondered about extending the coax line from the tuner to the balun to try and contain the field, but would this work? and how far could i use the coax for, before the balun and then ladder line? I dont have access to any kind of remote tuner.

I need to resize the ladder line anyway, as i have recently cut the antenna to 88ft from 102ft, and the tuning is now badly out!


thanks for any advice guys

Martin G7MRV
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N3BIF
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« Reply #1 on: November 30, 2007, 08:45:36 AM »

 IN many of the mfj tuner manuals they have a section which defines lengths of feedline to avoid , often adding 8-16 feet will correct such problems. If your manual doesn't have this info go to mfj site and look at the manual for the 949e it should be in the troubleshooting section i believe.

Good luck!
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G7MRV
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« Reply #2 on: December 01, 2007, 04:06:17 PM »

Thanks, I have checked the lengths, and added 6ft as required, but it shows no change in performance. 20m still shows 1.3:1 across the band with 50W or less forward power, despite the tx being on 100W (tested in FM to ensure power level).

other bands will tune at 10W, but show the same problem as 20m if on 100W,

does this suggest a fault on the antenna somewhere rather than just a problem with line length ? (which should now be ok anyway)
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N3BIF
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« Reply #3 on: December 01, 2007, 11:04:37 PM »

1.3:1 should be of no concern .
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G7MRV
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« Reply #4 on: December 02, 2007, 03:24:19 AM »

i agree, it shouldnt be, but the lack of decent power transfer is. As is the heavy SWR increase at higher power
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WW5AA
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« Reply #5 on: December 02, 2007, 07:05:13 AM »

Martin,

Baluns built into a tuner are the worst obtians for a doublet. Wind a current balun (10-6" turns) at the point that the coax is connected to the ladder line. Since you are using 300 Ohm feeder, 75 Ohm coax would be a better choice.

73, de Lindy
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G7MRV
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« Reply #6 on: December 02, 2007, 07:16:24 AM »

ok, bit of further info here. Firstly, my 4:1 balun is a homemade coaxial 'collins' balun, i dont use the built in toroid in the ATU at all. I tried also a home made coax 1:1 balun (10t) and that made no difference.

80m, 40m will tune but the settings are strange and the tuningpoint is very sharp. 20m wont tune properly, it gives a flat SWR of 1.7:1 (misread it before as 1.3, sorry) on 10W, but >6:1 when on higher power 100W, all the other bands do this on 100W as well.

What i have discovered today from checking the antenna is a hole in the sealant at the feedpoint, exactly over one of the joints, and that side of the feeder has black copper oxide all the way down to the point it joins the rest of the feedline (this is the original G5RV section). I cut out about 4ft of line trying to find a clean section but the oxide is all the way through.

So im convinced this is the cause of the problem. But why would it only show after i recut the doublet top section? it was straight after as well, not days later!
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WW5AA
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« Reply #7 on: December 02, 2007, 07:17:06 AM »

I should have spelled options correctly! In reading your post again, It seems that you are using outboard baluns. Are you sure that they are voltage baluns? Voltage baluns will not work well in your application.

73, de Lindy
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KF4ZGZ
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« Reply #8 on: December 03, 2007, 08:20:04 AM »

Have you tried the built in balun w/ the jumper on the back?

I've used MFJ tuners for a while, and they work well when used as designed.

I also use an MFJ 901b ( compact )tuner w/ an 88ft. doublet w/ no trouble. TV twinlead as the feeder.

Matt
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NB1U
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« Reply #9 on: December 05, 2007, 12:54:00 PM »

You may have RF being induced onto the D.C. supply cable to your router.  Ungrounded wall-wart type supplies can be bad news in high RF enviroments.  You might try a toroid wound onto the D.C. supply cable as close to the router as can be managed. The more turns the better.

73, Mark NB1U
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KY6R
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« Reply #10 on: December 09, 2007, 06:54:33 AM »

I have an 88' doublet fed with 450 ohm ladder line - up 40 feet with a little less than 40' ladder line that is connected to a DX Engineering 4:1 "tuner balun":

http://www.dxengineering.com/Parts.asp?ID=152&PLID=50&SecID=10&DeptID=9&PartNo=DXE%2DBAL200%2DH10%2DAT

and high quality coax running into the house and shack.

Works like a champ - tunes easily on all bands with a Ten tec 238B tuner (1.0 :1 on all bands) and no RFI whatsoever.
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WD4HXG
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« Reply #11 on: December 14, 2007, 12:24:24 AM »


---I use a Cebik style 88ft doublet, fed with 300ohm ladder line, via either a 1:1 or 4:1 coaxial balun from a MFJ 498(?) tuner---

Not sure what type of tuner you are using. I was not able to find that model number. Tracked back to a Keyer.

In any case a lot of tuners use ferrite toroid baluns  to match the feedline. If the mismatch is over 2 or 3 to one then losses in the core can be quite high. Also the balun will act more like an inline inductor then a transformer under high swr conditions. If you have a decent match then the ferrite core baluns work well. I discovered that the hard way. That may be your source of problems. Good Ole Johnson Matchbox is still the cat's meow in the antenna tuner realm in my opinion when using balanced line feeders.

---i think because the ladder line is routed along the wall close to the shack rather than directly away----

I would try using "Standoffs" to support the wire in the shack.  See http://www.radioshack.com/product/index.jsp?productId=2104003&cp=&sr=1&origkw=standoff+insulator&kw=standoff+insulator&parentPage=search

for a picure of the standoff used on normal antenna twinlead used in the states. Sorry I am not familiar with what is used across the pond.

This will allow the balanced line to function near normal inside the shack.

Balanced line and a Johnson Match box are a hard combo to beat.

73

Chuck WD4HXG
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G6KIZ
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« Reply #12 on: December 17, 2007, 07:38:46 AM »

I'm surprised that nobody has picked up on...

Quote

20m wont tune properly, it gives a flat SWR of 1.7:1  on 10W, but >6:1 when on higher power 100W, all the other bands do this on 100W as well.


If the vswr is changing radically between 10w and 100w then something is very wrong indeed. Unfortunately I have no idea what. Perhaps now that I've highlighted this somebody who does have a clue (unlike me) will assist.
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K8KAS
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« Reply #13 on: December 17, 2007, 11:32:05 AM »

You are saturating the balun core, you need a high quality balun with 2 or 3 large t200 cores. I use the 88 foot W4RNL or whatever antenna, there are about 5 editions of this same antenna mentioned in RSGB/ARRL/ and other handbooks and articles. I have 39 foot of 300 ohm feedline direct to 50 ohm coax (no balun) on the ground (about 70 feet) into the shack, the coax shield connects to my ground system (30----50 foot radials) where it comes into the shack. My shack is dead cold to RF at 1 Kw, it is even hard to see signal on my little diode RF sniffer setting on top of the 1 Kw amplifier.
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G7MRV
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« Reply #14 on: December 17, 2007, 12:49:21 PM »

thanks chaps,

that would seem a good explanation - the different impedences of the 88ft probably mean the collins coaxial 4:1, which is built from RG-58, cant handle it. I plan to replace it, first with a 1:1 choke balun from RG-213U, then a ferrite sleeve balun just as soon as i can source good ferrites here in the UK at reasonable cost. Im going to completely replace the feeder run as well with new 450ohm slotted line and change the route to make the unavoidable parallel section of the run as far from the antenna as is possible (about 25ft at most)

I have had advice from LB Cebik himself on this, for which i am very grateful, and from all you guys as well its been very helpfull

Martin G7MRV
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