Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
The Carolina Windom uses a coax feedline from a 4:1 balun at the feedpoint to a 1:1 current-type unun at the 22-foot point. Do you guys think that some number of snap-on ferrite chokes of the type Radio Shack sells at http://www.radioshack.com/product/in...entPage=search (Radio Shack part number 273-105), wrapped around the RG-8X feedline at the 22-foot point, will be suitable in place of something like the MFJ-915 "RF Line Isolator" (unun)? If so, how many should I use? I have four of them now and the local Radio Shack claims to have some more in stock. Each of those ferrite cores is about 1.25" long by 1" square. I have no idea what type or grade of ferrite material they're made out of. Thanks... |
#2
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
On Thu, 30 Aug 2007 22:40:58 -0400, "Rick (W-A-one-R-K-T)"
wrote: The Carolina Windom uses a coax feedline from a 4:1 balun at the feedpoint to a 1:1 current-type unun at the 22-foot point. Do you guys think that some number of snap-on ferrite chokes of the type Radio Shack sells at http://www.radioshack.com/product/in...entPage=search Of undetermined parentage. (Radio Shack part number 273-105), wrapped around the RG-8X feedline at the 22-foot point, will be suitable in place of something like the MFJ-915 "RF Line Isolator" (unun)? Could be. (There is every possibility that the UnUn that comes with the commercial unit is not suitable - in spite of testimonials to the contrary.) If so, how many should I use? I have four of them now and the local Radio Shack claims to have some more in stock. Put a short length of wire through it, use an antenna analyzer to measure the Z. Now, divide that number into as much Z as is necessary to isolate the end of the 22-foot feed point. That is how many you will need. Each of those ferrite cores is about 1.25" long by 1" square. I have no idea what type or grade of ferrite material they're made out of. The previous suggestion renders that moot. The real question devolves to the parenthetic statement made earlier: How much Z do you need? It should be 3 to 10 times the Z found at that drive point. However, with a wild card of the extra radiator found in the 22 foot drop, it is no longer a simple 4:1 relationship found at the upper end. So, we again enter into the exciting world of amateur discovery through experimentation. Keep choking up until you get the performance you desire - then stop choking up. This, then, raises the next real question: What performance are you expecting? 73's Richard Clark, KB7QHC |
#3
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a CarolinaWindom
Richard Clark wrote:
... This, then, raises the next real question: What performance are you expecting? 73's Richard Clark, KB7QHC Well, I'd kinda like my ford explorer xlt 4WD with a 4.0L engine to perform line a maserati, any chance? Any tweaks you could possible suggest? Possibly new paint? :-) Regards, JS |
#4
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
On Thu, 30 Aug 2007 20:08:07 -0700, Richard Clark wrote:
This, then, raises the next real question: What performance are you expecting? Good morning, Richard. I would like it to work well. :-) OK, more specifically, I'd like it to show a decent match (that is, a match that's close enough for my TS-940S internal tuner to handle) on 80, 40, and 20, and work at least as well as my 160-meter inverted V that I feed with ladder line and a manual tuner. I have no way to define "expected performance" any way other than that. Of course that leaves all kinds of unanswered questions regarding exactly what I mean by "at least as well"... regional / NVIS? (Carolina Windom may not work as well as the V) ... DX? (Carolina Windom may work better than the V at least on 20) ... etc. I also have no antenna analyzer and no means to acquire one any time soon... best I can do is my transmitter (cranked way back in power) and an SWR meter. No doubt 3/4 or more of my questions on this forum over the last several weeks would have been answered by an antenna analyzer but that's a luxury that's not currently available. You mention the "UnUn that comes with the commercial unit"... not sure if you meant a commercially-available CW antenna, but this one is being built from scratch using a W2AU-type 4:1 balun and some lengths of insulated copper-clad steel wire I have here. |
#5
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a CarolinaWindom
Rick (W-A-one-R-K-T) wrote:
If so, how many should I use? I have four of them now and the local Radio Shack claims to have some more in stock. You are trying to create a w2du current-choke-balun. Use the RS devices along with some #77 or #43 ferrite beads from Amidon. (Known characteristics for less than $5 seems to me to be superior to unknown characteristics for more than $5.) http://www.amidoncorp.com/aai_ferritesforrfi.htm -- 73, Cecil http://www.w5dxp.com |
#6
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a CarolinaWindom
Rick (W-A-one-R-K-T) wrote:
I also have no antenna analyzer and no means to acquire one any time soon... best I can do is my transmitter (cranked way back in power) and an SWR meter. What you are trying to do is minimize common-mode currents between the choke and the transmitter. On the eznec.com web page, Roy describes an inexpensive method of detecting common-mode current amplitude using a simple toroidal pickup coil. You could actually measure the relative effect of your RS RFI devices. -- 73, Cecil http://www.w5dxp.com |
#7
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
On Fri, 31 Aug 2007 11:13:41 -0400, "Rick (W-A-one-R-K-T)"
wrote: I also have no antenna analyzer and no means to acquire one any time soon... best I can do is my transmitter (cranked way back in power) and an SWR meter. Hi Rick, Then using what you have, replace the antenna analyzer with the SWR meter. That is, pass a short length of wire through the core you intend to invest in (which begs the question: Why don't you simply go to the right source and buy the right product?) and pour enough power to this "load" to see what it measures in SWR. If it is the right material (Amidon 73, 75, or 77 material). and the right size (101 size bead) and the right frequency (60M or 12M); then you should read something less than 2:1. Make it two beads, and it would still read something less than 2:1 (but closer to 1:1). You should note there are three variables above: material, bulk, and frequency. Bulk adds isolation. Material and frequency go hand-in-hand. As you have ferrite of unknown parentage you are adrift on two counts and must accept what this ******* will have to offer. You might be lucky, but it will undoubtedly cost more than doing it the right way. Anyway, returning to the practical investigation.... A 101 size bead will support perhaps a Watt or two at 100% duty cycle and you can raise that by the inverse proportion of DC. If you pour 100W into this load to read the SWR, I doubt your meter will have a 10mS peak reading capture time (unless you have a Daiwa). If you pour 1W into it, you probably are not going to get a reading at all (that lends any confidence at being accurate). 10W excitation should be adequate and lest anyone complain that the SWR is indistinguishable from that arising due to X (we are seeking R) - I will anticipate that as I have in the past and ask you to grab onto the bead immediately after the reading. A blister on your finger and thumb will stand in telling corroboration to the reading of nearly 1:1 that the bead exhibits an R of 50 Ohms. A cool bead will be silent testimony to high X. It is sometimes a wonder here how commonsense has to be spelled out. Now, as you have none of the "right" variables, you need only judge what you DO HAVE in relation to them. Merely repeat this with the Radio Shack item. You need to balance the consideration that bulk=Resistance and that your SWR reading may rise because it is either: 1. Doing TOO good; or 2. Doing TOO poorly. The Radio Shack bulk may lend itself to presenting 100 Ohms (too good) or is just sputtering along with 10 Ohms (too poor). As with resistors, you can judge this through series/parallel combinations to seek improved indications. If two loads in parallel bring the SWR reading into 1:1, then you have your baseline to build out to the proper sized Isolator. If two loads in series bring the SWR reading into 1:1, then you have your baseline to build out to the proper sized Isolator. (Amazing what knowledge can do when you have just a little of it.) This, of course, returns us to just how many Radio Shack clip-ons do you need? My guess is they could handle 4W to 8W at 100% D.C. and if one of them exhibits 50 Ohms you can do the math - depending on your choking requirements. I have no way to define "expected performance" any way other than that. Then we once again enter into the fascinating world of Amateur experimentation, an arena that comes with no guarantees and the prospects of blistered fingers. As you have no way of quantifying your goal, you have to judge it subjectively (no one else can do this for you, as you are the subject in subjectivity). Not knowing what Z is present at the distal end of the 20 Foot transmission line drop can demand isolation from 500 Ohms to 50,000 Ohms. It is unlikely that the isolation will absorb all the power your rig has to offer (but if it does, it will make a nice SWR match, one of those glowing rewards for focusing on the wrong goal). So, by these metrics, you can get along fine with 10 to 1000 of the mythical 50 Ohm Radio Shack clip-ons. Start with 10 (adjusted in proportion to their actual R) and add more until you have met your expected performance. Simple, n'est pas? 73's Richard Clark, KB7QHC |
#8
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
In article ,
Rick (W-A-one-R-K-T) wrote: You mention the "UnUn that comes with the commercial unit"... not sure if you meant a commercially-available CW antenna, but this one is being built from scratch using a W2AU-type 4:1 balun and some lengths of insulated copper-clad steel wire I have here. For what it's worth - last year, I home-brewed a coaxial RF isolator/choke, in an attempt to fix an RF-incursion problem involving our RACES 40/80 trap dipole and the phone system in the building at which the dipole is installed. I used some fairly large-diameter ferrite beads - probably 43 material, if I remember the impedance readings I took correctly. They were an inch or so long... I superglued a bunch of them together (around 8 or 10, I think) to form a long ferrite cylinder. The ID was large enough to let me get a length of RG-8X through the cylinder a total of three times... in effect, making a very long, thin three-turn choke. The whole thing was installed in a length of PVC pipe with end caps and N connectors. The common-mode impedance along the braid of the coax turned out to be quite high. If I recall correctly, it was at the upper limit of my MFJ-259's ability to read at 3.5 MHz (probably upwards of 1500 ohms, mostly reactive) and was off-the-scale at 40 meters and all higher frequencies. Whatever capacitive coupling occurred between the turns didn't amount to enough to matter. [Unfortunately, it didn't fix the RF incursion problem, which later experiments proved was caused by near-field pickup by the phone lines of RF from the antenna itself, and not due to feedline RF at all.] You could probably build a similar unun using the Radio Shack ferrites. If they aren't big enough to allow multiple turns of RG-8X, you could use a short length of RG-58 or an even smaller-gauge 50-ohm coax. Losses in this short a length of thin coax shouldn't amount to enough at HF to matter, or to cause appreciable heating unless you're running very high amounts of power. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#9
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
Thanks to all for your help on this ferrite core question. The objective was to see if I could put up the antenna with only what I have on hand... and I have a 50-foot length of RG-8X and four of those radio shack ferrite cores. I also have an MFJ unun and so I could cut the coax at 22 feet and put in a couple of coax connectors, and use the unun, I just hadn't gotten around to doing that and anyway I don't have the connectors (but, the Radio Shack is just down the street). Someone said "try it and see what works", so I did that... put up the antenna this afternoon. Unfortunately it started raining and the XYL started yelling at me to get my @$$ off of the roof before I fell off :-), so I didn't get the unun OR the ferrite cores installed yet. I figured what the heck, may as well try it without the choke and see what happens. I inserted the unun into the line where the coax comes in the window and fired it up. It actually seems to work better than my inverted V on 20 meters (only band I've tried it on so far), though the difference isn't exactly what one might call dramatic. Also the SWR is pretty poor but nothing the rig's internal tuner can't handle. I'll get the cores or the unun up there tomorrow and see how it acts then. |
#10
|
|||
|
|||
Ferrite cores instead of a 1:1 current-choke UnUn for a Carolina Windom
"Rick (W-A-one-R-K-T)" wrote in
news .... Someone said "try it and see what works", so I did that... put up the .... Ah, you are well on the way to exploring the meaning of the term 'works' as in 'my antenna works real well'. Don't forget collecting a few DX QSL cards, that is often regarded as the most conclusive evidence. Owen |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
FS: Carolina Windom 75 Meter Ant | Swap | |||
Ferrite Choke Cores | Shortwave | |||
Carolina Windom | Antenna | |||
TV type Ferrite Cores / Ferrite Cores / Magnetic Longwire Baluns (MLBs) and more | Shortwave | |||
carolina windom vs dipole | Antenna |