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KF7CJJ
03-23-2012, 02:43 AM
Hello, I am homebrewing an off-center-fed dipole for 40m and shorter. It hangs between two poles about 100 feet apart, about 32' above ground. The two limbs are roughly 2/3 and 1/3 length of half-wave at 7.100 MHz. I took the design from W8JI and a little from W0HC. W8JI explained how the placement of the feedpoint allows you to change feedpoint impedance, to find best compromise in impedances for multiple bands. A third ham pointed out in an article that performance on 15 and 10m could be improved with a second set of limbs cut to 30m, fed from same feedpoint and hanging below the main limbs with some separation angle.

I'm trying that with my OCF and running into problems. My strategy has been to adjust overall length of the dipole to get resonance at the longest band, by taking up slack at the feedpoint. Then use that slack to move the feedpoint back and forth to get workable SWR on as many bands as possible. Using Autek RF1.

When I adjust overall length of the dipole to minimize SWR on 7.100, the SWR instead heads for the roof, so high a tuner can barely bring it in. Tried this with and without the accessory dipole for the higher bands connected to the feedpoint.

Note that I don't have a balun at the feedpoint. Instead, I have a section of twin-lead running from the feedpoint down to near the shack, and a balun right where coax comes out of the house from the radio equipment.

I am guessing that the length of the twinlead is adding to the length of the dipole as I get the dipole close to resonance. Would prefer not to have the balun up there at the feedpoint, with the weight of it and the coax to feed it. Any ideas on what I'm doing wrong? Would a specific length of twinlead feedline, some function of lambda at 7.100 MHz, prevent this?

Am also considering a means to get this antenna about 25' higher, it is probably an NVIS in 40m band.

73 de KF7CJJ

WB2WIK
03-23-2012, 02:49 AM
OCF's vary in feedpoint impedance all over the place based on where the feedpoint is. With a 33% off center fed as you describe, a 6:1 balun is the norm to try to match impedance to 50 Ohm coax. Some of the "great" OCF's 33% off center fed used 9:1 baluns.

Why would you use twin lead to feed a very unbalanced antenna?

I'd use a 6:1 balun or so at the feedpoint, and then coax. But in all cases, this is an antenna that will require a tuner.

KF7CJJ
03-23-2012, 02:57 AM
yessir, am using 6:1 balun and LDG tuner.

The antenna is made of 17ga aluminum electric fence wire. Strong, but I don't want to push my luck using it to hold that balun and the weight of RG58 (let alone RG8) up there. The lower loss of the twin lead is also desirable. My QTH has very strong winds.

K6GB
03-23-2012, 12:59 PM
I think if you want to use twinlead you pretty much have to center feed, I did the center fed 80m dipole with ladder and it worked very well. Dale

K8JD
03-23-2012, 01:43 PM
The balun is part of the antenna and you need to feed that with coax or you will have another unspecified Z transformation with twinlead between ant and tuner !
I would rethink that design.
I never liked "all band" dipoles because of the radical difference in X and Z from band to band.
I finally decided to experiment with a 90 ft dipole fed with TV twinlead and a MFJ bal line tuner. It does ok on 60-20M for me but I also have a fan dipole for 30/20M that requires no tuner !
My butternut vert has traps and stubs to make it "all band" and also needs no tuner.

K4SAV
03-23-2012, 01:45 PM
An OCF will never work with twinlead as the feedline. The feedline will transform the impedance to something completely different from what you are expecting. It will also have very high common mode currents on the feedline and that feedline will be a radiating part of the antenna. If you want to read about what the feedline does, look at Cecil's website.

http://www.w5dxp.com/notuner.htm

You can either center feed it with twinlead, in which case the feedpoint impedance will be very high and you can use the tuner to match it, or use it as an OCF by placing the balun at the feedpoint and try to get the feedpoint impedance to be a reasonable match for 50 ohms.

Jerry, K4SAV

WX7G
03-23-2012, 03:13 PM
KC7CJJ to answer your original question yes there is a specific length of twin lead might improve things for the 40, 20, 15, and 10 meter bands. That electrical length is 180 degrees at 7.1 MHz.

What is the physical length? That depends on the VF (Velocity Factor) of your twin lead. Using a VF of 0.82 the length is 57 ft. The impedance at the OCF feedpoint will appear unchanged at the end of the 180, 360, 540, or 720 degree transmission line. IF it is resistive at the OCF feedpoint it will be resistive at the end of the transmission line.

Now what about the coax? That too will transform the impedance and it can be made 180 degrees at 7.1 MHz. For a VF of 0.66 (solid polyethylene coax) the length is 46 ft. For foam dielectric coax it is about 58 ft.

If you have an antenna analyzer, such as an MFJ-259, you can measure the VF of your coax and twin lead and cut them to the correct length. If you have only a VSWR meter you can still find the correct length by using a 50 ohm load.

K4SAV
03-23-2012, 04:28 PM
KC7CJJ to answer your original question yes there is a specific length of twin lead might improve things for the 40, 20, 15, and 10 meter bands. That electrical length is 180 degrees at 7.1 MHz.


If the antenna was at 70 ft height or higher and the half wavelength of feedline was routed straight down and there was a choke at the junction of the feedline and the coax (no 6 to 1 balun), you might have a chance of making this work on more than one band, although the SWR won't as good as with most OCFs. However with the antenna at 30 ft, that means more than half the feedline will not be vertical, and may be close to the ground. Since using twinlead with an OCF will result in the feedline being a radiating part of the antenna, that feedline will change the feedpoint impedance (in addition to the normal transformation due to impedance mismatches), and being close to the ground, and bent, it will change a lot, and that ground proximity will also reduce the gain of the antenna. If you play with the length of the feedline long enough you can probably get the SWR low on a single band, although you may need a small adjustment in antenna length.

You would be much better off constructing a conventional OCF.

Jerry, K4SAV

VE3EKJ
03-23-2012, 04:39 PM
yessir, am using 6:1 balun and LDG tuner.

The antenna is made of 17ga aluminum electric fence wire. Strong, but I don't want to push my luck using it to hold that balun and the weight of RG58 (let alone RG8) up there. The lower loss of the twin lead is also desirable. My QTH has very strong winds.

One of the guys already told you - using twin lead is just asking for trouble!

If you worry about the weight of the balun then why not use what's called a 'messenger' support wire, like the electrical service to the house? Get some 1/2" Dacron or similar rope and use that for the main span. Hang the balun from its top hook from the rope. Angle the antenna wire down slightly with maybe an 18" piece of 3/8 cheap grey PVC tubing with holes drilled where necessary to act as a spacer. This will handle the main wire and you can handle the 30 m wire the same as you've been doing.

Make sure it's a current balun! The ONLY OCF design that uses a voltage balun is the Carolina Windom, because it WANTS RF on the feedline! It uses a portion close to the feedpoint as a vertical radiator, isolated at the bottom with another balun to choke or block the shield RF.

Messenger wires work great! Takes all the worries away! Some guys just loosely twist the antenna wire around the rope, not bothering with the spacers. I wonder about rainy weather losses from the antenna to the wet messenger but if there's enough rope trailing from each end maybe I worry too much.

WX7G
03-23-2012, 06:25 PM
The best fit I can get in NEC for a 40, 20, 15 and 10 meter OCF is 68' long fed 27' from the end.

To keep the VSWR below 3:1 over these four bands it needs to be fed with a 4:1 current balun having a common-mode impedance of 2000 ohms or greater. The balun common-mode impedance was modeled as 1400 +j1400 ohms on each band. Higher is better. The balun is placed at the antenna and 50 ohm coax runs to the shack.

K0BG
03-23-2012, 08:18 PM
The article Tom did was more of a demonstration, than a real antenna design. The biggest issue with OCF antennas is common mode. Most light weight ratio baluns are voltage type, and do nothing to quell the common mode. To do the job right, requires two cores, and a lot of extra weight. Even then, depending on the environment they're erected in, you may still have enough common mode (regardless of the balun used) to cause you RFI problems.

WE6C
03-23-2012, 08:45 PM
If you want to use a 40m wire all bands (40 and up) and twin lead, feed it in the center. Read this...

http://vk1od.net/transmissionline/LOLL/index.htm

Very simple.

KF7CJJ
03-23-2012, 09:16 PM
Balun is RadioWavz 6:1 http://www.hamradio.com/detail.cfm?pid=H0-008705 which is sold as a current balun. It is weatherized so I can come to grips with yarding it up to the feedpoint. Point taken on the messenger.

I can't reach my SWR figures at the moment---am traveling---or I'd have posted those. SWR on shorter bands was actually tolerable. But RF in the shack on either 40 or 20 meters, depending on where I left the feedpoint. My cordless phones would ring if I keyed on SSB.

BTW, not doing any CW yet.

I fully expect to use a tuner on this antenna, on all bands.

If I move the antenna up higher, I will go longer and try for 75m as well, getting closer to that described in W8JI.

73 de KF7CJJ

KF7CJJ
03-23-2012, 09:18 PM
beg pardon, what is a conventional OCF, compared to what I'm trying?

WX7G
03-23-2012, 09:55 PM
Balun is RadioWavz 6:1 http://www.hamradio.com/detail.cfm?pid=H0-008705 which is sold as a current balun. It is weatherized so I can come to grips with yarding it up to the feedpoint. Point taken on the messenger.

I can't reach my SWR figures at the moment---am traveling---or I'd have posted those. SWR on shorter bands was actually tolerable. But RF in the shack on either 40 or 20 meters, depending on where I left the feedpoint. My cordless phones would ring if I keyed on SSB.

BTW, not doing any CW yet.

I fully expect to use a tuner on this antenna, on all bands.

If I move the antenna up higher, I will go longer and try for 75m as well, getting closer to that described in W8JI.

73 de KF7CJJ

Using the 6:1 balun listed, and the dimensions I gave, it should work well with a tuner on 40, 20, 15, and 10 meters. It will not work well on the WARC bands.

With it 136' long and fed 54' from the end it should work well on every band but 18 meters.

VE3EKJ
03-23-2012, 10:32 PM
beg pardon, what is a conventional OCF, compared to what I'm trying?

A conventional OCF is always feed with a balun, never balanced line. Some guys can make balanced line work but it takes a lot of screwing around and is definitely rarely used.

A voltage or a current balun will work but there are different results. The feedpoint is unbalanced so if you use a voltage balun there will be RF flowing on the shield of the coax feedline. For years this was the norm, since few hams had even heard of current baluns or were aware there is a difference. Using a current balun forces the current to be as equal as possible in the two sides. It's not perfect but it's a quantum level better than using a voltage balun! Using the current balun in most installations where the two legs are spread at a good angle, either horizontal or as a inverted V, usually has so little RF on the feedline that there are no problems. For the few occasions where you end up with RF in the shack a 'choke" 1:1 balun just outside where the feedline enters the house is usually enough to fix it.

Only one design of OCF uses a voltage balun anymore, to my knowledge anyway. This is a Carolina Windom. The conventional OCF on 15 metres does not present an impedance anywhere in the ball park to give a good match through the balun into the coaxial 50 ohm line. So with the Carolina design they use a voltage balun to deliberately allow RF currents to flow on the outer shield. They drop down the feedline a resonant length for a 15 metre radiator and then use a feedline choke balun to cut the outer shield currents off at that point. The feedline continues down to the shack and can be any length. The portion near the balun at the feedpoint acts as a reasonably good 15 metre radiator and also emits a bit on the other bands.

Often the feedline choke is not good enough with the Carolina Windom and so many users have to do extra work to get rid of that RF riding down the shield and into the shack. Still, there is no doubt that it can work well and often works just fine "out of the box".

You already know that you can add a 30 meter extra "windom" that will also give 15 metres. If the extra two legs are spaced well away from the others many guys have reported excellent success but for some reason few guys seem to be doing it. Perhaps they simply rely on their tuner to work 15 metres. It may not be the most efficient but 15 metres is the type of band where if the band is open it doesn't make much difference.

The question about ratios of the current balun depends on height of the feedpoint. At the usual 35-40 feet most of us use a conventional OCF shows a load for all its bands of around 200 ohms, plus and minus a fair bit. A 4:1 balun transforms these different band loads reasonably well to deliver 2:1 SWR or less. Some bands will be much better than others.

When the antenna feedpoint is higher that feedpoint load averages around 300 ohms. At heights approaching 55 to 60 feet a 6:1 balun will usually work better. Some guys use a 6:1 balun at any height. Usually this is close enough, especially with rigs that have an automatic tuner. However, the height factor still stands as one of Nature's Laws of the Universe. If you are at 35-40 feet and you want to have a match close enough to not need a tuner, especially if you are using an old tube rig that is more versatile as to the SWR, then use a 4:1 balun at that height.

Hope this isn't too much information!

Wild Bill

K4SAV
03-24-2012, 01:24 AM
You already know that you can add a 30 meter extra "windom" that will also give 15 metres. If the extra two legs are spaced well away from the others many guys have reported excellent success but for some reason few guys seem to be doing it. Perhaps they simply rely on their tuner to work 15 metres. It may not be the most efficient but 15 metres is the type of band where if the band is open it doesn't make much difference.


I have never seen anyone accomplish this. What kind of dimensions would you need and how would it be arranged?

You can't just add a center fed half wavelength wire the same way you do to a fan dipole. The feedpoint impedance at the OCF's feedpoint is relatively high compared to that of a fan dipole. The 30 meter element will cause significant loading on all the other bands, enough to mess up the SWR on all bands.

I thought maybe adding the 30 meter element on the 50 ohm side of the balun might work. I had the elements at about 35 degrees from each other and I couldn't get the SWR on 30 below about 3.4 to 1. (I used WX7G's suggested dimensions for the OCF). The longer OCF element loads the 30 meter element and messes up the SWR there too. When I moved the two elements to right angles, the SWR on 30M came down to about 2 to 1, however the 15 meter SWR went to about 5 to 1 (actually moved it above the band).

Jerry, K4SAV

VE3EKJ
03-24-2012, 02:38 AM
I have never seen anyone accomplish this. What kind of dimensions would you need and how would it be arranged?

You can't just add a center fed half wavelength wire the same way you do to a fan dipole. The feedpoint impedance at the OCF's feedpoint is relatively high compared to that of a fan dipole. The 30 meter element will cause significant loading on all the other bands, enough to mess up the SWR on all bands.

I thought maybe adding the 30 meter element on the 50 ohm side of the balun might work. I had the elements at about 35 degrees from each other and I couldn't get the SWR on 30 below about 3.4 to 1. (I used WX7G's suggested dimensions for the OCF). The longer OCF element loads the 30 meter element and messes up the SWR there too. When I moved the two elements to right angles, the SWR on 30M came down to about 2 to 1, however the 15 meter SWR went to about 5 to 1 (actually moved it above the band).

Jerry, K4SAV

This might help, Jerry!

http://kh2d.net/windom.cfm

It shows how this ham did it!

Wild Bill

K4SAV
03-24-2012, 05:59 AM
This might help, Jerry!

http://kh2d.net/windom.cfm

It shows how this ham did it!

Wild Bill

Thanks for the link.

For some reason I was thinking he wanted to add 30 meters to a 40 meter OCF. Anyway, the link is interesting. It's a modified 80 meter OCF and supposed to add 15 meters by adding another set of wires, however there must be some undocumented features in what he built that lets him get those low SWR readings. I modeled the same thing he documented and at the same height (with no feedline). Here is a comparison of his measured to my calculated.

Freq ___ His SWR ___ My SWR
3.8 MHz ___ 1.5 ______ 8.7
7.0 _______ 2.2 ______ 4.6
10.125 ____ 3.4 ______ 9.9
14.0 ______ 1.9 ______ 2.5
18.11 _____ 1.5 ______ 2.5
21.1 ______ 1.5 ______ 2.1
24.9 ______ 1.4 ______ 2.8
28.5 ______ 2.1_______ 2.9

The difference could be due to taking the SWR readings from the shack when using some very lossy coax, or from a balun that really doesn't look like a balun. It could also be due to using a voltage mode balun which allows large common mode currents. One manufacturer of OCF antennas does exactly that in order to get a low SWR on 80 meters (with the right arrangement of height and feedline length).

A better choice for an 80 meter OCF that includes 15 meters would be W8JI's version - much simpler and lower SWR.
http://www.w8ji.com/windom_off_center_fed.htm

Jerry, K4SAV

G3TXQ
03-24-2012, 10:34 AM
Jack Belrose co-authored a 1990 QST article which showed a "2-fan OCFD" - or a "double windom" as he called it - which covered 80m, 40m, 30m, 20m, 17m, 15m, 12m and 10m. One dipole was a horizontal 45.28ft+90.88ft and the other was an inverted-V (100 degree included angle) 15.39ft+30.77ft. He used a 6:1 balun at the common feedpoint. SWR was below 1.5:1 on all band except 40m where it was below 2:1.

The same article shows a 9 band "double windom" covering 160m thru 10m.

Steve G3TXQ

K4SAV
03-24-2012, 01:58 PM
Jack Belrose co-authored a 1990 QST article which showed a "2-fan OCFD" - or a "double windom" as he called it - which covered 80m, 40m, 30m, 20m, 17m, 15m, 12m and 10m. One dipole was a horizontal 45.28ft+90.88ft and the other was an inverted-V (100 degree included angle) 15.39ft+30.77ft. He used a 6:1 balun at the common feedpoint. SWR was below 1.5:1 on all band except 40m where it was below 2:1.


Thanks Steve.

The link before used a 4 to 1 balun, not a 6 to 1. I'm not sure what height was used for the antenna with the 6 to 1 balun. Changing the model to a 6 to 1, the SWR numbers don't come close to "1.5 to 1 on all bands except 40". Here are the numbers from the model using an ideal 6 to 1 balun with the antenna at 40 ft, and choosing the frequency in the band where the SWR is the lowest.

Band ____ SWR
3.5 MHz ___ 4.8
7.0 _______ 4.2
10.1 ______ 5.3*
14.0 ______ 3.6
18.1 ______ 2.7
21.0 ______ 1.8
24.9 ______ 1.5
28.2 ______ 1.3*
* SWR curve is very steep at these frequencies and so could be a lot different for small variations in the antenna.

It's obviously a case of the model not including everything that was built. I can only guess as to what that is. The large errors at low frequency makes me suspicious of a balun that has a lot of loss at low frequencies, or the use of a voltage mode balun.

Jerry, K4SAV

KF7CJJ
03-25-2012, 07:42 PM
The KH2D article mentioned a Canadian firm that made a double windom, with second set just to improve 15 and 10m. I found another article that described the double windom, can't find it just now.

I did find the Belrose & Bouliane article though http://n9wn.com/data%20files/OCF%20Dipole.pdf, and by golly it shows the antenna separated from the balun by 300-ohm feedline.

I had read W8JI, N9WN, and some other articles on OCFs before I attempted this antenna.

K4SAV
03-26-2012, 12:53 AM
I did find the Belrose & Bouliane article though http://n9wn.com/data%20files/OCF%20Dipole.pdf, and by golly it shows the antenna separated from the balun by 300-ohm feedline.


The 300 ohm line version was something from the 1940's. The version they built and tested used twin coaxial cables to implement a balanced 190 ohm line. Initially there was no balun. They measured some parameters of the antenna including current in the wires of the feedline and common mode currents into the ground and there are graphs showing that. Notice the huge ground currents. They were surprised at this. Because of that they recommended using a current mode balun to match the antenna. Unfortunately they didn't test it that way, or just decided to not publish the results, so there is no data on what it does with a current mode balun.

In my opinion, the large ground currents is the cause of the antenna being broadbanded on the low frequency end. It also decreases the antenna gain. The SWR and gain will be a function of feedline length, routing, and termination, which means it is highly unlikely that you will be able to duplicate their results. If you add the current mode balun, the response will probably be completely different.

HOWEVER, in the appendix of that article there is more data, done by someone else, and there is a version of the same antenna that is supposed to be fed with a 6 to 1 current mode balun. They show an SWR on 3.5 MHz of about 1.1. A quick gross calculation says that can't happen. If you take a 136 ft wire at 25 ft height (the height they used) and feed it at the 1/3 point, the impedance on 80 meters should be about 66 ohms. (It would be much higher at higher heights). Run that thru a 6 to 1 balun and you have 11 ohms. There is no way this gives an SWR of 1.1 in a 50 ohm system. The other wires do change the impedance, and an analysis of the complete antenna shows the feedpoint impedance on 80 meters to be about 17 ohms, including the balun.

Bottom line is that good experimental trumps good analysis every time. However the difference is usually due to not including everything in the antenna model. In this case it is probably that the balun is a voltage mode balun or it has a lot of loss at low frequencies.

Jerry, K4SAV

KF7CJJ
03-26-2012, 02:01 AM
Jerry, thanks. The plan is shifting to putting the wire up at closer to 50 feet, 137 feet total length, fed at 1/3 length. First I'll try with half lambda of 300 ohm feedline. Meanwhile I will spend more time with the Extra class handbook's antenna chapters.

VE3EKJ
03-26-2012, 09:07 PM
Jerry, thanks. The plan is shifting to putting the wire up at closer to 50 feet, 137 feet total length, fed at 1/3 length. First I'll try with half lambda of 300 ohm feedline. Meanwhile I will spend more time with the Extra class handbook's antenna chapters.

There are always a lot of arguments using modelling software and theory but we should keep in mind that it is always easier to explain after the fact than to predict in advance!:D

That's because there are often factors we did not take into account and assumptions we make that are wrong.

Meanwhile, there are a LOT of users of OCF dipoles that are getting consistent and positive results! This cannot be ignored. A computer model can never refute such fact.

Also, most of these arguments use models and data based on perfection, when the entire idea of an OCF dipole is a compromise! It can never work any better than any 80 metre dipole, since the only difference is the loads presented at the feedpoint. On the higher bands, like any multiple of a half wave dipole the antenna may show gain in some directions but only at the expense of nulls in others. There ain't no free lunch! Overall, the losses at the feedpoint seem to be more than reasonable, if not trivial.

What are we looking for here, a dipole feedpoint trick that will make it perform like a rhombic, in all directions?

An OCF dipole can work on multiple bands as well as any other dipole. The match will be low enough for a tube final and well within the range of a solid state transmitter with a tuner.

What more do we need? What more do we expect? If we weren't in a non-perfect location we would simply put up antennas all over the place!

Me, my OCF dipole is a great real-world compromise that gives me much better performance than my old ground mounted vertical surrounded by all the high fences and buildings. I could put up a simpler 80 metre dipole fed with balanced line and use a tuner, but I just don't want to screw around with more adjustments when the OCF dipole will work essentially the same anyway!

KF7CJJ
03-26-2012, 10:28 PM
There are always a lot of arguments using modelling software and theory but we should keep in mind that it is always easier to explain after the fact than to predict in advance!:D

That's because there are often factors we did not take into account and assumptions we make that are wrong.

Meanwhile, there are a LOT of users of OCF dipoles that are getting consistent and positive results! This cannot be ignored. A computer model can never refute such fact.

Also, most of these arguments use models and data based on perfection, when the entire idea of an OCF dipole is a compromise! It can never work any better than any 80 metre dipole, since the only difference is the loads presented at the feedpoint. On the higher bands, like any multiple of a half wave dipole the antenna may show gain in some directions but only at the expense of nulls in others. There ain't no free lunch! Overall, the losses at the feedpoint seem to be more than reasonable, if not trivial.

What are we looking for here, a dipole feedpoint trick that will make it perform like a rhombic, in all directions?

An OCF dipole can work on multiple bands as well as any other dipole. The match will be low enough for a tube final and well within the range of a solid state transmitter with a tuner.

What more do we need? What more do we expect? If we weren't in a non-perfect location we would simply put up antennas all over the place!

Me, my OCF dipole is a great real-world compromise that gives me much better performance than my old ground mounted vertical surrounded by all the high fences and buildings. I could put up a simpler 80 metre dipole fed with balanced line and use a tuner, but I just don't want to screw around with more adjustments when the OCF dipole will work essentially the same anyway!

Before I started tweaking the length and moving feedpoint, this antenna would tune up only on the lower half of the phone portion of 40m. It ran quite well on 20m, 17m, and 10m. It would tune on 6m but I couldn't hear any traffic there to tell whether I could make contacts.

I started tweaking just to get all of the 40m phone band. If I could put the antenna back to where it was, maybe I would, and instead change the length of the feedline, to get it as close as possible to half lambda at 7.100, then re-measure SWR on all bands 40m and shorter. That ship has sailed, am now concentrating on increasing height to ~50', increasing fundamental resonance length to 75m, and cutting feedline to half-lambda.

Eventually I'll have all the paperwork in place to put up some Rohn 25G and the K4KIO hexbeam, but I'll still rely on the OCF to detect what I want to point the hexbeam at.

73 de KF7CJJ

VE3EKJ
03-27-2012, 01:02 AM
Before I started tweaking the length and moving feedpoint, this antenna would tune up only on the lower half of the phone portion of 40m. It ran quite well on 20m, 17m, and 10m. It would tune on 6m but I couldn't hear any traffic there to tell whether I could make contacts.

I started tweaking just to get all of the 40m phone band. If I could put the antenna back to where it was, maybe I would, and instead change the length of the feedline, to get it as close as possible to half lambda at 7.100, then re-measure SWR on all bands 40m and shorter. That ship has sailed, am now concentrating on increasing height to ~50', increasing fundamental resonance length to 75m, and cutting feedline to half-lambda.

Eventually I'll have all the paperwork in place to put up some Rohn 25G and the K4KIO hexbeam, but I'll still rely on the OCF to detect what I want to point the hexbeam at.

73 de KF7CJJ

I might have missed it but what sort of tuner are you using to feed that balanced feedline? The reason I ask is that many tuners today simply offer a balanced line output by sticking a 4:1 voltage balun inside the case. That sort of arrangement cannot possibly handle the range of loads that an old-fashioned true balanced line tuner could do.

If this is the case perhaps you could consider rolling your own or scouting around for an old Johnson Matchbox with a true balanced line output.

Another thought that just occurred to me would be to replace a voltage balun inside a modern matchbox with a current balun. Because a true current balun has two cores you might find things too cramped but if you wind your own so that you only have to worry about fitting the cores and not the whole usual casing...

Wild Bill

KF7CJJ
03-27-2012, 01:28 AM
Tuner is LDG Z100, balun is RadioWavz B16C. I looked for tuners that output directly to a balanced pair of bananas or binding post, would consider a kit.

The place I'm considering for higher elevation of this antenna will not give the full 137' length end to end, it is only about 100'. I'll be drooping the two ends by ~18' or more.

VE3EKJ
03-27-2012, 04:02 AM
Tuner is LDG Z100, balun is RadioWavz B16C. I looked for tuners that output directly to a balanced pair of bananas or binding post, would consider a kit.

The place I'm considering for higher elevation of this antenna will not give the full 137' length end to end, it is only about 100'. I'll be drooping the two ends by ~18' or more.

I'm not familiar with that brand of tuner. It might be worth opening the case to have a look. A pair of banana jacks or binding posts is no guarantee that it is a true balanced tuner. A voltage balun inside would be fed from the coaxial line output and then be connected to those binding posts!

This is just not the same as a circuit specificially arranged to feed a balanced transmission line.

As far as drooping the ends - I wouldn't worry about it. Making it a bit of an inverted V will not likely make any significant difference in loading or matching. As far as radiating, it might actually work a bit better! The inverted V arrangement adds a bit of different polarization off the radiating wire. This can be an advantage during band conditions with signals fading down and peaking up. Different polarizations can combine to reduce QSB.

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