Author
Message
dammac751
Member
Offline
Posts: 24
Feedback: 0% (0)
Posted: 8/14/2013 6:05:07 AM EST
[Last Edit: 8/17/2013 4:42:38 PM EST by dammac751]
I've built a multi band ocfd antenna and having a heck of a time with swr.

Antennawas built with a Unadilla 4:1 balun and 14 gauge uncoated Flexweave wire.
Antenna is 38' at peak. Each leg is at 11-12' above ground.
Antenna is hung from a tree but there are no limbs within 3' of the antenna. The closest it comes to my house is about 20'.
Antenna is fed by 75' of new RG8x coax. The coax acts as a section of vertical radiator beneath the balun with snap-on ferrite chokes at 22' to stop the rf traveling to the shack inside.

I started with the wire legs at 45' and 90' based on a Buxcomm commercial antenna. Only bands usable were 40m and 17m. SWR was below 2, the rest were 2.5on up.
I then tried another formula(done lost where I found it) with legs at 40.6' and 82.5'. The only usable band was 80m. the rest were reading SWR over 3.5.

Third try, I come here and search the archive and find AFreeMans post about building this type of antenna. I use the measurements he suggested of 41' and 89'. That measurement will give me 1:1 SWR all across the phone portion of 40m. The 80m phone band ranged from 2 at the bottom to 3 at the top. The SWR reading on the remaining bands were well above 3.

I read article after article where people use thes without a tuner but I am having a heck of a time.
If anybody would have an idea where to start I'd appreciate some help.


I found the high SWR problem I was having. The vertical radiator portion of the coax was the culprit. After trying different wire leg legnths and feed point locations and gaining very little I decided to move the ferrite beads to the top of the coax right beneath the balun. Instantly saw a big improvment in swr. I now have 6 bands under 3:1 with 4 of them under 2:1 with no tuner. Big thanks to everyone with the suggestions. I also got my first ever HF QSO, then bagged a few more in a few minutes time. All before I realized I had the power set at 15 watts.
I was frustrated after a day or two but remembered this is a hobby and supposed to be fun. I've certinly enjoyed experimenting with this antenna.

Jupiter7200
Offline
Posts: 427
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 6:31:07 AM EST
[Last Edit: 8/14/2013 6:35:33 AM EST by Jupiter7200]
The "works all bands without a tuner" thing is advertising hype. But what you can do is work a lot of bands with an antenna that is tunable.

The 90' + 45' Buxcomm version I think uses bare wire, which is always a little longer than insulated wire for the same resonance. I've used 88' + 44' for insulated wire.

If you have one leg cut to 82' now, put 50' on the other side. That is the ratio for the 80 m Carolina Windom. Since that leg is shorter than 50' now, you don't have to use all new wire. Just add on another section of wire and insulator, and add a jumper around the insulator joining the two wires.

A few things I've learned about OCFD's (they are NOT true "Windoms"):

(1) Cut the antenna for the lower end of the lowest band so that the harmonics will like up correctly with the higher bands. If you make the overall length for the 75 m phone portion of the band all of the higher harmonics will be up over the desired bands. Ideal is to make overall about 132'-135' so it will resonate at 3.5 - 3.55, which make the harmonics at 7.1, 14.2, 17.8 (but good enough for the rest of the 17m band), 24.9, and 28.4 mhz.

(2) 90' + 45' or 88' + 44' (1/3 - 2/3) will give the best overall split for most bands, but will not work 30 and 15 meters.

(3) OCFDs will read high on the lowest band if you only connect the coax to an analyzer. You must ground the analyzer or coax to get a correct reading. Even with the coax and analyzer grounded, the lowest band may read higher than some of the other bands. It is influenced by height over the ground.

How many ferrites did you use for the choke? For 80 meters you'll need more than a few, more like a dozen or so.

If you have 3.5:1 SWR or less on any band that band is tunable and usable. Just use your tuner.
Elijah1
Member
Offline
Posts: 4048
Feedback: 100% (1)
Link To This Post
Posted: 8/14/2013 8:41:10 AM EST
Are you sure your coax is ok? And the balun? I am guessing something is not right there. I have one double your size 90'X180' and I only need a tuner on 75m, 30m and the phone part of 15m.

My guess is that either the balun or coax has an issue.
"Dupe - FBHO & FPM" -SYSTEM
Jupiter7200
Offline
Posts: 430
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 9:00:36 AM EST
And I just noticed, you are using the Unadilla 4:1 balun. That's a voltage balun, which allows a lot of RF on the shield.

You will be more successful with a 4:1 current balun, which will keep the RF in the antenna where it belongs.
BlammO
Onomatopoeia incarnate
Offline
Posts: 6916
Feedback: 100% (2)
Link To This Post
Posted: 8/14/2013 9:21:39 AM EST
[Jump To Reply]Originally Posted By Jupiter7200:
And I just noticed, you are using the Unadilla 4:1 balun. That's a voltage balun, which allows a lot of RF on the shield.

You will be more successful with a 4:1 current balun, which will keep the RF in the antenna where it belongs.
Methinks this be the key.
Never send a man to do a neutron's job. -- BlammO
كافر
BlammO
Onomatopoeia incarnate
Offline
Posts: 6917
Feedback: 100% (2)
Link To This Post
Posted: 8/14/2013 10:31:40 AM EST
Also, if you have a 200 ohm non-inductive resistor, you can temporarily connect that to the balun in place of the antenna and test. You should have reasonably low SWR on all bands. If not, then you have a balun and/or feedline issue.

That won't tell you if you should be using a current balun instead of a voltage one, but it's a good verification test.
Never send a man to do a neutron's job. -- BlammO
كافر
Frank_B
Offline
Posts: 628
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 11:47:40 AM EST
[Last Edit: 8/14/2013 11:48:41 AM EST by Frank_B]
Don't become overly obsessed with SWR. While under 2:1 is nice, using your antenna tuner to bring higher ratios down to where the rig is happy will still often produce results. My five-band trap dipole gets pretty squirrely on some frequencies, but I still make contacts. Not too long ago I nailed New Zealand on 10 Meters with 50 Watts into a dipole with a 6:1 SWR.

I'd had my license over 20 years before ever got an SWR meter. I'd cut them to length, string them up, and work the world. But back then the transmitters had tunable Pi-network outputs and could handle a wide range of loads.
Jupiter7200
Offline
Posts: 431
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 12:37:47 PM EST
Add 10' back to the short leg, making it 50'. Leave the long leg at 82'. That will give 132' overall, and resonance down about 3.54 mhz. Then the upper bands will fall in line.

Ground (to earth ground) the analyzer, which grounds the coax shield, and see what SWR measures at 3.5, 3.8, 3.9 and 4.0 mhz. Check SWR at 7.0, 7.2, and 7.3 mhz, 14.0, 14.2, and 14.35, 18.1 and 18.16 mhz, 21.0 and 21.4, 28.0 - 28.5 - 29.0 mhz.

That should give you a pretty good idea of what this antenna will do. Report results here.
dammac751
Member
Offline
Posts: 25
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 2:53:00 PM EST
[Last Edit: 8/14/2013 3:52:04 PM EST by dammac751]
Ijust got back from a fine family outing, I appreciate all the info you've offered up. I just went out and realized there is not 75' of coax. There is roughly 23' from the shack to outside, then a Diamond brand static arrestor. From the arrestor it is 75' to the balun. I also realized that the ground wire from the arrestor was not clamped tight to the ground rod, just kinda hanging in the clamp. I'm going to get it secured tight and take new readings before proceding any further. Please stand by, and thanks for all the replies.

Edit, I bypassed the arrester and the short coax by moving the radio to the deck to help eliminate any issues they may have.
Jupiter, I did the cut that you suggested of 50'-82'. That was actually one I had thought of doing at first. I don't have an analyzer available, just an external swr meter.
. 80 meter started at 2.3 and raised to 5:1 at the top of the band. the rest of the bands showed read from 2.5:1 on up.
This antenna building is all very new to me, and very frustrating. I am going to lower it down and go over all measurements and connections before trying any thing else.
Jupiter7200
Offline
Posts: 433
Feedback: 0% (0)
Link To This Post
Posted: 8/14/2013 4:09:07 PM EST
[Last Edit: 8/14/2013 4:12:54 PM EST by Jupiter7200]
[Jump To Reply]Originally Posted By dammac751:
... I bypassed the arrester and the short coax by moving the radio to the deck to help eliminate any issues they may have.
Jupiter, I did the cut that you suggested of 50'-82'. That was actually one I had thought of doing at first. I don't have an analyzer available, just an external swr meter.
. 80 meter started at 2.3 and raised to 5:1 at the top of the band. the rest of the bands showed read from 2.5:1 on up.




At this point, are you tied to earth ground? If you don't have earth ground now, you will get some high readings.

The 50'-82' with 20' or 22' "vertical radiator" is the double size (80m) Caroline Windom, a 38% - 62% split. It is supposed to work 15 meters.

You want to stay around 132' overall, regardless of the split. Different splits will bring SWR on various bands up or down. Here's a thing from the OCFD yahoo group.

The following is related to a 80m OCF dipole (~135 feet total length):

* If you feed it at 8.9%, you don't lose any ham band (80m-6m)(no in
ham band sine nulls). Although the SWR will be higher on 80m.
* If you feed it at 16.6%, you lose 15m band (sine equals zero).
* If you feed it at 20% (20/80), you lose 17m and 6m bands (sine
equals zero).
* If you feed it at 25%, you lose 20m and 10m bands (sine equals
zero).
* If you feed it at 28.89%, you lose 12m band (sine equals zero).
* If you feed it at 30%, you don't lose any ham band (80m-6m).
Although the SWR will be a little higher on 12m.

* If you feed it at 33% (1/3:2/3) you lose 30m, 15m, and 6m bands
(sine equals zero).
* If you feed it at 36%, you don't lose any ham band (80m-6m).
Although the SWR will be a little higher on 30m.
* If you feed it at 41.6%, you don't lose any ham band (80m-6m).
Although the SWR will be a little higher on 17m and 12m.
* If you feed it at 50%, lose 40m, 20m, 15m, and 10m bands (sine
equals zero), but should do very well on 80m, 30m, 17m, 12m, and 6m bands.
This is a normal dipole, where you only get odd-harmonics.

By "lose," I mean the antenna will not be useable and will have very high
impedance (Sine goes to zero).



This antenna building is all very new to me, and very frustrating. I am going to lower it down and go over all measurements and connections before trying any thing else.


I enjoy experimenting with antennas. There is no magic "one antenna for all bands".
dammac751
Member
Offline
Posts: 26
Feedback: 0% (0)
Link To This Post
Posted: 8/15/2013 8:58:36 AM EST
I've lowered the antenna, check and remeasured the legs. 50' and 82" plus maybe and inch where they wrap at the insulator. I removed a few limbs that were about 4' over the long leg and restrung the antenna. I was able to get that leg about 5' higher in the air after removing the overhead limbs. No other changes were made.

SWR across a few of the bands as follows:
3.5mhz 1.8
3.7mhz 2.0
3.8mhz 2.7
4.0mhz 4.5

7.0mhz 4
7.1mhz 3.3
7.2mhz 2.6
7.3mhz 1.9

14.0mhz 4.5
14.1mhz 4.0
14.2mhz 3.5
14.3mhz 2.9

17meter was 6:1 all across the band
The upper bands all ranged from 4 to 6 swr readings. I can post more detail of needed.

It seems that there are definate dips in swr now. I'm assuming that because the one leg is higher than it was when I first started. Would it hurt to fine tune the wire length any to move swr dip? Would I add or shorten?

p.s. thanks Jup for the reading list. I'm already thinking of a new antenna project soon as this on is finished.

Jupiter7200
Offline
Posts: 436
Feedback: 0% (0)
Link To This Post
Posted: 8/15/2013 7:49:04 PM EST
[Jump To Reply]Originally Posted By dammac751:
I've lowered the antenna, check and remeasured the legs. 50' and 82" plus maybe and inch where they wrap at the insulator. I removed a few limbs that were about 4' over the long leg and restrung the antenna. I was able to get that leg about 5' higher in the air after removing the overhead limbs. No other changes were made.

SWR across a few of the bands as follows:
3.5mhz 1.8
3.7mhz 2.0
3.8mhz 2.7
4.0mhz 4.5

7.0mhz 4
7.1mhz 3.3
7.2mhz 2.6
7.3mhz 1.9

14.0mhz 4.5
14.1mhz 4.0
14.2mhz 3.5
14.3mhz 2.9

17meter was 6:1 all across the band
The upper bands all ranged from 4 to 6 swr readings. I can post more detail of needed.

It seems that there are definate dips in swr now. I'm assuming that because the one leg is higher than it was when I first started. Would it hurt to fine tune the wire length any to move swr dip? Would I add or shorten?

p.s. thanks Jup for the reading list. I'm already thinking of a new antenna project soon as this on is finished.



OK, so what if you shorten the antenna to move resonance up?

Then the bottom end of 80 m will likely go higher SWR, and the top end, 3.8 - 4.0 mhz (aka 75 meters) will improve.

But at the same time, 40 meters, the whole band will increase SWR.

Ditto on 20 meters.

What I would do now is shorten the short leg from 50' to 49'. That's easy enough.

On the other end I would leave the insulator, and add a 4' section and another insulator, jumper around the insulator from the 82' to the 4' section, which makes it now 135'.

This brings resonance down, but also shifts the feedpoint so that you should have lower SWR in the 80/75 m band.

Now see what happens. If not satisfactory, you can make the add on section another foot longer, and subtract another foot from the short leg.

But lets see what happens with 49' and 86'.
dammac751
Member
Offline
Posts: 27
Feedback: 0% (0)
Link To This Post
Posted: 8/17/2013 4:44:49 PM EST
Original Post Updated.
K9-Bob
Member
Offline
Posts: 3824
Feedback: 100% (42)
Link To This Post
Posted: 8/17/2013 5:40:40 PM EST


Find a friend with an antenna analyzer.......this is what I use.

People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf. -- George Orwell
Jupiter7200
Offline
Posts: 449
Feedback: 0% (0)
Link To This Post
Posted: 8/17/2013 5:54:35 PM EST
[Jump To Reply]Originally Posted By dammac751:
I found the high SWR problem I was having. The vertical radiator portion of the coax was the culprit. After trying different wire leg legnths and feed point locations and gaining very little I decided to move the ferrite beads to the top of the coax right beneath the balun. Instantly saw a big improvment in swr. I now have 6 bands under 3:1 with 4 of them under 2:1 with no tuner. Big thanks to everyone with the suggestions. I also got my first ever HF QSO, then bagged a few more in a few minutes time. All before I realized I had the power set at 15 watts.


So, what did you end up with for feedpoint, overall length, height over ground, and can you be more specific on your SWR measurements for each band? Will your tuner easily tune the antenna down to a good, usable SWR?



I was frustrated after a day or two but remembered this is a hobby and supposed to be fun. I've certinly enjoyed experimenting with this antenna.


That's what it's all about! And you learned some things, what works and what doesn't. Now YOU'RE the expert on OCFDs.
pcsutton
Member ....but not a 3rd member
Offline
Posts: 32340
Feedback: 100% (1)
Link To This Post
Posted: 8/17/2013 6:46:33 PM EST
I use true ladder line for feeding my dipole...and don't have those issues.
"I am compensating. If I could kill stuff with my dick from 200 yards I would not need a firearm would I?"-Zanther
"You sound like a man who would try to feed cats to ATM machines." - Shane333
dammac751
Member
Offline
Posts: 31
Feedback: 0% (0)
Link To This Post
Posted: 8/21/2013 7:52:39 AM EST
Sorry for the delay on the antenna.
Balun heigth 37'
Short leg height 12'
Long leg height 15'
Ferrite chokes placed on coax directly beneath balun. I have 10 on but need more, I have a touch lamp that will turn on magicly when I transmit on 80m.
Long leg is 90', short leg is 45'. Feedpoint is 33.33% from end.
SWR:
3.5mhz 1.2:1
3.8 2.7:1
4.0 3.:1

7 mhz 1.8:1
7.2 1:1
7.3 1.7:1

14mhz 2:1
14.2 1.7:1
14.3 1.5:1

17 meter is nearly flat at 1.5:1 across the band.
12 meter is flat at 2.9:1 across the band

My tuner has had no problem bringing these readings below 1.2:1. I havent played with 10m or 6m bands with it yet but don't forsee any problems.
I figured out with Jupiter7200's help that textbook measurements are just basic starting points. Start from there and make it work to the surroundings you have, (tree locations, height above ground etc.).
I am going to replace the voltage balun with a current type balun and see how that works. Also, the flexweave wire I used was very forgiving to all the twisting and untwisting I put it through.


Jupiter7200
Offline
Posts: 462
Feedback: 0% (0)
Link To This Post
Posted: 8/21/2013 12:32:00 PM EST
[Last Edit: 8/21/2013 12:37:05 PM EST by Jupiter7200]
Congratulations!

Nail it down right where you have it! Don't touch a thing! That's just about textbook perfect for an OCFD.

Check 10 meters, I'll bet it will work fine there, too.

With this split 15 meters won't work, even with a tuner. If you are able to tune anyway it will just overheat the balun. If you need 15 meters (a nice band) a simple dipole with 11' of wire on each side will put you in the middle of that band.

BlammO
Onomatopoeia incarnate
Offline
Posts: 6931
Feedback: 100% (2)
Link To This Post
Posted: 8/21/2013 7:12:48 PM EST
Good job! Just curious, at what frequencies is it resonant (Z = 0) and what is the R at those frequencies?
Never send a man to do a neutron's job. -- BlammO
كافر