First ever home brew antenna project....

[W5LZ]

A 3:1 or 4:1 SWR? Odd, normally your radio wouldn't particularly like that and would 'cut back' on power. I'd think you'd see that on the watt meter? Changing the positioning of the feed point, sliding that thing up/down, would make quite a difference in the indicated SWR. Probably not a 'hear-able' difference, but the transmitter would sure tell a difference. And that 1/4" shouldn't make much, if any, difference, sort of, and certainly 'curable' by changing the feed point.
After a quick look I can't find it, but what's the antenna next to the 'J'-pole? It isn't for a marine band, is it??
- 'Doc

 

[Robb]

Question, Doc:
Are you saying that the proximity of the Marine band antenna is affecting the SWR?
Could it also be the feed point position?
N2ITH is having fun doing this; but I think he is getting frustrated. What should/could he do next?
You see, I want to build on of these too, and I will probably run into the same problem he is having - so what can be done?
Certainly a balun isn't going to fix it; there isn't a ground on it like there is on a dipole.
?????

 

[W5LZ]

Rob,
I'm looking for probably causes mainly. The marine antenna being close to the 'J'-pole shouldn't really be a problem as such, but getting them further apart certainly wouldn't hurt either. Knowing how I've sort of 'goofed' with the wrong feed line before, I was trying to 'suggest' that without making it sound too silly. (Who, me? I ain't never... and probably will again though.)
The 'hardest' thing, but also the best thing to do is to get away from it long enough so that you can take another look to see if you've got things at least 'ball-park' close. A "forest" and "trees" thingy, you know?
You're right about a balun, wouldn't do any good at all, a 'J'-pole isn't a 'balanced' antenna at all. But, there -is- a 'ground' to it, that short stubby part is the 'ground', the 'other half' sort of.

This part is a bit 'off topic' but really would make things much simpler to understand.
A 'ground' is a terrible name for what people typically call the 'other half', or the 'mirror image', or the 'Image plane' of a vertical antenna. 'Ground' has too many definitions, or it's just used too much to describe things that are not 'grounds' at all. A 'ground' can mean one thing in a DC circuit, which isn't necessarily the same thing in an AC circuit, and is definitely not the same thing as far as RF is concerned. Keeping all those 'definitions' straight can really get confusing. Especially if you aren't familiar with all of it's definitions! If you ain't on the same 'page', understanding isn't something I'd count on. It makes very good sense to 'safety ground' a 'J'-pole antenna. It makes no sense to 'RF ground' one, it just doesn't need it, neither do most other type antennas. You see where I'm going with that? Okay, and now I'll get off that soap box and use it for kindling.
- 'Doc

 

[Highlander_821]

I was reading your description of your efforts so far, and I can't see anywhere where you tried to add a little length to the antenna.

Have you tried to add a little length to the antenna yet?

 

[WX2MIG]

I was reading your description of your efforts so far, and I can't see anywhere where you tried to add a little length to the antenna.

Have you tried to add a little length to the antenna yet?

I added length to the second J-Pole I built, however, and assuming the weather cooperates today, I will try adding length to this one with some solid copper wire clamped to the verticals and adjust them to see what changes I get.

Also I should note that since those photo's were taken, I removed the marine antenna, and the closest tree is approx 10 to 12 feet away.....

 

[N9RZD]

Well I'm at my wits end with this experiment, I built a second J-Pole from scratch, make absolutely certain my measurements were right on the money, made 50 trips up & down the ladder adjusting the damn thing, and just like the first one, it radiates almost flat up in the VHF Marine bands, but I just couldn't get those SWR's down on the 2m ham frequencies. The best this one will do is just under 3.1:1.....(the first one ran close to 4.1:1)......:headbang

I changed feed lines, raised it up another foot, changed out the jumper between the radio and meter, ran a ground wire from the antenna pole to a copper grounding spike that's 4 foot deep.....no matter what I try, or how many adjustments I make, I just can't the the SWR's down to a reasonable level......

Everyone I've talked to that built one of these antennas has gotten no more than 1.2:1, and most get 1.1:1 on them......I don't get it......
I get great signal reports, was even on a simplex net this evening, and on medium power I was pinning everyone's "S" meters, so the thing is working, but with a load of reflected power.......

Any thoughts....????

do you have access to another swr meter ?
are you sure your swr meter is able to do 2 mtrs ?

 

[WX2MIG]

do you have access to another swr meter ?
are you sure your swr meter is able to do 2 mtrs ?

The meter is rated for 140 to 500 Mhz....

I doubt it's faulty since it does give me good readings in the VHF Marine bands....1.2:1 and below, at the upper end of the ham band (148.000Mhz) it's just a hair over 3.1, and at the lower end (146.000Mhz) it's close to 4.1.....

If the weather holds out, a friend is coming over with a pre-assembled and tuned Ringo, we'll pop that up in the exact same spot as this J-Pole to see what happens.....

As always....I'll report my finds here.....

 

[Highlander_821]

Sounds to me like you are making this harder than it needs to be. The SWR curve that you are reporting screams of needing a longer element. I'll be shocked if you can't get it to resonate on 2 meters by adding a little at a time and checking your meter.

Do you have a pair of handheld radios you can use? Then you can have one guy make changes to the antenna, and have another guy in the shack with the SWR meter. N9RZD and I have tuned lots of antennas that way. Just make sure not to transmit while the antenna monkey is touching or near the antenna!

 

[N9RZD]

The meter is rated for 140 to 500 Mhz....

I doubt it's faulty since it does give me good readings in the VHF Marine bands....1.2:1 and below, at the upper end of the ham band (148.000Mhz) it's just a hair over 3.1, and at the lower end (146.000Mhz) it's close to 4.1.....

If the weather holds out, a friend is coming over with a pre-assembled and tuned Ringo, we'll pop that up in the exact same spot as this J-Pole to see what happens.....

As always....I'll report my finds here.....

it was worth a shot ,Highlander and i built two virtually identical
w5gi antenna's earlier this year , and no two standing wave meters gave the same reading with either antenna ,
i agree with Highlander , sounds like it is a bit short then !
dont know if this has been suggested , but you could also try
loosening the feedpoint and slide it up and down and see if that has any
effect .

Don't Give up !!!! this is how you learn !!!!!

73

 

[Beetle]

I may have overlooked it, but I couldn't see any mention of you changing the feedpoint location. That's critical, and as QRN said, measuring with a tape might get you close, but rarely if ever will it get you right where it needs to be. And all antennas/installations are different.

 

[WX2MIG]

I'm aware of the feed point adjustment, and have had it up and down the verticals, and everywhere inbetween, it will change the SWR's, but not low enough.

I did some more research, found some alternate J-Pole information and design drawlings, and discovered that the Bible of all things Amateur Radio is flawed.

That's right folks, the ARRL, and their Antenna Book has the measurements all wrong on the J-Pole, so if you want to build one, don't go by that book.

The ARRL book has the main radiator listed at 56 3/4", the tuning stub at 18 3/4", and the two separated by 1 1/4".

BuckComm has the main vertical at 57 3/4", tuning stub at 19 1/2", and a separation of 1 7/8"

I artificially lengthened the verticals 1" and 3/4" perspectively, and the SWR's dropped significantly, they aren't the 1.1:1 I keep hearing about, but they came way down, I'm now happy, and not afraid to run this radio at full power.

BuckComm also offers plans for building a 6m J-Pole, and a calculator to determine radiator measurements for any frequency, far better and more accurate information than the ARRL Antenna Book that I just wasted 40 bucks on......

 

[N9RZD]

I'm aware of the feed point adjustment, and have had it up and down the verticals, and everywhere inbetween, it will change the SWR's, but not low enough.

I did some more research, found some alternate J-Pole information and design drawlings, and discovered that the Bible of all things Amateur Radio is flawed.

That's right folks, the ARRL, and their Antenna Book has the measurements all wrong on the J-Pole, so if you want to build one, don't go by that book.

The ARRL book has the main radiator listed at 56 3/4", the tuning stub at 18 3/4", and the two separated by 1 1/4".

BuckComm has the main vertical at 57 3/4", tuning stub at 19 1/2", and a separation of 1 7/8"

I artificially lengthened the verticals 1" and 3/4" perspectively, and the SWR's dropped significantly, they aren't the 1.1:1 I keep hearing about, but they came way down, I'm now happy, and not afraid to run this radio at full power.

BuckComm also offers plans for building a 6m J-Pole, and a calculator to determine radiator measurements for any frequency, far better and more accurate information than the ARRL Antenna Book that I just wasted 40 bucks on......

Not in defense on the "Antenna Bible " , but i thought i noticed
in your build the main vertical was 3/4" or 1", and the tuning stub was
a samller diameter , or did it just appear that way because it was older and
had a patina ?

anyway , if you didnt get er figgered i was going to suggest
looking into your original design ,

looks like your heading inthe right direction with it now

73

 

[WX2MIG]

Not in defense on the "Antenna Bible " , but i thought i noticed
in your build the main vertical was 3/4" or 1", and the tuning stub was
a samller diameter , or did it just appear that way because it was older and
had a patina ?

anyway , if you didnt get er figgered i was going to suggest
looking into your original design ,

looks like your heading inthe right direction with it now

73

You got it right, the first one I built came out a 1/2" short on both elements, so I built a second J-Pole that came out to the exact specifications outlined in "the Bible". That second antenna was still too short compared to the other spec's I got from "BuckComm". After artificially lengthening the elements to BuckComm's spec's, the SWR's dropped significantly, which indicates that the spec's printed in "The Bible" are off by an inch on the main radiator, and 3/4" on the tuning leg.
The Bible also suggests a starting point for the tuning clamps @ 4" above the "T" joint, and BuckComm specifies a starting point @ 1 7/8" above the "T" joint.

Presently my second antenna stands at 57 3/4" on the main element, 19 1/2" on the tuning leg, the contact (feed point) at 1 7/8" above the "T" joint, the only spec I haven't changed yet to match the BuckComm spec's is the separation of the two elements, it still stands @ 1 1/4", where BuckComm's spec's ask for a separation of 1 7/8". However, in it's present configuration my SWR readings are far lower than those represented on the original ARRL Antenna Book spec's for the 144 to 148 Mhz band.

This leaves me with the firm belief that the ARRL dropped the ball on this particular design. The rest of the build spec's for other antenna projects may in fact be correct, but after spending 3 days on this, and having the results I obtained, I feel confident in saying The Bible of Antenna Design is flawed.....

 

[W5LZ]

This is going to sound like a 'defense' of the ARRL, but it isn't by any means, please don't take it as such.
There are no 'one' fixed, never changable specifications for a 'J'-pole, or any other antenna for that matter. Most dimensions are variable to some extent, the primary reason being the diameter of the conductor used in the antenna's construction. A 'larger' diameter typically means a shorter length in one instance. Where the nearness of one conductor to the other contributes some particular characteristic that's desired, making the diameter larger can also mean making the separation larger when a 'special' interaction is required. I've seen working 'J'-poles with element separations ranging from only an inch to over four inches (mine's about three inches, give or take a few 16ths). There can be some 'wiggle-room' with anything, antennas included. I still think the ARRL ought'a have said that somewhere though.
The main thing is that it's working, and it sounds like it is, so good! The only thing I'd add, if you haven't already thought of it is to seal the top ends of that tubing. Plumbing can 'rust out', so can antennas. (Believe it or not, you eventually find the same kind of 'crap' in both of them .)
- 'Doc

 

[Beetle]

The Antenna Book also says that for building a dipole, you use 468/f to determine the overall length. This gets you pretty close, but there will still be the need for some fine tuning. Each antenna, each antenna installation, and each antenna builder is different.

Similarly, I just Googled for jpole information and took ten 2m/70cm designs at random. What worked for one was several inches different from what worked for another. Why? Nature of the beast.