PDA

View Full Version : Doublet SWR when wet



W8JI
01-09-2011, 10:47 AM
My boatanchor antenna is a doublet 130 feet long and about 100 feet high. It is out in the clear fed with #14 or 16 gauge ladder line. The dipole ends are well-insulated with long ceramic insulators to long ropes.

The feedline drops right to the tuner. It's a good tuner.

When the line gets wet from rain, the 40 meter SWR goes up from 1:1 to just over 3:1 and the antenna gets very broad!! Is there a good way to guess the additional losses without measuring the actual impedance change?

I'm a little shocked by that large change.

W0BTU
01-09-2011, 12:01 PM
...When the line gets wet from rain, the 40 meter SWR goes up from 1:1 to just over 3:1 and the antenna gets very broad!!...

Another reason why I dislike window line, besides the wind loading issue.

Why don't you fabricate some ladder line like you used to make your Beverages out of, Tom? And with the spacers farther apart.

VE3FMC
01-09-2011, 12:56 PM
Any antenna I had up that was fed with ladder line was effected by rain, snow or any ice. I could have the tuner set for the lowest SWR on a given day. If it was raining the next time I came into the shack the SWR would not be what it was when the ladder line was dry.

I really can't see how you can change anything Tom. Unless you went to wider spaced ladder line. Maybe that would help.

K9FV
01-09-2011, 02:40 PM
Tom said "ladder line" - I'm thinking he mean ladder line rather than window line. Tom, would you clear that question up please - ladder line or window line (450 ohm?). "IF" truly ladder line, what are the spacers?

I was thinking (wrong thinking caused by poor memory or understanding) - that window line - be it 300 ohm, 450 ohm - would change dry vs wet while ladder line (usually 4" to 6" spacing) would not change SWR when wet.

Tom, do you think there is some other element at work here?

I will be following this thread to learn from my "elmers" :)

73 de Ken H>

K7MH
01-09-2011, 02:57 PM
I have a 102 ft. doublet fed with 450 ohm window line.
I don't see any significant change when it is wet.
The tuner settings don't even vary all that much.

W5DXP
01-09-2011, 05:04 PM
When the line gets wet from rain, the 40 meter SWR goes up from 1:1 to just over 3:1 ...

I'm assuming you can re-tune the SWR to 1:1. Rain decreases the velocity factor and the Z0 of the ladder-line. I have to re-adjust the length of the ladder-line on my no-tuner antenna system by about two feet on 40m when it rains to obtain a minimum SWR. I never noticed that rain lowered the Q but that is not a surprise. Question is: What would you do about it if you knew the additional loss due to the rain?

W0BTU
01-09-2011, 05:46 PM
Rain decreases the velocity factor and the Z0 of the ladder-line.

With two parallel #14 THHN wires with Teflon spacers every 3 feet, why should it? (Just an example).

I smell lots of solid dielectric between the wires in Tom's feeder. Lousy water-absorbing dielectric, too.

W8JI
01-09-2011, 06:33 PM
It's window line. I just don't ever recall it changing so much but I did cut the length back from being about 170 feet to 130 feet. Must be the high impedance now.

W0BTU
01-09-2011, 06:38 PM
It couldn't have been anything BUT window line. :-)

There was a thread with links to some plastic spreaders that conveniently snapped over #14 THHN stranded insulated copper wire (to make ladder line), and another link to a guy who sells REAL open wire line. But for the life of me, I can't find it. Maybe it was on eham.net, but I don't think so.

AE1PT
01-09-2011, 06:38 PM
I just had a brush with ice and 450 ohm window line--but ultimately I would not sacrifice the performance I get with my doublet for any sort of coaxial fed system. That's just my position though.

Perhaps some day I will build something using open line of my own construction--not on the immediate horizon though. What I have done is to create two different spreadsheets of tuning positions--one for dry days and another for rainy days. On my setup, there is not a tremendous difference--but I like to be very close to the mark so that my tuneup does not take but a few seconds to finalize on the tuner and the amp.

K9FV
01-09-2011, 06:43 PM
Is this the "true ladder line" you are referring to?

http://www.trueladderline.com/

I built my on open ladder line using 4" spacers and #14 THHN stranded - not but about 33 ft so it was easy. (33 feet of feed line shows how low my 80 meter loop is! That is 33 ft to a point 5 ft high where it comes thru wall - loop is perhaps 28 ft at that point?

Ken

K8JD
01-09-2011, 06:59 PM
I have dipoles and loops fed with coax and I note a shift in SWR in a few of them during rainy or snowy WX, some of them go from under 1.5 to over 2:1.
I attribute it to changes in ground moisture/conductivity under them.

VK1OD
01-09-2011, 07:05 PM
...
When the line gets wet from rain, the 40 meter SWR goes up from 1:1 to just over 3:1 and the antenna gets very broad!! Is there a good way to guess the additional losses without measuring the actual impedance change?



I am not quite sure of what you mean here Tom.

If you are saying that the antenna system was adjusted for VSWR=1 into the ATU when all components were dry, and that it is greater than 3:1 when wet, I offer the following thoughts.

Firstly, we lack comprehensive peformance data on the commecial window line products.

We do have some excellent work by Wes Steward, N7WS. He made some measurements of four of the Wireman products and published those in a paper, quite likely the only measurment data that we have for these types of lines. Wes's paper also includes measurements of ladder line in a fairly controlled wet condition.

TLLC uses data derived from Wes's work.

The dry line data is fairly straight forward apart from my concern that Wes's measurements at VHF would not have captured the effects of inadequate copper cladding at low HF.

The wet line data has some further issues in that IIRC, Wes tested new line with a controlled application of water. My expectation is that older line that wets differently, especially where it has pollution deposits, surface degradation, lichens etc, will exhibit different characteristics.

The TLLC models derived from Wes's data show lower velocity factor for wet line, and higher loss. The wet characteristics are in TLLC, and are interesting in that they show how sensitive line parameters are to being wet, but I caution in thinking that they apply literally to all wet lines because of the reasons above. Try some cases, you will find them interesting if not definitive for your own application.

Everyone who has used lines of this type knows that the 'tuning' of the system changes when wet, more so that say a coax fed dipole, so there are effects that seem attributable to changes in the feed line when wet.

Owen

W5DXP
01-09-2011, 11:14 PM
Wes's paper also includes measurements of ladder line in a fairly controlled wet condition.

If I remember correctly, Wes used a detergent to wet the ladder-line. Under normal conditions, the surface tension of rainwater is not broken down by ladder-line, but it is broken down by detergent so IMO it was not a logical thing to do. Insects survive rainwater very well but if you squirt rainwater mixed with detergent on them, they will die. Apparently, so will ladder-line. :)


My expectation is that older line that wets differently, especially where it has pollution deposits, surface degradation, lichens etc, will exhibit different characteristics.

Some of my older ladder-line has lost a very thin transparent coating. It seems to be some sort of anti-wetting coating. Without that coating, the ladder-line does not resist wetting nearly as well as new ladder-line.

EI4GMB
01-10-2011, 12:29 AM
Hi Tom,

We get a lot of rain here in Ireland. The SWR on my 300 ohm ladderline fed G5RV Jr also increases slightly on 40m when its wet but never to the extent of 3:1.
I reckon, other factors must also be at work.

Kind Regards

Fred EI4GMB

K8BEC
01-10-2011, 01:31 AM
I was having issues with my 450 ohm ladder line when it would rain. It had probably been up 2 to 3 years. Took it down last summer, and took a can of scotch guard and sprayed all the old line with it. I put it back up and moisture does not seem to bother it as much. Now of course the heavier the rain the bigger the change, but not near what it was before.

W0BTU
01-10-2011, 06:06 AM
Is this the "true ladder line" you are referring to?
http://www.trueladderline.com/

Yes. And the spacers are at http://cgi.ebay.com/Ladder-Snap-600-OHM-LADDER-LINE-G5RV-KIT-31-FOOT-KIT-/170587568217?pt=LH_DefaultDomain_0&hash=item27b7cfb859

I prefer a closer line spacing, but that stuff is still bound to be better than window line.

VK2FXXX
01-11-2011, 11:49 AM
Is there a good way to guess the additional losses without measuring the actual impedance change?
calibrated field strength dry vs wet?
Get another length and measure loss wet and dry?
Im sure you will come up with something Tom.Let us know what please.
Brendan.

WA4OTD
01-11-2011, 12:30 PM
These are the questions we always expect you to answer! THis is interesting problem.

I could be totally offbase....but, I would guess the rain as an element is just a parallel resistor across the feed line, think that is a given. So the feedline series inductance in each leg and capacitance across it that forms the characteristic impedance is shunted by the resistance of the rain. Increasing the SWR, and making it lossy and broadband. WIth the antenna modelling I think you could match the SWR curve with a resistor across the feedline or make it very lossy feedline.

I feel like I'm submitting an answer to the teacher!

W8JI
01-11-2011, 01:06 PM
I'm trying to figure out how to easily turn what happens into useful information.

I'm sure it is the transmission line changing, and not the antenna or earth. My dipoles don't change any measureable amount when wet. They barely changed from the ice we have.

I guess I could put a sense antenna up and meaure the field strength change after the SWR is renulled. That might be the easiest.

73 Tom

K1DNR
01-11-2011, 02:41 PM
My boatanchor antenna is a doublet 130 feet long and about 100 feet high. It is out in the clear fed with #14 or 16 gauge ladder line. The dipole ends are well-insulated with long ceramic insulators to long ropes.

The feedline drops right to the tuner. It's a good tuner.

When the line gets wet from rain, the 40 meter SWR goes up from 1:1 to just over 3:1 and the antenna gets very broad!! Is there a good way to guess the additional losses without measuring the actual impedance change?

I'm a little shocked by that large change.

I'm suspicious you are giving us an academic exercise just to see who will respond! As OTD stated, you are usually the one answering more complex questions than this!

Could you not measure the power at the load side on transmit dry and wet?

Could you not measure at the source the strength of a know signal generator injected at the load, dry and wet?

If a field strength meter were used, is the loss proportional in all elevation and azimuth?

That's how I'm thinking of this... most humbly of course... ;)

VK6ZGO
01-11-2011, 03:19 PM
[QUOTE=AE2CS;2147980]I'm suspicious you are giving us an academic exercise just to see who will respond! As OTD stated, you are usually the one answering more complex questions than this!




That was my suspicion too,---about as close to a troll as a gentleman like

Tom would produce.:D

Then again,if something weird has happened,Tom is just the person to track it

down ,& find a logical cause.


73,VK6ZGO

VK1OD
01-11-2011, 06:25 PM
...
I guess I could put a sense antenna up and meaure the field strength change after the SWR is renulled. That might be the easiest.

It would be a very interesting experiment to capture some data over time about not only the apparent change in VSWR (which tells us little as you well know), but the effect on EIRP before and after retuning the ATU.

Going one step better than VSWR and measuring impedance looking into a known line would allow estimation of the line loss under the new conditions, but the problem is that it is likely that the main contribution to the change in input impedance is that the line characteristics have changed, and are no longer known.

A field strength probe (perhaps just a detector on another on-site antenna) would provide some interesting information, but probably not definitive as conditions close in may change due to changed soil parameters. Observations during rain, then after rain when the feedline has dried but the ground hasn't might help to de-noise those observations.

My experience has been that the ribbon and windowed ribbon type of constructions are more affected than wider space open wire construction with spreaders. For a period of time, I had the same 40m inverted V fed variously with coax with balun, newish windowed 300 ohm TV ribbon, and home made open wire with 100mm line spacing and spreaders. This antenna was rigged near (but not in) a line of trees, and VSWR did vary with rain, but much more so in the case of the TV ribbon. Was that loss in the TV ribbon? I don't know, it is possible that the line length was such that the impedance transformation was more sensitive to velocity factor.

An interesting project Tom, a question worth answering. Even if the answers seem specific to your own configuration, to flag the magnitude of the issue is to understand whether or not it is a significant downside to that type of feedline. Your next observations might just be the second step in drilling down on understanding the apparent problem.

Owen

AD7WB
01-12-2011, 02:18 AM
Hi Tom,

Seems like you are seeing the effects of the narrow match provided by the tuner when presented with the high impedance of the doublet through 130' of window line. If I run your setup through EZNEC and add 130 feet of Wireman 554 (dry) using Zplots I calculate an impedance of 1941-j1993 ohm at 7.1 MHz. With the wet line modeled in Zplots the impedance changes to 346-j709 ohm. I don't know the details of your tuner but from your writings I imagine it is a pretty beefy T-match and the lowest loss match arrangement might not be very tolerant to the impedance change. I could easily imagine the SWR between the rig and tuner going to 3:1 or worse with the wet line. But I wouldn't assume the losses in the line are that much greater. The velocity factor and impedance change caused by pure water on the line could change the load seen by the tuner greatly but if one just re-tuned everything might be fine.

130 feet seems a particularly bad line length for 40 meter, if you had 150' or so you don't hardly need the tuner. And I would guess the sensitivity of the circuit to the effective change in line length would be much less.


Lance

WA4OTD
01-12-2011, 02:56 AM
Field strength is the way to go. I do this all the time at work for our wireless products, don't know why I don't think of doing this ham radio.

W8JI
01-12-2011, 03:08 AM
I'm going to install a sense antenna on a tower close by (but in the far field) when I climb up to repair the ladder line fed doublet. The thing came down yesterday when I had about a half-inch of ice on the towers and antennas.

I think old ladder line is a whole lot worse than fresh stuff, based on the wet SWR change.

AD7WB
01-12-2011, 03:19 AM
I'm interested in how one goes about bounding the error in a field strength measurement like that as the boundary conditions (ground conductivity) change (wet vs dry weather). I don't suppose there is a way to get the line wet under dry conditions to eliminate that variable?

Lance

VK1OD
01-12-2011, 03:19 AM
I'm going to install a sense antenna on a tower close by (but in the far field) when I climb up to repair the ladder line fed doublet.


Sounds interesting, I wish I could send you some rain!!!

Owen

N3JBH
01-13-2011, 12:11 AM
Tom i do not know if the 14 gauge 450 ohm stuff Davis RF sells is any different
then any thing any one else sells. But that is what i use here in Latrobe PA today we have 10 inches of snow and plenty of ice on the ladder line. and i really did not see any real variations that where what i would call amazing.

honestly during rain showers it never changes i do not know if my tuner a palstar is just that forgiving??? of slight swr changes that it does not show it or not but it has been up for a few years and i have been very happy with it.
i will add it is very robust stuff this spring i had a 6 inch thick maple limb snap in the wind and fall across it. made one heck of a bow in it but never broke it.

I understand you love empirical data i really can not offer you any of that just good old fashion this stuff really works for me. So sorry for your troubles in the new year and i hope you will get your ladder line situation fixed in due time. and many thanks for your many kind anwser you have given me over the years. Jeff N3JBH

W8JI
01-13-2011, 12:42 AM
I'm interested in how one goes about bounding the error in a field strength measurement like that as the boundary conditions (ground conductivity) change (wet vs dry weather). I don't suppose there is a way to get the line wet under dry conditions to eliminate that variable?

Lance

Hi Lance,

My dipole is 100 feet high over medium soil that has dozens of radials plowed just under the surface. Even bare dirt would hardly allow rain to affect pattern or loss with the low field concentration in the soil. With all the wires, I'm sure it is stable even for a low dipole. My low 80 meter dipole is over a similar ground near another tower, and even though 35 feet high it doesn't budge in SWR or resonant frequency from rain to dry.

I would not put the sense antenna near earth, I was thinking of mounting it broadside about 150 feet away and about 60-70 feet high on a 70 foot tower. That should make the second antenna stable.

I think the impedance shift is due to the high impedance and very high SWR on the ladder line. My doublet is almost perfectly a full wave on 40. I did not notice this shift when it was 170 feet long, so I expect it is aggravated by the very high SWR in the feeder.

73 Tom

ad: Vari-Ten-1