Hi All,
I did get the,160M dipole in the air. It's a "Z" shape where the ends of the "Z" are closer to 90 degrees. Due  ( I think ) to the fact that it is really low, along the last 60 ft. on each side,10 to 15 feet above ground, my new NANO VNA tells me that it is near resonance at about 1.24 MHz. I am pretty sure this is due to ground effects and because of those effects I just can't trim it for the difference in frequency. Any body have a clue on how much to trim off each side? I can go slow and do a foot or two at a time but I could sure use some help from somebody that has been there.
73,
Rob

how long is it and what is the feed line

how high is the center off the ground?

What type wire are you using. Did you take into account the velocity factor if it is insulated wire?

My guess is since the antenna is going to be way low vs wavelength  unless you have a 200 foot tower, having ends 10 feet off the ground isn't going to affect the tuning any more than if the ends were 50 feet

Have you considered feeding it with ladder line and just tuning it to the 160m band. There will be almost no loses due to the ladder line and the antenna will be inductive so it can be tuned mostly with capacitance.


My 160m dipole also has ends low to the ground, but I feed it with 600 ohm ladder line

I've never tried it to see if it actually works, but I found this method: http://tore.aasli.no/dipoles.php

I did some rough math using an assumed desired freq of 1.95mhz and an antenna length of 240ft, and I came up with something like 46ft you need to remove from each leg which seems waaaaaay off, so I probably screwed something up.

Another method: https://mocars.org/tuning-a-dipole/

Edited to add: I did the math using the second method and came up with a 44ft reduction per side. So they must be more accurate than I thought...

Yet another edit: since the ends of your dipole are the lowest it might make more of an effect than desired so trim slowly!

If it was in free space and bare wire it would need to be trimmed about 42 feet for side, but it is not in free space.
There is no way to calculate it that I know off because it is coupled to the ground and the coupling varies with wire length and height and every time you take some wire off, not only does the natural resonance change but so does the coupling to the ground and the coupling to everything else that is in the huge near field of the 160m wire which on my 1/2 acre yard includes several neighbour's houses and cars in their driveways.

So I think you are in for a tedious process of trial and error in very short snips, but it sounds like you know all this already

Don't trim. Roll each end up in a 6" loop. That way if you go to far you can unroll it. When you get it where you want it, unroll it 10 feet, trim, then do fine adjustments. Or just leave the ends rolled.

that is what I do, i twist the ends and use the loop to tie off the wire. It also adds a sort of negative corona effect at the end of the wire I think

Hi @Mach,
Thanks for the reply, the antenna is cut for 1.8 MHz and 468/ 1.8 MHz yields 260 ft. at 70 ft on each side the wire are tied off and the remainder goes off at 90 deg. roughly for the remaining 60 ft on each side. One end goes east and the other to the west. It's that 60 feet of length on each end of the dipole that is 15 to 10 feet above the ground, so there is some length parallel to the ground. The mast is mounted through the deck where there is a basement walk out.  Due to the slope of the yard the mast is higher in the back vs the front. The impedance on 160 M is off the chart on the MFJ 259B. The antenna is ladder line fed ( 450 ohm ) with about 105 to 110 feet of line, I do want this to work as an all band doublet so I'll have to be careful cutting it. When I couldn't find a resonant point with the MFJ 259B I got out my new NANO VNA and found the near resonant spot at 1.24 MHz. I couldn't read the VNA screen very well, the print is too small, but there was a pronounced dip.  LOL I do think the antenna is coupled to the earth enough to cause the frequency shift but I don't have enough experience with 160M antennas. If this doesn't make sense , ask some more questions and we'll get if sorted out. Thanks!
73,
Rob

Hi All,
Thanks for the responses, I'd thought about using some split bolts to set the length with out cutting the wire. And yeah, it looks like cut and try will be the order of the day. I'll keep you guys posted, I do want to be on 160M this winter, but I don't want to give up 40M, 80M and the higher bands.
73,
Rob

@robmkivseries70

You are not measuring the antenna, you are measuring the antenna and the feedline length combined which is why you are getting such a low resonance. Those devices are looking for a 50 ohm feedline and you are giving it all wire.

and you are starting with a 50 ohm to 450 ohm impedance mismatch.

I would try connecting the tuner and seeing if you can tune it. If you can it is good to go. If not, try hooking up the VNA through the tuner and trim until you get a 1.0 SWR and you should be good to go.

That length should be a good length of ladder line as far as I know.

also when you tune other bands with it be careful, you can get what looks like a good match but it will generate high voltage and arc at high power if you run an amp. If that happens, you can probably find another match combo that generate less voltage in the match box, then make a note for the good match and bad match so you don't do that again. I have a problem with 80m on my 160m doublet.

I could be wrong about some of this, I am no expert.

ETA: also since you have an unbalanced doublet just like me because both sides of the dipole are not the same horizontal
length or different heights above the ground, if you can't get a good match with the tuner and are not using a true balanced tuner, like I am, switch the connections to the tuner and you may be able to counter the unbalanced legs with the unbalanced tuner for a better match. I tried that in my case and it worked. I put the down sloping wire and much lower to the ground on the grounded side of the "balanced" output of the tuner and got a much better balanced feedline and eliminated a bunch of RF coming off the feedline due to it being better balanced..... I think.

If your wire was all nice and straight and equally high above the ground using the math found in the links above you would need 179 feet total, or 89.5ft per leg to be resonant on 1.8mhz. Since the low ends are probably having the majority of the coupling effect it would behoove you to trim those sections less than predicted and see where you end up. The good news is the shorter you make the ends the less oddly shaped it will be

I get the same as the OP. 468/1.8=260 feet, minus about 4% if it is insulated wire due to a higher velocity factor.

I meant using the math mentioned in those links for trimming the dipole once up

Before you cut anything, try to measure SWR with your radio at 5 to 10 watts. SWR meters use very low power and sometimes get screwed up from RF induced into the antenna from other stations or whatever is there near the measured frequency.
Check SWR at both ends of the band. This will tell you if the antenna is to short or to long.

@Mach, and everybody else.

My Apologies,
I don't know why I didn't put this in, there is a Balun Designs 1:1 current balun at the end of 3 feet of coax to the analyzer, ladder Line goes right out from the balun. Normally, at the rear of the tuner there is a 6 inch piece of RG-8. The tuner is a Palstar AT2K. When I saw it wasn't in the range of the tuner I knew I had  a problem. Thanks for all the ideas, this almost made me reconsider the Cobra, I've got enough of the flat rotator wire build one and would be a neater install. Believe me, I did try the tuner, with the radio, it was way out of range. On balance, I'm going to have to rely on the balun to force that.
73,
Rob

@Gyprat The resistance and reactance were out of range on the MFJ 259B at both ends of 160M. Thanks!

I am not saying that won't work, but I have never heard of anybody doing that. All the doublet designs that run coax to a balun to the ladder line I have seen use a 4:1 balun from the coax to the ladder line, not a 1;1 balun. A good doublet design is supposed to be about a 200 ohm impedance at the beginning of the ladder line. This is determined by the proper length of ladder line that is not resonant at any freq to be used and not a very high impedance cut of the actual dipole wires.

You seem to have a 50 ohm cable is that attached to a 1;1 balun ( no impedance matching ) connected to 450 ohm ladder line that is around 450 ohms for a 9:1 imbalance. That is going to get the SWR meter to read an SWR of 9:1. and the loss will be most of the radiated power in the coax and even well designed manual tuners will have a problem with that. Stand alone Auto-tuners will probably not be able to match it and auto-tuners in a radio will not have a prayer.

The VNA is reading the impedance mismatch of 9:1 ( 450 / 50 =9 ) at the output of the balun. Most designs use at least a 4:1 matching balun and try to get close to a 200 ohm impedance of the end of the ladder line going into the 4:1 balun for a match based on length of dipole and length of ladder line relative to freq. Now the dipole is supposed to be producing a 50 ohm impedance at the feed point, but it isn't because it is low relative to wavelength and not in free space. So the impedance will be lower and the resonant freq will be lower, lets assume about 25 ohms.  So the 450 ohm ladder line at the antenna feed point is a large mismatch of 450 / 25 = 18 or an SWR of 18:1. But for ladder line this really doesn't matter much because the reflections have very low loss on the ladder line compared to coax where the loss is converted to heat due to the dielectric between the wires and close proximity of the wires to each other. For insulated ladder line there will be a little more loss than uninsulated line as the insulation acts as a dielectric and results is a small amount of loss as heat.

But it is more complicated than that because the impedance that the balun sees is a combination of ladder line impedance and dipole impedance  and impedance isn't just resistance it is also capacitance and inductance and I have no idea how to calculate the impedance that the balun sees. but I do know that a 1;1 balun has no impedance matching. A 4:1 balun will reduce impedance seen by a ratio of 4:1 for the coax.  Or in other words the feed line sees the coax as a 200 ohm impedance instead of a 50 ohm impedance and the coax sees the ladder line as 1/4 the impedance.  So lets say the impedance that the balun sees from the ladderline antenna connection is 1000 ohms. With a 1;1 balun, the coax to ladder line junction will be a 1000 to 50 mismatch or a 20:1 SWR. With a 4:1 balun that same junction will see a 1000:200 mismatch or an SWR of 5:1 which can be matched with a tuner.


The first thing I would do is use a 4:1 balun instead of a 1:1 balun or take the balun out and use a true balanced line manual tuner or use a manual tuner that has balanced line output and connect the ladder line directly to the tuner which will have a 4:1 balun built into the output. If you don't have a manual tuner with a balanced line output , then replace the 1:1 balun with a 4:1 balun. I bet that works just fine.



@robmkivseries70

ETA: I just re-read your post and see you have a AT2K. Did you get the 4:1 balun with it? If so use that and you should be good to go. The 1:1 balun is not going to force a match. If using the 1:1 balun you are going to have to play with the feedline length and antenna length to try to get the impedance to the point that the tuner can match it but you will still have losses in the coax for a big mismatch. How much I don't know. I don't know if SWR loss is dependant on coax length or not but I don't think it is. A standing wave is a standing wave and heat is heat, but I am not sure.

For what it is worth, I have  160m doublet at 50 feet that runs through trees, one side is almost horizontal the other has a short run and then angles down and turns more than 90 degrees

I have an MT-2000 tuner that has been rewired with a 1;1 balun on the front end and then free floated to balanced line output which is supposed to be a semi-better balanced tuner compared to an unbalanced tuner that has a 4:1 balun on the output to balanced line.

I also an MT-3000A tuner with a 4:1 balun on the output to balanced line.

The MT-2000 will not match my doublet and the MT-3000A will due to the 4:1 balun.

What the actual impedance of the feedline and antenna combo is, I have no idea.

Spot on with the split bolt connectors! They save a lot of twisting and untwisting as you home in on the right length.

I enjoy playing around with trap/fan dipoles where there's a lot of interaction between the various element lengths so I have a whole pocket full of split bolts. They're worth their weight in gold -- or in these days, toilet paper.

My AT2K is too new to have the internal balun, with 6" of coax to the balun, I'm not worried about loss there.

@Mach

I can't claim to be a Radio Engineer; though, I've worked in both broadcast radio manufacturing and the paging TX manufacturing industry. My education is in Computer Electronics and Industrial Electronics. Most of my radio antenna theory comes from following an engineer or three on QRZ.com, W5DXP, Cecil, W2DU Walt, Kurt N Sterba of World Radio fame, and a friend and Elmer, Bob, K9QLL who did radio while dancing with the little people in the black pajamas. Still, the whole 160M dipole is new to me. Thanks for the input, First, I'm going to try the 4:1 balun, shortening the dipole to see if that helps is next. If I do get it to load up I'll have to worry about ground loss.   I wonder if some loading coils on the ends of the 80M section would have as much loss. I hope I've explained myself thoroughly.  I'll keep the thread going so the progress and solution will be available for the next OP.
73,
Rob

@Mach
Thank you! Thank you! Thank you! The 4:1 balun got it close enough that I could reduce the length of the dipole 1 foot on each end and it will take power at 1.825 MHz with out arcs and sparks. Here is what I forgot, when it comes to current baluns and maybe voltage baluns. On the analyzer, especially at the higher frequencies I had some values of resistance ( Real "R" ) of 10 Ohms. I had blown a foil on a PCB in a Ten Tec 238B dealing with this in the past. Anyhow, while the same value of "R" appears at the analyzer the Impedance is reduced by 4:1 just as Mach said. My error was thinking I would be reducing the value of that Real "R" with the balun. I can get the antenna closer in the future. Thanks again Mach!
73,
Rob

I'll have to see how it is with RFI in the shack and the TV.  

glad I could help.

antennas and feed line are what I love about this hobby

I can make all kinds of stuff come alive using 160 and 80.

Fans come on
smoke detectors chirp
speakers come alive with my voice with the stereo OFF

RFI is a magical thing!

@ Mach
You are Welcome! My favorite RFI story, during the time I was a Tech class and we had just gotten Novice CW privileges, I was spending the summer in a cheap apartment while school was out. I was running a Heath HW-16 on 80M, The antenna was the shield on the cable TV lead in, it went out the window around the front of the house to the street where it was disconnected. I used an old Dentron random wire tuner and ran a ground wire to a pipe under the sink. This was the day of new home phones with electronic ringers. Every time I keyed the TX, the ringer went off. When I shut off the ringer, I'd forget and miss calls. LOL
73,
Rob

I blew a hole in my thumb with a 50 watt 6 meter AZDEN mobile once. RF burns are no joke. It was also fun to watch a co-worker dance after I lit him up when transmitting on a UHF Astro Spectra at 110 watts on 418 MHz while he grabbed the STICO antenna.