Input Impedance of off center fed dipoles

[Baluncore]

It is not a dipole, it is not a multi-band antenna. It is an amateurs random wire against a counterpoise.
It cannot be modeled generally as it is highly dependent on ground and nearby structures.
It will work sometimes if you have a good antenna tuner.

 

[davenn]

It is not a dipole, it is not a multi-band antenna. It is an amateurs random wire against a counterpoise.

the best statement so far made in this thread

Well I guess that's the beauty of it. Yes it is a complicated antenna, but yet simplistic to build and works very well. I know you say compared to what? If I wanted to build a single antenna for one frequency then I wouldn't worry about all of this and design it by the book. If I had a huge area to put it on, or a tall tower or tall trees to hang it.

Hi Jackson

Have you considered a probably much better alternative ... a trapped dipole ?
They are great for multiband use. And you can please yourself if you use it as a flat ( horizontal )
dipole or as an inverted V. The inverted-V just gives a better non-directive radiation pattern than a horiz. dipole that you cannot rotate

EDIT: or if you don't like trap building ... go for a G5RV broad band antennaDave

 

[Jackson Richter]

Thank you Dave for your input. I have considered many antennas for my projects. I do believe strongly that the antenna is the most important part of the communications process. With in the last year I have been using this OCF antenna. There are so many positive aspects about it. 1st, its very broad band. 2nd, its multiband. 3rd, it has a very flat VSWR across these bands. 4th, it actually blows the g5rv and others like it, out of the water. It appears to be a very efficient antenna. I talk to many others that have this same antenna, who sound strong and do much better now since they have changed to this antenna. So what I am trying to do is to quantify it and put the data in a form that makes it more predictable. I guess you would have to see for yourself, being from Missouri, lol. All kidding aside, There are a few quirks about this antenna that have to pinned down and analyzed. So with that being said, that's what I am trying to do. There are 4 things that make this antenna work. 1st, A very good current balun, designed for high power and built unbalanced to balanced configuration. (to help with common mode currents) and impedance matching. 2nd, This antenna is put in an inverted V configuration. 3rd, The height is also very important, because of the near field radiation. This antenna if mounted 30 to 40 ft high, the OCF input impedance falls within limits of the 4:1 balun and nicely transfers that impedance down to a proper value that output transmitter and coax can drive. 4th, This antenna must be clear of any large metal objects within the near field proximity. That's pretty much it. Sorry, I hope this makes sense to you. Seeing is believing and I should have tackled this antenna years ago. I talk every day on this antenna and have worked Europe on 20 meters many times even with the terrible band conditions. I also talk to California every night on 80meters with this antenna. Its like having a super nice car, you want to keep it fine tuned. I am also trying to fine tune the OCF as well.

Jackson WB0USA

 

[tech99]

If you drive it at one end and place a terminating resistor at the other, it forms a half rhombic, or Bruce, antenna, and will have gain and F/B ratio for frequencies of about 14MHz upwards. It will also have a fairly uniform driving point impedance.

 

[Baluncore]

I do believe strongly that the antenna is the most important part of the communications process.

You have a lot to learn. Without propagation, communication is impossible. With propagation, just about any antenna will work when tuned.

There are so many positive aspects about it. 1st, its very broad band. 2nd, its multiband. 3rd, it has a very flat VSWR across these bands.

If that flat VSWR is close to 1.0 then it indicates that you do not have a resonant antenna attached, you simply have high losses in your feedline, tuner and/or environment.

A dummy load also has a flat VSWR. It radiates in the IR band as heat. A low SWR may indicate that little energy is returning to the SWR meter because it is lost as heat before it gets back.

 

[davenn]

I do believe strongly that the antenna is the most important part of the communications process.

You have a lot to learn. Without propagation, communication is impossible. With propagation, just about any antenna will work when tuned.

indeed

There are so many positive aspects about it. 1st, its very broad band. 2nd, its multiband.

There's so many negative ones too, as have been discussed in this thread, It's broad band. NOT multiband as you have already been told. And broadband immediately brings about very low gain compared to any other resonant antenna eg. a dipole. The OCF will never have the gain of a dipole on any given frequency.

If that flat VSWR is close to 1.0 then it indicates that you do not have a resonant antenna attached, you simply have high losses in your feedline, tuner and/or environment.

A dummy load also has a flat VSWR. It radiates in the IR band as heat. A low SWR may indicate that little energy is returning to the SWR meter because it is lost as heat before it gets back.

again agreed

actually blows the g5rv and others like it, out of the water. I talk to many others that have this same antenna, who sound strong and do much better now since they have changed to this antenna.

really ? ... when being compared to another antenna from the same location and transmitter ? I doubt it has been done

I wouldn't believe that statement till actual side by side tests were done ... hearsay isn't worth diddly squat

It appears to be a very efficient antenna

appears being the operative word

So what I am trying to do is to quantify it and put the data in a form that makes it more predictable.

Seeing is believing and I should have tackled this antenna years ago

that IS the ONLY way and as I said earlier, you cannot compare it to another antenna unless they are being used from the same location and with the same gear

DONT believe till you see actual results

I guess you would have to see for yourself, being from Missouri, lol. All kidding aside,

Ohhh, and I have probably traveled more highways and byways of Missouri and the other mid-western states than you

Storm chasing takes you anywhere and everywhere
Dave

 

[Baluncore]

A beginner will claim perfection of a bent wire coat hanger if he QSLs the antipodes with 300 watts, on HF, just once. A professional will claim imperfection if they fail to communicate on one day in the year.
Nearby, more experienced amateurs will communicate with the antipodes daily, over a decade or more, with 1 watt QRP. They will not praise their antennas because it is the choice of the right band for the available propagation that really makes the real difference. They will use open wire feed lines with a transmatch and an antenna tuner.

 

[Jackson Richter]

First of all, I don't clam to be a professional! Maybe I am over zealous on this antenna, because I hear it and use it EVERY DAY. I am reporting what I have observed. Yes you are right, the propagation has to be there to have consistent communication. Dave I am sorry but I haven't heard a good sounding G5RV yet. That doesn't mean there isn't one. I have also had others report the same thing about the G5RV. If it looks like a bad antenna, runs like a bad antenna, maybe it is a bad antenna. Pardon the expression "bad", There are a lot of antennas out there, some that WORK most of the time and some that work during good propagation "with 1 watt" and if your lucky get to another state. The real amateurs today know that the propagation is on a decline and probably won't get better for quite a few years, unlike the 50's and 60's where a watt of power could get you around the world. Yes changing bands to get the proper MUF has already around for years. Sorry If I offend anyone, that wasn't the intent. All I wanted to do was ask questions about an antenna that "appears" to work EVERY DAY! even when the propagation is poor. By the way Dave, we have a Dr.Gupta here in Cedar Rapids that is also a storm chaser and loves it.
I hope everyone has a good day.

JR Richter WB0USA

 

[davenn]

,HI Jackson

First of all, I don't clam to be a professional!

Nor do I, tho I have worked in the telecommunications industry for the last 40 yrs

Maybe I am over zealous on this antenna, because I hear it and use it EVERY DAY. I am reporting what I have observed.

God forbid, I'm sure Baluncore and I definitely are not trying to squash your enthusiasm ... something that is severely lacking everywhere.
What we both are saying is, DONT let it override empirical science/testing

BUT have you actually, As I said in a previous post, tried it in a side by side comparison with any other antenna, where you just antenna switch between the two ?
probably not ... and that is where the real test comes

Dave I am sorry but I haven't heard a good sounding G5RV yet. That doesn't mean there isn't one. I have also had others report the same thing about the G5RV. If it looks like a bad antenna, runs like a bad antenna, maybe it is a bad antenna. Pardon the expression "bad",

The G5RV was just an example ... Any non-resonant antenna is a compromise and I doubt it's any better or worse than your OCF which is also a serious compromise. The G5RV works in that it provides wide band coverage for minimal construction effort

There are a lot of antennas out there, some that WORK most of the time and some that work during good propagation "with 1 watt" and if your lucky get to another state.

If it is a resonant system like a fixed band dipole, a trapped dipole, or a yagi. it will WORK, ALL the time and all else being equal, the ONLY thing that will affect operations will be the propagation. I can guarantee my trapped dipole will out-perform your OCF or a G5RV any day.

The real amateurs today know that the propagation is on a decline and probably won't get better for quite a few years, unlike the 50's and 60's where a watt of power could get you around the world

anyone in long haul telecoms does ... amateur or professional

Sorry If I offend anyone, that wasn't the intent.

there's been no offense anywhere

All I wanted to do was ask questions about an antenna that "appears" to work EVERY DAY! even when the propagation is poor.

As already said, any respectable antenna will work if the propagation exists

By the way Dave, we have a Dr.Gupta here in Cedar Rapids that is also a storm chaser and loves it.
I hope everyone has a good day.

Not familiar with the name ...
And an awesome day to you

Dave
and since we are exchanging callsigns

de VK2TDN, ex ZL4TBN, licenced since 1980 and crazy about the microwave bands in particular
where antennas and plumbing go hand in hand

 

[Jackson Richter]

Well Dave, A lot of testing is basically being done every day. You are right, there has been no side by side comparison from the G5RV and the OCF. There hasn't been any RF field strength testing either. I don't have any empirical data to show comparisons of the many antennas, except for the many stations that I talk to every night. I guess I have failed to say it properly. Being from the old school, I like to do things by the book, believe it or not. If an antenna works well, the empirical data should show it, along with the design data proving that it works. Also the field testing with the countless hours should prove something too. I am getting older and sometimes the grey matter doesn't kick in right away. lol. It takes a lot of thought process to hammer it in.
I think a resonant dipole performs very well. There is no argument here. Your trapped dipole I'm sure works well also. I would like to mention again that this antenna is not only broadband but also multiband operation too. It works on 80m, 40m, 20m and 10m and 6m. I am not sure why you said it isn't. I have used this antenna on these bands when the propagation was there. I have many antenna plots of this antenna with a AIM4170 impedance analyzer. If you would like to see them, I would be glad to send them to you. I also would like to mention that I agree about the VSWR. Just because your VSWR is flat that doesn't mean your antenna is efficient. It just means your matched to the load. I also know from old school that a center fed antenna with open wire feed line has been used for multiband operation for many years. I also know many operators that have used this type of antenna all of their lives with success but a tuner is required to match resistive and reactive properties of the feed line and antenna. I personally don't like the open wife feed, its cumbersome and pretty hard to hide, but it is very low loss. The prior studies of the OCF antenna left so many holes. I am trying to gap these holes and make it more palatable, for guys like yourself and Baluncore, lol. I will refrain from the over zealous remarks. This OCF has a lot to offer, its a wide bandwidth and a multiband antenna, which "appears" to work well because of the consistent daily reports from other amateurs. I will say now that it may not out perform a standard dipole, but it will be very close. Your right Dave, it may not out perform even your antenna but it will go where yours can't and do it surprising well. Sometimes I feel like the little Dutch boy with his finger in the dyke plugging all the holes. Maybe someday I'll have all empirical data needed to show the how and why of this antenna. Thanks again to you and Baluncore for keeping my head straight, we need that every now and then.

JR Richter

 

[Baluncore]

Because it is horizontally polarised at the design wavelength and is close to the ground, any induced ground currents will cancel the transmitted signal. A vertical over a ground plane does not suffer that problem.

This OCF has a lot to offer, its a wide bandwidth and a multiband antenna, which "appears" to work well because of the consistent daily reports from other amateurs. I will say now that it may not out perform a standard dipole, but it will be very close.

Not very close. The pattern will be dipole shaped but several dB down on a true dipole.

The resonant dipole is designed so that impedance of the wire remains constant along the dipole. An inverted V has height variation. If the diameter of the wire, (or the cage of wires), is not adjusted to be proportional to height from ground, (and nearby conductive features), then there is constant impedance mismatch and smear of the phase along the wire. That kills the Q of the resonance and goes a long way to explaining why it has low SWR, especially on the shorter wavelengths. For that reason it can never match or even get close to a true dipole.

 

[davenn]

It works on 80m, 40m, 20m and 10m and 6m. I am not sure why you said it isn't.

Baluncore and I are telling you it isn't multiband because it isn't resonant on other bands without tuning
That is the difference between broad band and multiband. They are not the same thing ... very different meanings
Where as my trapped dipole is truly multiband, it will work on multiple bands WITHOUT tuning for each of those bands
because it is already resonant on them and any other bands that are harmonically related

I built my trapped dipole to cover 40, 20, 15 and 10. I couldn't care about 80m it's way too noisy to use from the burbs of a large city
and I also have huge 220kV transmission lines about 1 km from home that add to the S9+++ noise on 80m ... even 40m is pretty noisy
rarely gets below an S5

for 6m I have always used yagis, in the days when I was really active on 6, I had 6ele on a 6m boom, see the ARRL antenna handbook from ~ the '70's
an amazing performer. Cant use that from my current location within the depths of Sydney city ... am restricted to a 3 ele yagi and its 6dBi gain

I suspect your OCF won't be producing any signif. gain on 6m ( someone can prove me wrong )

Your right Dave, it may not out perform even your antenna but it will go where yours can't and do it surprising well.

not sure what you mean by that ?

My ultimate HF antenna would be the SteppIR ... I can only dream

http://www.steppir.com/4-element-yagi
cheers
Dave

 

[Jackson Richter]

Baluncore and I are telling you it isn't multiband because it isn't resonant on other bands without tuning

I what I am telling you "nicely" no tuner is required on the other bands.

 

[Jackson Richter]

and I also have huge 220kV transmission lines about 1 km from home

Being that close to the power line is certainly a determent on 80m. What I can suggest is a listening antenna like a beverage type, that is directive for only one direction. We work a station in Florida every night on 80m with similar problems. He receives on the beverage and transmits on a different antenna.

 

[Jackson Richter]

My ultimate HF antenna would be the SteppIR ... I can only dream

Dave I do agree, very nice antenna. I also have one for you and think you will get a kick out of it. If you can go on You Tube and type in KB9RBL, (who is one of the antenna installers), you will witness the installation of a 80m 3 element array (3000 lbs) at 140 ft. The gentleman who owns this is a retired pastor.

 

[Baluncore]

I am wasting my time when the OP sees brand as more important than functionality.
I must abandon this thread as it is not about physics.

 

[davenn]

I what I am telling you "nicely" no tuner is required on the other bands.

HUH ??

you have already admitted in your post #27 and agreed with Baluncore's post #23 that because of the constantly changing feedpoint impedance with freq
that a tuner will be required

So you now don't think that it will be ? how are you going to match your TX with the widely varying input impedance without a transmatch ?
Dave

 

[Jackson Richter]

So you now don't think that it will be ? how are you going to match your TX with the widely varying input impedance without a transmatch ?

I don't mean to lead you astray. There is variance in the impedance. This variation is not extreme. The feed point impedance is between 100 and 200 ohms when the feed point is located about 1/3 the distance from the end of a half wave dipole. For instance, if your 1/2 wave antenna is designed for 3.6 MHz and fed the antenna in the center 1/4 wave length position the input impedance will be approx. 73 ohms when mounted a wave length or higher. This have wave length is about 40m. If we feed this antenna at 13.6m from the end, a finite impedance can be calculated. The impedance is around 100 ohms at 3.6 MHz. The impedance formula will support this as well. As Baluncore mentioned, the near field also effects the input impedance and he is right, it does. If this antenna is raised higher, this near field will effect it less, but will still affect it as same as the dipole. Now by keeping the feed point at this location, this antenna can be operated at 2nd, 4th, 8th harmonic with pretty much the same impedance. The equations will also support this. A 4:1 balun is used here at this feed point making the impedance more manageable. I do have all the data to support this. When this antenna is put into a inverted v configuration the impedances of course change. The findings so far that this antenna has a optimized height of around 13m. No, the VSWR is not perfect but it seems to broad band, even across other bands. That's what I know Dave. The highest VSWR is about 2.0 : 1. If this is too hard for you and Baluncore to except well I can't do anything about that. My hope here was get help not be chastised. Again I may find out its all muddied and skewed because of the type of antenna I'm using. That's the purpose of this thread, was to learn and not to be cut off at the knees because I didn't agree with you. Again sorry Baluncore for wasting your time. At least Dave your personality didn't resemble that of a rock. Thank you for that.

JR Richter

 

[davenn]

If this is too hard for you and Baluncore to except well I can't do anything about that. My hope here was get help not be chastised.

Please don't think you are being chastised ... far from it

As I said earlier a couple of times, just don't let your enthusiasm override the science
comments like "this antenna sounds better than that one" is not scientific evidence
antennas don't affect the audio quality, but I'm sure you really do know that

when you have done real testing, including side by side performance tests,
I encourage you to show us the results
regards
Dave

 

[Baluncore]

The highest VSWR is about 2.0 : 1. If this is too hard for you and Baluncore to except well I can't do anything about that.

I accept that is the case. Indeed that is exactly what is expected of a high loss system.

You should understand that SWR works both ways.
When the antenna is perfectly matched you have an SWR of 1.0
When because of feedline losses, no energy gets back from a mismatched antenna, you will also have a low SWR.

By de-tuning a resonant antenna it should be possible to reflect almost all the energy. If you cannot get a high SWR of 10:1 from a resonant antenna, mismatched to it's feedline, then you have problems with your feedline.

It is always interesting to short circuit the antenna terminals at the balun, then look for the maximum SWR.