Simple portable antenna

[KOA4705]

This antenna has over 9db of gain

[KOA4705]

Radiation pattern

[295 antenna]

do you need a tuner with this or is it already resonant ?

[The DB]

A few notes... To start with, the question that was asked...

do you need a tuner with this or is it already resonant ?

This isn't resonant, and if you adjust the balanced feed line to make it so this antenna is resonant, the impedance is over 150 ohms. This antenna, by itself, will not safely plug directly into your 50 ohm radio.

I'm sorry to say, the modeling results presented above are over reporting gain.

When it comes to the radiation patterns presented above, I modeled this antenna myself, and initially got similar results. However, this was before running something called AGT, which stands for "Average Gain Test". Essentially, AGT is used to check models for accuracy. When I correct the model to correct for this failed test, I get the following patterns. (Click on them if you can't read the gain listed in the lower right corner.)




On my model the antenna was 36 feet above ground. If you mount the antenna below that, or even a little higher than that, you won't see the claimed 10 dBi in gain that the original models predict, it just isn't going to happen. A word of advice, when someone posts a model as part of a claim about an antenna, ask about it. Get a second opinion. See if someone else can get consistent results while checking for known but often overlooked problems with the model that can drastically affect its accuracy, in this case a change of 2 dB.


The DB

[KOA4705]

3D radiation pattern
I will give the feed point impedance and the impedance transformation at the send end of
300 ohm line.

[KOA4705]

DB good catch the 300 ohm feed line length so be 11.81 Feet in length LOL. At a Antenna height of 30 feet.
From 27- 27.4 MHZ at the send of the 300 ohm line the Max SWR= 1.21:1, min is 1.10:1
Joe

[The DB]

How long and what type of coax was used between the radio and the twin lead?


The DB

[KOA4705]

DB lMR400
Joe

[KOA4705]

DB Here the antenna feed point impedance and the far end of the 300 ohm line (send end) impedance vs frequency

FREQ (MHZ) ANTENNA FEED POINT (Z) SEND END (Z) 300 ohm line (SWR) SEND END 300 ohm line, 50 ohm input

27.0 194.1 +j 497.5 48.28 -j11.32 1.21:1
27.1 201.1 +j 517.1 47.22 -j5.88 1.15:1
27.2 208.4 +j 538.6 46.02 -j1.14 1.10:1
27.3 216.0 +j 556.6 44.81 +j2.97 1.13:1
27.4 224.1 +j 576.6 44.07 +j6.61 1.21:1

[KOA4705]

Portable antenna 3D radiation pattern. The antenna wire runs parallel with the Y axis

[MDYoungblood]

What modelling programs are you using? I'm interested because I want to build a large horizontal loop in my yard, it will be about 20ft off the ground average and about 200ft long, 4 segments at 50ft each. Creating it on the computer first will give me an idea of if it will work or not.

3's

Greg

[The DB]

What modelling programs are you using? I'm interested because I want to build a large horizontal loop in my yard, it will be about 20ft off the ground average and about 200ft long, 4 segments at 50ft each. Creating it on the computer first will give me an idea of if it will work or not.

3's

Greg

KOA4705 is using EZNec. From personal experience, I didn't really like using that program. It just wasn't intuitive, or easy for me. On another note, he seems rather new with using it.

I use 4Nec2. It was far easier for me to pick up, and far more intuitive as well. It can also do things that only the very expensive EZNec Pro can do, and its free.

Having used both in the past, even if all of the EZNec versions were free, I would recommend using 4Nec2. Also, having used both in the past, the results from the two programs for identical models are extremely close, the testing I did puts them at less than 0.1 dB difference when it comes to patterns.

If you go the 4Nec2 route and have questions I will be happy to answer them for you.

With your loop, are you planning on feeding the center of one side or one of the corners?


The DB

[MDYoungblood]

Cool, I was checking both of those out and will probably download both, anything now a days is better than the old trial and error method. On my loop it will be fed at one corner, I'm thinking of using 450Ω ladderline as a matcher to a balun with 50Ω into the house. The 200ft is the max I can lay out, shorter can be done just as easy. Looking at working as much of the HF bands as possible with it.
3's

Greg

[KOA4705]

This isn't resonant, and if you adjust the balanced feed line to make it so this antenna is resonant, the impedance is over 150 ohms. This antenna, by itself, will not safely plug directly into your 50 ohm radio.


The DB
[/quote]

The antenna 300 ohm feed line can be adjusted to a length 11.81 feet for a SWR of less than 1.22:1 across the band. The antenna can be plug directly into your 50 ohm radio. How did you come up with a real part of over 150 ohms?
Joe

[The DB]

The antenna 300 ohm feed line can be adjusted to a length 11.81 feet for a SWR of less than 1.22:1 across the band. The antenna can be plug directly into your 50 ohm radio. How did you come up with a real part of over 150 ohms?
Joe

At the time, I didn't know you were using 300 ohm feed line. I modeled wires that don't resemble 300 ohm feed line in the model I made, they actually had a higher impedance, which would throw off the resulting numbers.

I can confirm that if you get the numbers you said you are getting it is possible to get a match that is very close to 50 ohms with some length of 300 ohm coax. My calculations show a bit longer than you are using, however, I did not factor in a velocity factor, that will be present with a 300 ohm twin lead feed line matching transformer like you plan to use. It is hard to say based on the specific twin lead you are using, but I'm guessing a velocity factor of 80% to 85% (most sources use these numbers for generic 300 ohm twin lead), which puts the results in the ballpark of the numbers you are posting.

The two things that has me concerned is, 1) your model seems to be over reporting gain, I still suggest running AGT and seeing what the result of that test is, and 2a) I'm not sure if you are using the default "perfect conductor" or have changed the conductors in the model to copper (or whatever metal you used), which can make a difference when it comes to how long it needs to be when tuning, and 2b) I don't know if you are using bare wire, or wire with a PVC or other covering, and if you are using wire with such a covering was that included in the model? Both 2a and 2b will add a velocity factor to the antenna which will need to be accounted for if you haven't already. A friend on another forum tried to make an antenna from a model, failed to take into account 2a and 2b I mentioned above, and ended up with a wire that was almost 50% of the length the model said he should use...

Hey MDYoungblood, in the next few days, if I get a chance, I plan on making a video for you about modeling such an antenna. It will include what I do at various points in the process and why. The intent will be to show you things to look out for that someone new to modeling won't know to consider. It is also intended to show some good habits, and perhaps point to a bad habit or two to avoid. While the video is intended to be aimed at you and your project specifically, it would also be useful to anyone who might have an interest in modeling. I will also give some advice based on years of working with such software that I had to learn the hard way... That is coming when I have time.


The DB

[Bobcat]

The DB, I jumped in here to thank you for that! I had no idea that such even existed. Playing with antennas is one of my favorite aspects of hobby radio and this looks like a handy thing to have! Thanks!

Bob

KOA4705 is using EZNec. From personal experience, I didn't really like using that program. It just wasn't intuitive, or easy for me. On another note, he seems rather new with using it.

I use 4Nec2. It was far easier for me to pick up, and far more intuitive as well. It can also do things that only the very expensive EZNec Pro can do, and its free.

Having used both in the past, even if all of the EZNec versions were free, I would recommend using 4Nec2. Also, having used both in the past, the results from the two programs for identical models are extremely close, the testing I did puts them at less than 0.1 dB difference when it comes to patterns.

If you go the 4Nec2 route and have questions I will be happy to answer them for you.

With your loop, are you planning on feeding the center of one side or one of the corners?


The DB

[KOA4705]

The antenna 300 ohm feed line can be adjusted to a length 11.81 feet for a SWR of less than 1.22:1 across the band. The antenna can be plug directly into your 50 ohm radio. How did you come up with a real part of over 150 ohms?
Joe

At the time, I didn't know you were using 300 ohm feed line. I modeled wires that don't resemble 300 ohm feed line in the model I made, they actually had a higher impedance, which would throw off the resulting numbers.

I can confirm that if you get the numbers you said you are getting it is possible to get a match that is very close to 50 ohms with some length of 300 ohm coax. My calculations show a bit longer than you are using, however, I did not factor in a velocity factor, that will be present with a 300 ohm twin lead feed line matching transformer like you plan to use. It is hard to say based on the specific twin lead you are using, but I'm guessing a velocity factor of 80% to 85% (most sources use these numbers for generic 300 ohm twin lead), which puts the results in the ballpark of the numbers you are posting.
The two things that has me concerned is, 1) your model seems to be over reporting gain, I still suggest running AGT and seeing what the result of that test is, and 2a) I'm not sure if you are using the default "perfect conductor" or have changed the conductors in the model to copper (or whatever metal you used), which can make a difference when it comes to how long it needs to be when tuning, and 2b) I don't know if you are using bare wire, or wire with a PVC or other covering, and if you are using wire with such a covering was that included in the model? Both 2a and 2b will add a velocity factor to the antenna which will need to be accounted for if you haven't already. A friend on another forum tried to make an antenna from a model, failed to take into account 2a and 2b I mentioned above, and ended up with a wire that was almost 50% of the length the model said he should use...
Hey MDYoungblood, in the next few days, if I get a chance, I plan on making a video for you about modeling such an antenna. It will include what I do at various points in the process and why. The intent will be to show you things to look out for that someone new to modeling won't know to consider. It is also intended to show some good habits, and perhaps point to a bad habit or two to avoid. While the video is intended to be aimed at you and your project specifically, it would also be useful to anyone who might have an interest in modeling. I will also give some advice based on years of working with such software that I had to learn the hard way... That is coming when I have time.


The DB

Good to meet you both! We all have a common interest antenna systems modeling and transmission line theory. Most people are bored out of there minds if you talk to them about this stuff. Your as only as good as your tools. I got tired and frustrated using a noise bridge, antenna analyzer and a General Radio RF bridge for impedance measurements. Due to limited range of impedance that can be measured and or the inaccuracies. I broke down and bought a Vector Network Analyzer which solves all these problem. No more guessing with active and passive devices including common mode chokes, antenna systems, transmission lines, filters, group delay, S parameters, attenuators, directional couplers/ samplers, transistors, dummy loads etc. I could never afford one back in the day the cost was prohibitive. But due to new technology the prices have dropped dramatically.

[The DB]

An old HP network analyzer? Those are neat, I would have one if I had several thousand dollars burning a hole in my pocket. I don't need one, but I kind of collect older RF test equipment so it would be nice to have and play with...

I use something a little more modern, and AIM 4170c VNA. Here are some results from a friends mobile 2/6 meter ham radio antenna. They are to big to put directly in the message so I'll just link them...

http://hittman.us/pictures/3-29-14/3.jpg
http://hittman.us/pictures/3-29-14/4.jpg


The DB

[The DB]

MDYoungblood here is that video I mentioned. If you have any questions feel free to ask.

https://www.youtube.com/watch?v=I2glvVKZo2U


The DB

[MDYoungblood]

MDYoungblood here is that video I mentioned. If you have any questions feel free to ask.

https://www.youtube.com/watch?v=I2glvVKZo2U


The DB

Thanks DB

3's

Greg

[KOA4705]

An old HP network analyzer? Those are neat, I would have one if I had several thousand dollars burning a hole in my pocket. I don't need one, but I kind of collect older RF test equipment so it would be nice to have and play with...

I use something a little more modern, and AIM 4170c VNA. Here are some results from a friends mobile 2/6 meter ham radio antenna. They are to big to put directly in the message so I'll just link them...

http://hittman.us/pictures/3-29-14/3.jpg
http://hittman.us/pictures/3-29-14/4.jpg


The DB

Hi DB that's a amazing single port VNA at a incredable
price.
Did it come with a calibration kit too? Looking at the
reviews ratings are excellent. The display covers
all pertinent data with S11 inclusive. I also read
that you can also can display a Smith chart plot.

I got a calibration kit from ebay that's good
to 3 Ghz from Israel for $100. HP cal kits are still $1000
on ebay. Like you most all of my test equipment is old HP,Tek,
fluke, Wavetek etc it heavy and big but labratory quality.This
is the same stuff I used through out my career but could never afford
it back in the day. I remember in the 70's a HP141T spectrum analyzer with
a 8555A RF section and a YIG filter cost more that a house.
Joe

[The DB]

Hi DB that's a amazing single port VNA at a incredable
price.
Did it come with a calibration kit too? Looking at the
reviews ratings are excellent. The display covers
all pertinent data with S11 inclusive. I also read
that you can also can display a Smith chart plot.

I got a calibration kit from ebay that's good
to 3 Ghz from Israel for $100. HP cal kits are still $1000
on ebay. Like you most all of my test equipment is old HP,Tek,
fluke, Wavetek etc it heavy and big but labratory quality.This
is the same stuff I used through out my career but could never afford
it back in the day. I remember in the 70's a HP141T spectrum analyzer with
a 8555A RF section and a YIG filter cost more that a house.
Joe

It works very well, definitely worth the price, at least for those who can understand what that type of device is capable of.

It comes with the calibration loads. It also has a smith chart output, and can do far more than what is shown on that screen.

It can't cover anything near the frequency range of your HP equipment, but it is ok for my needs.

I see they now have a 4300 that covers a wider range of frequencies, and it is a lower price than I paid, $500 for that kind of device? Wish it was that cheap when I got mine...


The DB

[KOA4705]

MDYoungblood here is that video I mentioned. If you have any questions feel free to ask.

https://www.youtube.com/watch?v=I2glvVKZo2U


The DB

DB what is the feed point impedance vs frequency?
Joe

[KOA4705]

MDYoungblood here is that video I mentioned. If you have any questions feel free to ask.

https://www.youtube.com/watch?v=I2glvVKZo2U


The DB

DB what is the feed point impedance vs frequency? Why didn't you just transpose the antenna feed point to a 50 ohms system elegantly. Instead of first using close to a 1/4 wavelength 450 Zo OWL then a 1/4 wavelength of 75 Zo coax to get to a 50 ohms system
Joe

[The DB]

He said he wanted to match the system with 450 ohm line, so I used 450 ohm line to see what I would get. Is it bad that I followed his instructions to the letter?

He doesn't want to go any longer than that length of antenna, and shortening it some actually pushes the impedance match after the 450 ohm ladder line transformer further away. That being said, if he learned what he needed, and played with it some he may have found a better solution on his own, or I could have helped him in that direction. My goal was to give him a starting point and help him work towards that goal. In any case, their are longer and shorter antenna lengths that will tune much better when using 450 ohm feed line.

If he used a different impedance feed line, 300 ohm feed line would get him to 45 + j0, which is a near direct conversion for that antenna length. But that isn't what he wanted, so that isn't what I did.

Finally, I was trying to give him the basics on trying to eventually be able to figure this out on his own, again, which is what he wanted. Their is a big difference in doing something for some one and helping them learn to do it themselves. He posts in the antenna section on this forum a *lot*. Helping him gain additional knowledge will only help this forum in the long run.


The DB