Inverted "V" Attic Antenna Using Mobile Whips

[StrangeBrew]

Build your own to use with a digital multimeter.
http://www.zen22142.zen.co.uk/Circuits/rf/sfsm.htm

Thanks for the link.

At the time I was using one I was trying to map out the radiation pattern of different mobile setups, just getting relative readings from different directions. Most of the trouble I was having was reading the miniscule deflections that the analog meter I was using were giving me, this seems like an application where digital can be a very big advantage.

MOGVZ,
I'll probably try one someday to see how they compare. Will report back if I do.

May be time to build yourself a field strength meter.

And no this isn't just an attempt to make the above look like less of a threadjack.



Added on edit:

.... You can also easily experiment with various spacings too and actually see the results.

This is actually what I was doing with mine, somehow the common accepted knowledge concerning mobile phased antennas didn't seem to fit with my personal experience.

 

[fourstringburn]

Further, if you ask most CBrs, and even many ham's, that use antenna analyzers, they will tell you that the best possible tune is R=50 and X=0, which will get you that perfect SWR match people seek, and in most cases, if you are seeking performance, is not the best possible tune...

Why would you say that an ideal reading on a Analyzer like so wouldn't be the best possible tune or performance? PLEASE EXPLAIN YOUR STATEMENT!

Its a shame really. You can get a much cheaper tool to assist in tuning antennas, one that doesn't cost much more than a cheap SWR meter, and it will help you tune antennas for performance as well as a much more expensive antenna analyzer, but no one ever mentions them anymore. I'm talking about a field strength meter. If you are tuning pre-made antennas, it is a field strength meter that you should get experience working with first as they can teach you some valuable lessons that you can later apply to using an antenna analyzer, should you ever get one

Field strength meters can work for basic tuning and checking radiation patterns and such, but were talking about using analyzers for people who like to build there own antennas as well as check other antennas for an inside look on what an antenna is really doing. There are analyzers that are more intuitive that use computer modeling for Smith's charts for a more comprehensive analysis for the techno geeks. The MFJ259B is antiquated in my book, I couldn't sell mine fast enough when I saw better analyzers on the market.

Analyzers assists greatly in making antennas, matching with stubs, loading coils, baluns, chokes, etc. while field strength meters can't help you with that.

Analyzers can teach you more valuable lessons than just SWR readings if one really wants to learn. For the idiot CBer who only cares that his SWRRRR 'S is flat across the band for his over modulated export radio and his Davemade amp, then I would agree an analyzer would be a wasted expense since a good vintage D104 mic and a noise toy and a little left over for a truckstop Barjan SWR meter can be bought for the same amount. .

I presume you don't own an analyzer but still form an opinion on one and everybody that uses one.

 

[Rick330man]

Nice project, Riverman. Great thread.

Sometimes you gotta see it to believe it! That is what the old timer here said after seeing what his Shakespeare big stick looked like on one, lol! He ordered a new antenna that day! Anyway. Good day.

It is funny that you mention the Shakespeare Big Stick. I have two. One is the Shakespeare 176-1 marine version that I bought to mount on my boat (but it is sitting in my garage). The other is the Radio Shack 1/2 wave "Crossbow" base station version which Shakespeare made. Were it not for the ruggedness and reliability of these antennas in this harsh climate (Florida Keys) I would have gotten rid of these a long time ago.

The Radio Shack one is almost 35 years old. It is solidly built and is still giving me good SWRs - from 1.1 on channel 1 to 1.4 on channel 40. On the other hand, my A99 - and just about every other half wave I have ever tried - outperforms it in every way. I've been wanting to get an antenna analyzer, but didn't really think my little 4 watt AM/12 watt SSB, barefoot 11 meter set up justified the expense. I've been telling myself for years that I'll just take that money and upgrade my antenna. The problem is that the Shakespeare/ Radio Shack "Crossbow"/ "Big Stick" refuses to die.

That aside, I'm working on a simple project. I've got a bunch of 102" stainless steel whips. I'm going to set up 2 1/2 wave dipoles in phase and see what happens. They are mounted on marine stainless steel rail mounts which allow me to run them horizontal, vertical or anywhere in between. I've also got an antenna matcher in line to adjust for impedance variations as I experiment with different angles. I'm curious to see what this does.

 

[M0GVZ]

Why would you say that an ideal reading on a Analyzer like so wouldn't be the best possible tune or performance? PLEASE EXPLAIN YOUR STATEMENT!

50 Ohms at resonance is not the best possible tune for an antenna even though it is what the radio wants. The impedance, R, is made up of several things which we can't measure separately, one of which is ground loss, one is coil loss and one is radiation resistance.

Ground loss is bad for obvious reasons, it sucks up our signal instead of sending it out where we want. It also disproportionately affects signal strength at low angles where its important for DX.

Coil loss is an article on its own so I'll not get into it, suffice to say the less efficient a coil is, the worse your antenna works.

Radiation Resistance on the other hand is an indication of how well RF current is flowing within an antenna and is a function of electrical length. The longer an antenna is, the higher the Radiation Resistance will be and when working with shortened antennas, the more efficient a radiator it means the antenna is. It tops out at 36 Ohms for a 1/4 wave.


So back to why 50 Ohms impedance at X=0 is bad when we want our radio to see 50 Ohms.

Most antennas have an ideal impedance to be seen at resonance, especially with verticals. Each type of antenna is different., for example, the input impedance of a 1/4 wave vertical over a perfect ground is 36.8 ohms. Therefore in the case of the 1/4 wave vertical, if we have any reading other than 36.8 Ohms at resonance its because we have losses and ones we don't want. The most common cause is ground loss so usually if you're seeing more than 36.8 Ohms on a 1/4 wave vertical its because you've got a poor RF ground. You end up with the dim bulb effect, that is you still put 100W into the bulb but because of poor current flow and losses you end up with a dim light instead of a bright light. You can see this yourself using a table lamp powered from the wall socket. Disconnect the negative wire and turn the light on. It'll still light but will be very dim.

Anyway, back to our 1/4 wave vertical and the original question. We know that at resonance that over a perfect ground, the input impedance is 36.8 Ohms. That isn't going to result in a SWR of 1:1. This is an example of where the best possible tune and performance is not the ideal reading of R=50 Ohms at X=0 which is what the transmitter wants.

 

[fourstringburn]

50 Ohms at resonance is not the best possible tune for an antenna even though it is what the radio wants. The impedance, R, is made up of several things which we can't measure separately, one of which is ground loss, one is coil loss and one is radiation resistance.

Ground loss is bad for obvious reasons, it sucks up our signal instead of sending it out where we want. It also disproportionately affects signal strength at low angles where its important for DX.

Coil loss is an article on its own so I'll not get into it, suffice to say the less efficient a coil is, the worse your antenna works.

Radiation Resistance on the other hand is an indication of how well RF current is flowing within an antenna and is a function of electrical length. The longer an antenna is, the higher the Radiation Resistance will be and when working with shortened antennas, the more efficient a radiator it means the antenna is. It tops out at 36 Ohms for a 1/4 wave.


So back to why 50 Ohms impedance at X=0 is bad when we want our radio to see 50 Ohms.

Most antennas have an ideal impedance to be seen at resonance, especially with verticals. Each type of antenna is different., for example, the input impedance of a 1/4 wave vertical over a perfect ground is 36.8 ohms. Therefore in the case of the 1/4 wave vertical, if we have any reading other than 36.8 Ohms at resonance its because we have losses and ones we don't want. The most common cause is ground loss so usually if you're seeing more than 36.8 Ohms on a 1/4 wave vertical its because you've got a poor RF ground. You end up with the dim bulb effect, that is you still put 100W into the bulb but because of poor current flow and losses you end up with a dim light instead of a bright light. You can see this yourself using a table lamp powered from the wall socket. Disconnect the negative wire and turn the light on. It'll still light but will be very dim.

Anyway, back to our 1/4 wave vertical and the original question. We know that at resonance that over a perfect ground, the input impedance is 36.8 Ohms. That isn't going to result in a SWR of 1:1. This is an example of where the best possible tune and performance is not the ideal reading of R=50 Ohms at X=0 which is what the transmitter wants.

Thanks but I wanted DB to answer it.

 

[The DB]

Fourstringburn, apparently you ignored the post I was responding to, then read a ton into what I said while ignoring other key aspects of my post. Lets start with what I was responding to and part of what I said in response...

It's not all about vswr!! There are other factors.

Then I responded with...

This is true, yet when many people get an antenna analyzer all they use the increased abilities of the antenna analyzer to help them tune SWR...

So I started that post agreeing with you that the other variables are important, and should be looked at. I was in fact complaining about those who don't use an antenna analyzer in such a way. In essence I agreed with your entire premise of antenna analyzers right here.

Now to get to your question, the one you specifically stated you want an answer from me on... Not a problem, here is the quote and my response...

Why would you say that an ideal reading on a Analyzer like so wouldn't be the best possible tune or performance? PLEASE EXPLAIN YOUR STATEMENT!

Easy enough. First off, what makes you think that R=50 and X=0 is an ideal tune in the first place? If you think it is an ideal tun in any way then you are yourself stuck on SWR. How about a situation where you have a vertical 1/4 wavelength antenna over a horizontal ground plane. With your fancy antenna analyzer you tune it to the point where you have that so called "perfect tune" of R=50 and X=0. Now lets look at the performance you are NOT getting out of the antenna.

That length of antenna has a natural "radiation resistance" of about 36 ohms. This is "the good resistance" as it directly represents the energy lost to radiation. But wait, your R variable is showing 50 ohms... Where is the rest of that coming from? Losses.

Losses in series with antenna, such as ground losses, will add to the R variable, making it read as higher on your shiny antenna analyzer. In this case about 14 of that R=50 is from loss. To get this figure you simply take the R reading you get at resonance, and subtract the radiation resistance from it. So now we figure in how that 14 ohms of loss affects the antenna... 14 ohms of loss is 28% of the R=50, so 28% of the energy the antenna is absorbing from the feedline is being lost to heat instead of being beneficial to the antenna's performance. This is an antenna efficiency of 72%.

If you, instead, tuned this same antenna to, say, R = 36 and X=0 you give up that so called "perfect" SWR match, and instead have an SWR of about 1.39 : 1. This will reflect 2.663% of the power that makes it to the antenna back towards the radio. Making the mistake of assuming that all of this reflected power is lost, (is isn't), that means now your antenna's efficiency is in the range of 97%, and in the real world slightly higher than even that.

So you choose, in this case do you go for that so called "perfect" SWR match, or do you go for a slight SWR mismatch shown above? On the plus side, very few people in the world will ever see the difference between these two possible tunes of this example antenna... Don't think this can't be a real world example, a ground mounted vertical antenna fits this perfectly, where some people add radials until the tune is that so called "perfect" SWR match and stop adding radials when there is more performance to be had from the antenna...

Field strength meters can work for basic tuning and checking radiation patterns and such, but were talking about using analyzers for people who like to build there own antennas as well as check other antennas for an inside look on what an antenna is really doing.

Assuming you can get the antenna for the field strength meter in the lobe of the antenna you can tune the antenna to its peak performance by directly measuring the field strength of the RF radiation from the antenna. Sure, an antenna analyzer can get you close to this point, but the field strength meter can get you even close. If you want to tune an antenna for peak signal, a field strength meter is the way to go.

The whole reason I brought up field strength meters to begin with was apparently missed by you. The intention is to get one and use it for a while before getting an antenna analyzer, at which point you would have learned that that so called "perfect" SWR match is rarely the best tune for an antenna performance wise.

Further, you don't have to have an antenna analyzer to build an antenna. Look back on HomerBB's earlier builds posted right here on these forums. He had built many antennas that worked well before he got his first antenna analyzer.

Continuing on...

Analyzers assists greatly in making antennas, matching with stubs, loading coils, baluns, chokes, etc. while field strength meters can't help you with that.

Great, you just agreed with the point I was trying to make in the first place. Thank you.

Analyzers can teach you more valuable lessons than just SWR readings if one really wants to learn. For the idiot CBer who only cares that his SWRRRR 'S is flat across the band for his over modulated export radio and his Davemade amp, then I would agree an analyzer would be a wasted expense since a good vintage D104 mic and a noise toy and a little left over for a truckstop Barjan SWR meter can be bought for the same amount. .

Here again, you state the whole point of me writing that post in the first place. So you demonstrate that we are in 100% agreement..

I presume you don't own an analyzer but still form an opinion on one and everybody that uses one.

You presume I don't own an antenna analyzer? Really. I own 2 actually, and I showed the output of one of them to you in a different thread, you know, the one where you pointed out several times that your RigExpert was better than the MFJ? Further, I also have a grid dip meter that I sometimes use as well just for fun.

The opinion I formed was just about those who spend the money to buy an antenna analyzer, even the as you see it, lowly MFJ259b, and then continue to use it for nothing more than SWR meter. Those are the ONLY people I was talking about, and I think I did a pretty good job making that clear in the context of the post. I was NOT talking about people who own antenna analyzers in general, and that is also clear in the aforementioned post.


The DB

 

[fourstringburn]

So what you are basically saying is purposely mismatch an antenna some so it will even out with radiation and ground losses that are apparent anyway and that will balance out the mismatch. Where did you learn all of this?

The losses are there any way. The reactance value measured of an antenna system ideally is a value of 0. This represents the resonant point, which may not be the lowest SWR in some cases. This is what should be achieved and can be found easily with an analyzer. Feed point impedance can vary with different antennas and types of feed-lines.

An RF antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency. This is the frequency where the capacitive and inductive reactances cancel each other out. At this point the RF antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

Two major factors associated with radio antenna design are the antenna resonant point or center operating frequency and the antenna bandwidth or the frequency range over which the antenna design can operate. These two factors are naturally very important features of any antenna design and as such they are mentioned in specifications for particular RF antennas. Whether the RF antenna is used for broadcasting, cellular telecommunications, or any other application, the performance of the RF antenna is paramount, and the antenna resonant frequency and the antenna bandwidth are of great importance.

Get your antenna matched as good as possible, then the other losses are there anyway.

 

[The DB]

So what you are basically saying is purposely mismatch an antenna some so it will even out with radiation and ground losses that are apparent anyway and that will balance out the mismatch. Where did you learn all of this?

The losses are there any way. The reactance value measured of an antenna system ideally is a value of 0. This represents the resonant point, which may not be the lowest SWR in some cases. This is what should be achieved and can be found easily with an analyzer. Feed point impedance can vary with different antennas and types of feed-lines.

An RF antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency. This is the frequency where the capacitive and inductive reactances cancel each other out. At this point the RF antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

Two major factors associated with radio antenna design are the antenna resonant point or center operating frequency and the antenna bandwidth or the frequency range over which the antenna design can operate. These two factors are naturally very important features of any antenna design and as such they are mentioned in specifications for particular RF antennas. Whether the RF antenna is used for broadcasting, cellular telecommunications, or any other application, the performance of the RF antenna is paramount, and the antenna resonant frequency and the antenna bandwidth are of great importance.

Get your antenna matched as good as possible, then the other losses are there anyway.

First off, you didn't say anything in this post I didn't already know, and with all due respect, the example I came up with and the math behind it requires that I know at least that much.

In essence you are correct, the example I used demonstrates the effects of changing one type of loss for another, and it made a difference when it came to antenna efficiency of over 24%. Yes, there is a slight SWR mismatch, but to be concerned about any SWR of less than 2.0 : 1, a reading that is far beyond the point that any remote station will be able to notice the difference, is simply wasting your time. Other changes that effect the complex impedance of the antenna will have far more of an effect, such as removing the significant amount of ground losses that were present in the example I gave.

I have recommended reading for you that doubles as one of multiple sources you requested for where I learned the information given in my example given above, M. Walter Maxwell's "Reflections Transmission Lines and Antennas", any of the three editions printed. If you can't find one of those, you can get an early version of the first seven chapters for free by searching for a .pdf called "Another Look at Reflections".


The DB

 

[fourstringburn]

Thanks for the references, I will do some research on them and I will see what I can learn from them. My antenna knowledge is mostly mobile antennas since I live in a HOA neighborhood and my base antennas are simple but effective for what I can get away with.

 

[DuckSoupe]

Well I bought a 11 meter ' dipole ' with a ' balun ' and it is a perfect Swr. I just don't hear much chatter; just a lot of ground noise which I wrote the manufacturer; but have not received a reply . I am not looking for distance as trucks are always in the area. I ordered a indoor no ground antenna with and if it does better I will take dipole down.

 

[M0GVZ]

Well I bought a 11 meter ' dipole ' with a ' balun ' and it is a perfect Swr. I just don't hear much chatter; just a lot of ground noise which I wrote the manufacturer; but have not received a reply . I am not looking for distance as trucks are always in the area. I ordered a indoor no ground antenna with and if it does better I will take dipole down.

OK, conditions on 11m are crap so you're not going to hear a lot of DX. Because a dipole is horizontal and local and mobile stations are using vertical antennas you will find that signals you receive are a hell of a lot weaker than they should be and likewise you will be to them due to the fact your signal is horizontally polarised and theirs is vertical so there's an automatic -20dB loss due to cross polarisation. Think 6 S points or more on a typical CB so something that would normally be a S9 may now be a S3.

An "indoor no ground antenna" doesn't exist unless its at least 18ft long. It has a ground, just not one supplied with the antenna. The coax that goes to it, your radio, the wiring to your power supply, the house wiring all end up being the ground. Expect more noise and to find things like your TV or computer speakers being affected.

 

[zig zag]

OK, conditions on 11m are crap so you're not going to hear a lot of DX. Because a dipole is horizontal and local and mobile stations are using vertical antennas you will find that signals you receive are a hell of a lot weaker than they should be and likewise you will be to them due to the fact your signal is horizontally polarised and theirs is vertical so there's an automatic -20dB loss due to cross polarisation. Think 6 S points or more on a typical CB so something that would normally be a S9 may now be a S3.

An "indoor no ground antenna" doesn't exist unless its at least 18ft long. It has a ground, just not one supplied with the antenna. The coax that goes to it, your radio, the wiring to your power supply, the house wiring all end up being the ground. Expect more noise and to find things like your TV or computer speakers being affected.

soon DX conditions will be back in full swing on 11 meter,,im hearing it now,, then your dipole will make a difference,,,from a vertical,,,put both up they have different advantages,,

 

[Riverman]

On a half-wave wire horizontal dipole does it matter how the coax is routed away from the antenna if a balun is used? Or does it still need to be perpendicular to the antenna for the best match?

 

[BJ radionut]

Yes, it needs to be perpendicular(90 deg's) to the feedpoint of the antenna for the best match.
All the Best
Gary

 

[Riverman]

Yes, it needs to be perpendicular(90 deg's) to the feedpoint of the antenna for the best match.
All the Best
Gary

Okay. Thanks!