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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 05-28-2018, 5:13 PM
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Default No SWR meter/Antenna Analyzer/..., now what?

Seems like tonight there will be a local ARC net talking about "shack gear" and I suspect the SWR meter is one of those indispensables. But what do you do when you don't have one yet?

I was wondering if people have worked with tuning their (homemade too) J-pole, mono, or dipole, and what happens to your SWR when you're 1%, 2%, etc. off of your target wavelength (i.e. for 2m, you're 2cm, 4cm off) - how much does SWR increase? Would a 10% driven element length error double? quadruple? your SWR? As for me I'm trying to target 70cm for now as I do not have a usable 2m radio, so 1% of 70cm would be 7 millimeters which will be tough to get perfect.

Perhaps there's more to it than just this -- making a SWR meter absolutely necessary. The online homemade SWR meter for 2m and shorter seem quite complicated...

The radio I'm using is a LMRS radio, so no signal quality features in it, so I'm screwed there. Actually I do have the schematic that I could possibly jury rig something into the impedance protection circuitry to give me a rough indication of SWR... HMM!!! this might be the easiest avenue to pursue...
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Old 05-28-2018, 5:20 PM
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Running radios without a SWR meter, especially if you're building your own antenna, is like visiting a bordello without a condom.

Just.

Don't.

There's too many things that can go wrong without you having any clue.
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Old 05-28-2018, 5:24 PM
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Lower gain antennas, like quarter wave and half wave (J-Pole is a half wave) tend to be pretty broad banded, so if you are doing your math correctly, chances are you are pretty close.

Many commercial mobile antennas come "pre-tuned", especially some quarter wave antennas. Checking SWR on them is sort of a formality, making sure everything is connected and working as it should. For the pre-tuned antennas, I've never found one from any of the major US manufacturers that was out of line with where it should be.

Going into the commercial base antenna, and it can be pretty rare to even have a way to adjust them.

So, yes, an SWR meter is a good tool to have. An antenna analyzer is even better. But if you don't have one and want to get on the air, stick to the basic antenna designs, double/triple check your math, and make sure you have good connections.

Also, most commercial radios have high SWR protection circuits. The radio itself has a circuit that looks at reflected power and will turn back the transmitter power if it senses the SWR is high. This doesn't negate the need to check the SWR and your antenna system, but rumors of radios exploding in mushroom clouds because the SWR wasn't perfect are just fairytales. For most modern commercial gear, you've got to do something pretty egregious to them to kill the final stage. Don't take that as a challenge, though.
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Old 05-28-2018, 6:31 PM
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Yeah, after reading through many manuals, it seems like the commercial LMRS radios I have indeed do have that protection circuitry -- though it protects against self immolation, I can't use it to tell how far off my antenna is or which way to tune it -- without perhaps tapping into the system and measuring some voltages to see how bad the protection circuit thinks the mismatch is. If it's trying to severely limit the output stage, we can make a fairly good assumption that the SWR is very high.

I was looking at building an omnidirectional, no (formal) gain antennas and seeing if I can get it tuned "about" right as I do not have any 2m/70cm antennas -- only 11m. Probably a J-pole taped to a window, perhaps no more than 35W for now because that's my biggest transmitter. The velocity factor is the number I can't necessarily determine for an arbitrary piece of wire with or without insulation, so I have to make an educated guess based on tables or Google, and thus I can be off by a fair amount if the guess was wrong.

If I was wrong and off by say a 1% (that 7mm on 70cm), how much would SWR increase? It sounds like it won't be too much for a J-pole/monopole/dipole and very unlikely to be a direct correlation, but curious as to how much worse an "almost" versus a "perfect" antenna would be. As I don't have twin lead either, I'm thinking of just using some generic hook up wire or perhaps even zip cord -- hence the worry about not knowing the velocity factor...

(I was reading up on a really cheap J-pole design using TV antenna twin-lead http://www.iw5edi.com/ham-radio/file...-dual-band.pdf , and saw that despite it was tuned for 2m (SWR of less than 1.3 for most of the band) it can be used for 70cm and still mostly stay under SWR 2.0, which seems "good enough" for my purposes... except I'll trim for 70cm for now with no 2m radio, and supposedly it would be even better for 70cm.)
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Old 05-28-2018, 6:50 PM
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A passable 2 meter/70 centimeter SWR meter can be had on the cheap if you look around. Not a great meter, but will give you some useful info.
If you are serious about building your own antennas, investing the money in the right tools can be a good thing.
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Old 05-28-2018, 7:16 PM
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There was a time when very few Hams had an SWR meter and antenna analyzers were unheard of.
However, for serious antenna work you want to put a SWR meter on your wish list. A VHF/UHF SWR and power meter can be had for under 100 bucks. An analyzer and Bird 43 can come later if you really get into it.

For now, just build a simple 1/4 wave ground plane. Get the antenna up as high as possible and use good coax and get on the air. It will be close enough.
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Old 05-28-2018, 10:10 PM
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Heh. I was trying to avoid something that required a ground plane and thought the J-pole would fit that bill. Might have to do away with the J-pole idea?

I also realized that if I used my window as part of the antenna, that too (the dielectric effect of the silicon dioxide) probably adds a little to the uncertainty ... hahaha...

I don't think I'm going to be a "professional" antenna designer, but would like to make sure I fully understand the issues behind it.
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Old 05-28-2018, 11:29 PM
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Quote:
Originally Posted by needairtime View Post
But what do you do when you don't have one yet?
Buy one.

Ok, so you're mainly on 2m/70cm using an LMR radio. Don't dispair. Avoid antennas like jpoles that require tuning. A ground plane is primarily going to be dependent on physical lengths of elements for tuning. There's not much to go wrong.

Commercial made antennas usually don't require tuning.

The criterion as to whether it's ok is, does it work about as expected? If you don't have a meter, and can't get one, what else do you do? Just get on the air and try it. A modern radio isn't going to hurt itself if the swr is a bit off.

You can always get a meter later and start experimenting later. Don't let lack of an swr meter keep you from getting on the air.
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Old 05-28-2018, 11:32 PM
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The real 'point' of having a SWR meter is that the 'real world' can often differ from what the 'math' says. If your antenna is near other metal (like the mast it is mounted to or the car is it on), the 'extra' metal may (or may not) affect the antenna. Also a SWR meter supplies a quick check of the coax and connections. Remember the 'simple' formulas used for antennas have several 'assumptions' built in -one of them being the antenna is in 'free space' (not likely here on planet Earth). Most of us make the antenna a a bit longer than the formula indicates and then 'trim' it with an SWR meter (or analyzer) with the antenna placed where it will be used. This allows for the antenna to be 'tweaked' for best operation where it will be used.

Most of today's gear (especially handhelds) are designed to operate with a 'bad' antenna or at least not be damaged by one. That said, in a pinch a simple quarter wave with quarter wave radials for a ground plane is simple and if kept at least a half wave form other metal should be 'close enough'.

building your own antennas can be fun and rewarding-at first the cost of the test gear eats up savings but the learning never stops.

73
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Old 05-29-2018, 1:04 AM
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Have people found that the environment or element length to be a larger cause for SWR mismatch? I don't know how often people have adjustable element lengths other than cutting them, how much of a change in SWR are people seeing as a percentage of wavelength clipped?

I also wonder: are people trying really hard getting that 1:1 such that perhaps 1.05:1 isn't any good? (well, possibly contrived, but hopefully the picture is clear: getting that last bit of mismatch out.)

I suppose my goal isn't really to get all the way to 1:1 or even 1.5:1 or something, what I'd like to do is to someday eyeball a design, know frequency, measure loading coils/capacitors/lengths of any built antenna design and answer a simple yes/no: "that would produce better than 2:1 SWR" ... Or is a goal like this farfetched without measurement equipment?

I probably will need to acquire or build an analyzer or SWR meter someday. Just not yet, I'm still working on my PLL project (to solve my 2m radio problem)
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Old 05-29-2018, 5:53 AM
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Quote:
Originally Posted by needairtime View Post
Have people found that the environment or element length to be a larger cause for SWR mismatch? I don't know how often people have adjustable element lengths other than cutting them, how much of a change in SWR are people seeing as a percentage of wavelength clipped?

I also wonder: are people trying really hard getting that 1:1 such that perhaps 1.05:1 isn't any good? (well, possibly contrived, but hopefully the picture is clear: getting that last bit of mismatch out.)

I suppose my goal isn't really to get all the way to 1:1 or even 1.5:1 or something, what I'd like to do is to someday eyeball a design, know frequency, measure loading coils/capacitors/lengths of any built antenna design and answer a simple yes/no: "that would produce better than 2:1 SWR" ... Or is a goal like this farfetched without measurement equipment?
Environment and element length are both factors in antenna performance. As is coax and various other things. If you put a twin-lead J-pole in a PVC pipe for weather protection, the interaction between the antenna and PVC will change the resonant frequency of the antenna. If you have two antennas near each other, they will interact with each other. That affects SWR, and when you TX with one antenna, RF will feed into the other antenna and whatever is attached to it. Without a meter you have no way to verify that your homemade antenna is working as intended, and if you have multiple radios and antennas in close proximity, you have no way to know if induced RF levels feeding into a receiving radio are possibly high enough to damage the radio when you key up.

A basic power/SWR meter isn't that expensive, if you have money for radios, then you have money for a meter. And don't use protection circuits in a radio as an excuse not to get a meter. Running high SWR is still hard on the radio, and whatever is causing high SWR will negatively affect both TX and RX performance.
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Old 05-29-2018, 7:30 AM
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Not only are antenna analyzers good for building antennas, but they are an excellent tool for finding an issue that could make your radio lose all it's smoke.

A good SWR/Power meter will probably work fine for your purposes, but there is much more than just a good SWR when making home brew antennas, or checking used commercially made antennas.

I have an SWR/Power meter and an antenna analyzer, and I don't know at this point how I got along without them.
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Old 05-29-2018, 7:32 AM
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Another good addition to the meter is a dummy load. It helps understanding what your radio can do into a non-reactive load without the influence of an antenna.

Plan ahead for short adaptor cables and adaptors to allow all the pieces to fit together. Link is to one I use with my HTs and QRP rig.

Mike, KE5MC

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Old 05-29-2018, 9:07 AM
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Quote:
Originally Posted by needairtime View Post
I also wonder: are people trying really hard getting that 1:1 such that perhaps 1.05:1 isn't any good? (well, possibly contrived, but hopefully the picture is clear: getting that last bit of mismatch out.)
People do. People don't. Depends on the individual.
Likely no ears could tell the difference between a perfect 1:1 or a 1.05:1 match.

But then again, when you have the right tools, why not? For years I just used an SWR meter. If the SWR was down below 1.5:1, I was generally happy with it. When I had access to the equipment that would let me do a proper antenna analysis, it was easier to get it lower. On some installs, I've spent the time to get the SWR down as low as it will go.
Is it worth it? Probably not, but when it's easy to do…..

Quote:
Originally Posted by needairtime View Post
I suppose my goal isn't really to get all the way to 1:1 or even 1.5:1 or something, what I'd like to do is to someday eyeball a design, know frequency, measure loading coils/capacitors/lengths of any built antenna design and answer a simple yes/no: "that would produce better than 2:1 SWR" ... Or is a goal like this farfetched without measurement equipment?
As Jon stated, there are a lot of external variables involved.

For an antenna in a perfect location, just doing the math will get you close, but you are going to find it hard to get a perfect location anywhere. There will almost always be some sort of interaction with the cable, mounts, other structures, and other variables. You still need a tool to tell you if it's working as it should.

When I was much younger, my grandfather taught me about antennas. He'd worked for Lockheed for a long time and had worked with some of the radio guys. He said they'd measure down to the fraction of an inch, but in the end, pulling out a meter and doing the final adjustments by what the meter said was always done. He said it was like antenna design was a bit of a black art. The measurements will get you so far, but then the magic happens and you've got to tweak it a bit.

I think doing this just on paper is going to get you close. But understanding all the variables will be impossible.

A basic VHF or UHF base antenna or mobile 1/4 wave will likely work fine without a meter, if you pay attention to the install and what's around the antenna.
If you are going to be doing a lot of design and install of your own antennas, then yeah, you are going to need some tool to confirm your design.

Quote:
Originally Posted by needairtime View Post
I probably will need to acquire or build an analyzer or SWR meter someday. Just not yet, I'm still working on my PLL project (to solve my 2m radio problem)
You'll want to, sooner or later. A basic one can be had pretty cheap. But if you are really going to be designing a lot of antennas, you are going to want better than cheap. For an SWR meter, and especially if you'll be using it over a wide range of frequencies and power levels, it's hard to beat the Bird 43.
And antenna analyzers, or a spectrum analyzer/service monitor with a tracking generator and directional coupler really do make life easier. It would not be impossible to build your own.
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Old 05-29-2018, 9:49 AM
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A basic power/SWR meter isn't that expensive, if you have money for radios, then you have money for a meter. And don't use protection circuits in a radio as an excuse not to get a meter. Running high SWR is still hard on the radio, and whatever is causing high SWR will negatively affect both TX and RX performance.
Actually that's sort of the issue, the LMRS radios I have were acquired in "unknown condition" and I paid more to ARRL-VEC than the radios I have, combined. So far I'm completely trying to use whatever I already have to get them to work. (Programming the radios when I don't have the manufacturer gear was the first challenge. In fact if I couldn't figure out how to program them with the stuff I already had, I probably would not even think about antennas, or even getting my ham license at all )

The idea I had wasn't exactly to blindly 'use' the protection circuit, but actually go a step further and monitor the state of the protection circuit to understand how hard it's trying to prevent damage. I would have to make an assumption that it behaves linear with respect to the mismatch instead of being an on/off protection, which would not be useful. If I could find a voltage somewhere that's proportional to how much the protection circuit is trying to cutting power to the output stage and perhaps the current output power, then with some math (or op amp) I could get a number that's relative to the mismatch -- or is this just wishful thinking?

Granted yes, cable loss will not be accounted by doing this, but that's all right, I'm just curious how much is being sent back for now. Someday I'll get/build a SWR meter because it's best measured nearer to the feed point instead of at the driver anyway.

Just not now... I don't know how serious I want to be in antenna design yet.
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Old 05-29-2018, 1:14 PM
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If the goal is to just get on the air for now-I would have to suggest you make a 1/4 wave vertical with radials. It is not only fairly tolerant (broad band) but the dimensions are some what not critical. What I would do is use a small piece of wood and a S)-239 connector for the 'base'. 5 pieces of #12 solid wire 19 inches long (one for the vertical element and 4 for radials) bend the radials down at 30-45 degrees and you will be close. If you have a DMM, double check for shorts and opens. After you get it installed listen if you can to some of the traffic, If everything sounds good, use low power and transmit.

If you have a local club, someone likely has a SW meter you can barrow (same with DMM) Test equipment spends a lot of time on the shelve and hams as a group tend to be helpful.

Most of the J-poles I have built are real 'picky' as to the location of the feed point and are also more narrow banded.

Welcome to the hobby
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Old 05-29-2018, 7:32 PM
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That sounds like a plan, I had forgotten about the radials antenna, that sounds like a possibility as a noncritical antenna. Thanks for the warning about J-poles being picky, sounds like getting SWR down on those would be tough. Currently I've been using a ~ ¼λ piece of wire stuck into the radios' SO-239 and able to pick up some local ham repeaters hence the excitement that it actually works. I do wonder about this 'degenerate' antenna for transmitting, where there is no feed line loss, since there is no feed line, and how it could be possibly impedance matched in any sense of imagination... Instant output transistor kill most likely if it didn't have impedance protection!

Fortunately I have some test equipment, just not for radio. I think the only bit of radio-specific gear I have is an antique, AM signal generator which is not really any good for VHF testing nevermind UHF (old vacuum tube Colpitts signal generator to about 145MHz that squares off the sine waves so Eico calls that 145MHz "435 MHz" due to the third harmonic visible in the waveform... HAH, what a racket.

Doesn't matter that much I guess, my oscope doesn't go that high and will round off the corners anyway due to bandwidth limitations. However it'll be great for HF once I get around to it, perhaps even use it as part of a homemade tuner for HF.
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Old 05-31-2018, 8:52 AM
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Needairtime-

Another simple antenna is a 'vertical dipole'. I am using one I built a couple of years ago and still works great. Get about 4 feet of 1/2 inch copper pipe and a PVC 'Tee' Use the 'tee' as the center insulator and use a bit more PVC from the 'tee' (at least a foot or so) to what ever to mount it. Use metal screws for connecting coax and run the coax along the PVC stub. Make the overall length to the classic dipole formula (468/f (Mhz)=l(ft)). I get about 10 Mhz of bandwidth between the 2:1 SWR points and a nice low (about 8 degrees) angle of radiation. The antenna is up about 30 feet and a foot or so to one side of my tower(yes the tower does mess up the pattern and makes the antenna favor the direction away form the tower by a few db).

By your comments, coax will be an issue. You need to get that antenna up in the air about 15 feet minimum and away from you or other people (RF exposure and usability). You can use RG-6 type coax if you are careful with connectors and watch the power. You will not get perfect SWR but anything under about 2:1 (when protection circuits take note) is fine. 2 meter FM is by no means 'week' signal work and a db or two is mostly unnoticeable. RG213 type coax (or other good 50 ohm) would of course be preferred.

Coax may be something you can salvage or a local ham may let you have some.

All things considered, your antenna system will likely be what determines the 'range' or effectiveness of your station so remember weatherproofing. At VHF, the higher the better, but for the most part the repeaters do the heavy lifting so just over the roof line and 'in the clear' is a first goal.

Feel free to ask more or PM

73 and again welcome to the hobby

Last edited by wyShack; 05-31-2018 at 9:08 AM..
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Old 06-12-2018, 12:30 AM
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I think I have some interesting data collected from my TK810's output protection circuitry.

makeshift dummy 50Ω load (may be slightly inductive...):
Low power (nominally 2W):
Power Control output: 1V
Protection circuit: 0.62V

High power (nominal 10W):
Power Control output: 9V
Protection circuit: 2.6V

With a 4ft RG58 and a ~ 5/8 λ steel wire makeshift antenna with no attempt at matching:
Low power (nominal 2W):
Power Control output: 1V
Protection circuit: 1.2V

High power (nominal 10W):
Power control output: 5.6V
Protection circuit: 3.4V

Seems like the circuit is probably trying to make sure that the protection circuit voltage does not go past 3.4V and reduced power, represented by 5.6V. 9V may be the representation for full power, though I am not totally sure either why the protection circuit voltage increased with output power with the dummy load.

I will need to see what this circuit does with a CB antenna, and if I can find a real 70cm antenna to see the behavior with that attached.

Theoretically I should be able to correlate this data to SWR though I'm not quite sure how to get nominal power output value from the pot electronically without disturbing its functionality. I may not need to know this data and assume this is 20:1 or something like that when power is reduced and protection voltage is 3.4V ...

Calibration, calibration...
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Old 06-13-2018, 7:23 AM
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Okay for those who don't have SWR meters and think about using wrong antennas... more measurements!

My fiberglass stubby, supposedly CB antenna (mag mount), the fiberglass stick has a loading coil wound outside of it, covered in heat shrink.)
Low power (nominal 2W):
Power control output: 0.99V
Protection circuit: 1.796V (!!!) Well, this means using the CB antenna is *really* bad at UHF, worse than my makeshift 5/8λ antenna.

I unscrewed the fiberglass stubby and tried sticking things into the hole vacated by removing the fiberglass whip, continued with 2W:

A 1" bolt: 1.93V Well of course this is even worse. I was planning on drilling a hole into the bolt and attaching wire to it for use as an antenna with this CB magmount.

Brass 10" faucet stem leaning in the hole: 0.896V ... Woah...

Galvanized 8" bolt leaning in the hole: 0.588V ... Jackpot... sort of. But now I have to explain why this lower than the 50 ohm load (then again, different cable. The CB antenna cable is "better" RG58 I would think; I don't know what the true quality of the alleged "RG58" I had in the previous experiment, so this is not an apples vs apples experiment.)

Hopefully any antenna I optimize for this radio will work for others (at the same frequency of course).
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