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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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  #21 (permalink)  
Old 06-13-2018, 10:36 AM
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Needairtime-

At UHF a vertical dipole would be just over a foot (12-5/8") with a quarter wave being half that (call it 6-1/4") for the antennas described earlier. Get the antenna about 5 feet from anything conductive and you should be 'close enough' (interested in what readings you get with the above). that said, getting the antenna up over the roof line should still be a goal-both to eliminate RF exposure and for good range.

It is good to see someone new to the hobby that likes to tinker.

73
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Old 06-14-2018, 11:12 AM
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Thanks, will let you know if I come across new experiments.

I do have one new experiment: I am now using a 18 foot piece of "CB" RG58A/U with a PL259 on each end. Plugged one end into the TK810 and the other end... an alligator clip that has a banana jack hole (the PL259 pin is about the same thickness as a banana plug) clipped onto a 8" piece of wire since that was readily available. I hung this contraption onto a nail in my wall that's about 7 feet from the floor.

0.342V from the protection circuit!

Granted I probably am losing more and more of the reflections with longer and longer RG58, so this probably explains the lower voltages, alas I forgot to do a no-wire test (perhaps just with the alligator clip) as a control...

And yes, I wish there were more tinkerers around...I would think this should be the spirit of ham radio.
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Old 06-14-2018, 5:46 PM
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In reality there are issues like physical length versus electrical length and velocity factors for materials, so if the OP is using the world's finest plans for an antenna, and matches the measurements down to 0.1mm, he still won't know the SWR of the antenna without also exactly matching all the materials that were used.

And that's just how it is. "Close is good enough" in horseshoes and antennas. Yes, by all means, buy OR BUILD an SWR meter, they are dirt simple to build. Don't let that stop you in the meantime, because even if you are off by more than a few percent, odds are your radio will not be harmed by an SWR under 3, and you can beat that if you're using a hacksaw and crayons for your measuring and making.
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Old 06-14-2018, 6:45 PM
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20 feet of RG58U at UHF is still only 2 dB-UHF FM is not weak signal work. 20 feet will get the antenna away from the radio. you could take the 'vertical dipole' idea and just make one by taping the two pieces to a wall. Wrap the wire around your coax to make a connection and measure from the wrap 6i nches. that will get you close enough. Listen to the local traffic-it should be full quieting with anything for an antenna. See what you can hear and get the radio programmed (tx tones ect) -BTW I assume you have an odd frequency programmed for simplex to 'play' with the antennas? Remember to ID. Then try 446.000 Mhz simplex, then your closest repeater.

After that work on improvements -including getting with the local groups for more ideas.

73
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Old 06-14-2018, 9:54 PM
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Cool yeah, I was wondering about about that, how close the measurement and how much of an SWR reading change it would mean, sounds like fractions of a millimeter can still change the SWR significantly?

Anyway, yeah the problem is ... making or buying... a UHF SWR meter. From what I've seen on the web, sounds like most "CB" SWR meters just won't do UHF, and, well, UHF ones are not cheap. I did see some ideas to use a 11m SWR meter and sort of jury rig it to work at 2m, but not sure if I can keep going to 70cm.

In lieu of all that, I was just seeing what I can collect from the protection circuit in the TK-810L. After staring at the protection circuitry, and at least sort of comparing it to the design in a 11m SWR meter - gosh darn it, they looks uncannily similar - it IS a SWR meter inside the transceiver, used to protect the output in case someone hooks up or neglects to connect an antenna! However, I don't get a SWR number like what people normally get, I get a voltage that I'd need to translate to a SWR number... or don't bother with the SWR number and simply try to minimize the voltage which is equivalent of minimizing SWR.

I was listening in on 446.000 simplex and heard nary a peep on it, so I ID'ed and said test when doing the experiments (mainly because the LMRS radio is a PITA to program, have to pull the eeprom to program new frequencies), quickly picking up the voltage from the protection circuit before I unkeyed. Initially I started testing with 2W and 10W but found that I probably got what I needed at 2W, so discontinued the 10W tests. Nobody responded to any of the tests, so I guess it's okay (if a tree falls and nobody is around to hear it, did it ... fall?)

So that last antenna experiment I got the lowest voltage. I switched over and was able to open a channel with the local ARC repeater and apparently held onto it just fine. Seems like people heard the transmission fine! However they commented my audio was weak, kind of figures, had to hack a microphone for the radio too as I didn't get one with the TK-810L. But that's a different problem. Partial success I suppose! Just need to see if I can improve it some more, and find a solution for my car as well

On the other hand, either my scanner sucks or my LMRS radio is causing intermod with the scanner. The LMRS radio seems to break squelch on the scanner if the receive tuner matches up with the tuning on the scanner somehow. Again, an unrelated (or maybe "too-close") problem but not quite on topic...
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  #26 (permalink)  
Old 06-15-2018, 9:29 AM
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Needairtime-

Most simple antennas are not that fussy. SWR is overrated as a means of measuring antenna performance. SWR for a dipole will have a 'dip' where it 'bottoms out' at about 1-1.5 to 1. From the dip to 2:1 on each side you will have 5-10 Mhz. When trying to get the microphone going I would go back to the dummy load, pull the antenna off the scanner. What is happening is often refereed to as desense-the strong signal is driving the gain circuit of the scanner too hard.

Find out as much as you can about the microphone you are trying to emulate-then build to match. Many microphones have a stage of amplification built in or your input may be expecting a 'strange' impedance. I would stay off the repeaters until I knew I had TX audio figured out-you cannot ID without it.

Same with your control circuit -the zero point (meaning 1:1 for SWR) may not be zero it could be something else the next block in the chain is looking for.

sounds like you are getting things up and running.

73

Last edited by wyShack; 06-15-2018 at 10:11 AM..
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  #27 (permalink)  
Old 06-15-2018, 11:17 AM
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Actually the funny thing about the scanner/TK-810L: I don't need to be transmitting to break squelch! Something funky is going on, the TK810 is probably radiating something that the scanner is picking up. It doesn't appear to be the only device, my Garmin rino FRS/GMRS appears to also break squelch if I have the two near each other, again without needing to transmit - I just need to have both tuned to the same frequency.

I was finally looking more closely at the audio input circuit of the TK810 - looks like the microphone needs to be of "low" impedance type, as said on the specs... but more importantly, the 500 ohm sensitivity pot in the radio. No wonder the 2K ohm dynamic microphone I had tried to hook up to it was really weak sounding, have a 5:1 attenuation right there!

The makeshift copied microphone circuit that uses an electret was a bit better however. Its impedance is lower due to the transistor amplification circuit -- but has a muffle effect that the other hams report I need to resolve. I can't seem to find any LPF that I may have accidentally introduced. I haven't discounted perhaps using a bunk electret element, I have another that I should sub in real quick from a telephone I recently parted.

Indeed I was wondering about the fact it is technically illegal to transmit phone without a working microphone, but there was the other clause of "short tests" which I was hoping I fall under. Yeah I have to apologize to anyone who may be listening, it wasn't intentional but it's hard to experiment without experimenting.

And yes the "zero" (i.e. 1:1) point is clearly not zero. As far as I can tell, as long as the voltage (minus its attenuator potentiometer, which I am not touching for now) does not exceed the threshold to turn on the transistor that it hooks up to, the radio won't limit its output. In this respect that 3.4V limit I've seen is probably 0.7V (from the threshold of the transistor) with a 5:1 attenuation. Now based on the observations people have been reporting that transmitters tend to limit power at around 2:1, this would imply that 3.4V is near that 2:1 VSWR magical number. However, as there's that attenuating potentiometer there, all bets are off. Oh well, so much for calibration that way, but all I need to do is to keep it low as I can, something under, say two or so volts is probably "good enough".

But yeah I think I'm basically enabled now, learned a lot of "real" equipment stuff instead of just theory...which basically all that licensing exam was
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Old 06-15-2018, 5:18 PM
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Quote:
Originally Posted by needairtime View Post
Actually the funny thing about the scanner/TK-810L: I don't need to be transmitting to break squelch! Something funky is going on, the TK810 is probably radiating something that the scanner is picking up. It doesn't appear to be the only device, my Garmin rino FRS/GMRS appears to also break squelch if I have the two near each other, again without needing to transmit - I just need to have both tuned to the same frequency.
Local oscillators. A bit leaks out, other receivers will pick it up.
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Old 06-15-2018, 5:56 PM
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Yeah, the scanner receiver seems really sensitive to either of the transceivers, enough so that I can't squelch it out; however, neither the Garmin or the Kenwood seem to receive the other transceiver's presence.
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Old 06-16-2018, 8:57 PM
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Hey Needs .....
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I read thru your comments and those that answer'd you....and I am a bit perplex'd. Is it me, or are you trying your hardest not to use an SWR meter? I can appreciate your taking a scientific approach to your antenna issues, but by taking this route you're only compounding your efforts.
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Buy a SWR meter.
.

They are not expensive, especially if you go to Hamfests. It sounds to me like you're interested in 2 metre's- you can (usually) get by with a simple CB meter up to 150Mhz...... (70cm is another issue).... but look on Amazon- there is a plethora of 2/70cm meters there.
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If you are going to use anything more advanced than a quarter wave vertical (and even then) -- make your life easier and use an SWR meter to adjust it. Trying to build "J" etc. antennas by monitoring your radio's standing wave protection circuit voltages is... for lack of a better word..."Exotic." In my years I have never seen this done real world.....
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When you have some experience with commercial meters, then and only then should you try your hand at building them. Anything above +30 Mhz's will require a strict attention to design- something appreciated only after acquiring a familiarity with the Standing Waves.
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  #31 (permalink)  
Old 06-17-2018, 12:53 PM
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Oh no... it's not that I don't want to use one, it's more like, I don't have one and finding out what I and anyone else can do without one. Though it initially seems the goal is to do "without" one, now I find out apparently the circuitry/instrumentation is actually built into some of these LMRS/Commercial radios, just need to get that information out from the radio!

Unfortunately it's the 70cm that I'm currently looking at as I do not have other ham radios that I can play with at the moment. My 2m radio is yet another project since I don't have crystals for it.

Thanks, yeah, perhaps I'm a bit unusual in that I'm building equipment. I've always wanted to figure out how to get information over the airwaves from components, buying something off the shelf just doesn't do it for me
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Old 06-17-2018, 1:03 PM
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Quote:
Originally Posted by needairtime View Post
Though it initially seems the goal is to do "without" one, now I find out apparently the circuitry/instrumentation is actually built into some of these LMRS/Commercial radios, just need to get that information out from the radio!
Depending on how you go about it, extracting forward and reverse power information from the radio might alter the measurement, giving both you and the radio faulty information. If there's a meter to read, great. But if the sensors are used for power control and circuit protection, leave 'em alone.

Honestly, a suitable meter is cheaper than potential repair or replacement of a radio.
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Old 06-17-2018, 1:43 PM
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I'm amazed. Astounded even!

The exam requires a basic grasp of fundamentals, and clearly you can do the maths. They hobby is about self-learning and experimentation. If you build a ground plane antenna, then even if it is too short or long by iffy cutting, it will still radiate. Near to resonance, we are indeed talking for 2m/70cm very small amounts cutting off making a difference - but people often cut too much off, and it goes past perfect to being wrong again. The practical upshot is not noticeable - in terms of performance.

Buy a meter. There is no point NOT having one, because they're cheap new and dirt cheap 2nd hand. The cheap ones are not remotely as accurate as a Bird - but that doesn't matter. They show the critical things - a good match or not a good match, and your alterations move one way or the other. 1:1.23 as a reading means little - two decimal places is not required because we're dealing with ratios, so the amount of power being reflected is relatively small.

I think that I'd describe a VSWR meter as the only real tool you need. Once you know how it reacts and how it reads, you can predict performance when you swap antennas or cables.

Get a meter - even an inaccurate one gives a good clue. Many people chase the perfect 1:1 but I suspect that getting this reading on anything other than a Bird, that uses a rotating single diode, is probably the meter being optimistic. I'm happy enough with 2:1 or better in practice. My van has 3 roof top verticals. Cut for business band and marine. The marine antenna gives me 2:1 if I use it for amateur radio on 145, and 2.1:1 on VHF high band - and is resonant on 157MHz. That's fine for me.

2 to 1 has never caused me any grief with transmitters. A couple of the radios with meters show slightly less power output as a result of VSWR sensing. I'm happy with this.

Without the meter, you are just guessing - so spend a small amount and buy one.
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Old 06-17-2018, 1:52 PM
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Whatever you use for measurements should be trusted, be it homebrew or store bought. What you are trying to do is like maybe measuring a submarine you can't really see by driving along side it with your wheels spinning, then calculating your vehicle gear ratios vs tire circumference, etc to find the speed. But you really can't trust anything in that measurement.

If you could borrow a known accurate power meter I suppose you could run what you've been using through all its paces recording voltages with known loads and lengths of coax and maybe you would end up with something your could trust. But maybe not.

There are some really good RF sensors you can build, or just buy a really cheap but very accurate $30 Chinese power/frequency meter and buy or make some attenuators to extend its measurement range. Either way please verify your end result with a known and accurate meter so you can trust it from that point on. BTW, here is that super cheap and accurate power/frequency meter in the $30 range that could be the basis of a wide range wattmeter: https://www.ebay.com/itm/3GHz-1-3000...sAAOSw-kdXyVIG

I think there are some equal or cheaper FWD/REF power and SWR meters on Ebay that you can assemble and expand into other instruments.


Quote:
Originally Posted by needairtime View Post
Oh no... it's not that I don't want to use one, it's more like, I don't have one and finding out what I and anyone else can do without one. Though it initially seems the goal is to do "without" one, now I find out apparently the circuitry/instrumentation is actually built into some of these LMRS/Commercial radios, just need to get that information out from the radio!

Unfortunately it's the 70cm that I'm currently looking at as I do not have other ham radios that I can play with at the moment. My 2m radio is yet another project since I don't have crystals for it.

Thanks, yeah, perhaps I'm a bit unusual in that I'm building equipment. I've always wanted to figure out how to get information over the airwaves from components, buying something off the shelf just doesn't do it for me

Last edited by prcguy; 06-17-2018 at 1:58 PM..
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Old 06-17-2018, 2:01 PM
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Quote:
Originally Posted by needairtime View Post
Oh no... it's not that I don't want to use one, it's more like, I don't have one and finding out what I and anyone else can do without one.
Typically, you can't do much more than guess without one. Typically the solution to that problem is to get one.

There is certainly more than one right way to solve a problem, but in this particular case, doing anything other than getting a meter will result in more effort, less accurate data, and nothing of value learned. What's the point?
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Old 06-17-2018, 3:24 PM
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As a graduate student teaching assistant I once used to demonstrate frequency determination by the ancient and honourable technique of Lecher Wires. If I was really careful, at 60 or so MHz, I could get get the frequency to within a MHz or two either side. One of students could peg them to within 100Khz-- but I think she was cheating somehow...lol.. (like maybe a pocket'd portable frequency counter?)
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Antennas don't Absolutely *Need* SWR meters to be adjust'd... Field strengths, antenna current amp meters, drawing an arc off a conductor with a lead pencil, or my favorite (sarcasm) tuning for the dullest glow of a graphite plate element in a high power glass envelope transmitting tube (love the 3-500's for that )
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Great historical stuff, and it does make you appreciate where we have evolved from.... but why do it today? Rhetorical question- it does enliven discussion.
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Off topic: PRC, ZZ- some day if RR lets me out of Purgatory and I can Post on the Tavern section, I will respond to the 'mysteries'... but I don't have that privilege ..... smiles .............
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Old 06-17-2018, 4:18 PM
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It's always good to experiment with meters out of their guaranteed range, because the way they start to misread let's you see what is going wrong. It's also interesting to try those twin meter ones wired back to front. They rarely read the same! I agree with the premise that absolute accuracy is rarely required, just the trend - trimming an antenna and watching the meter go down, then pausing, then starting up again.
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Old 06-17-2018, 9:16 PM
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I was concerned that using a "CB" 11m SWR meter at UHF frequencies would result in the detector diode(s) end up behaving like a wire or open circuit and thus giving data that's completely unusable -- whereas the built in protection circuitry is indeed designed for UHF use!

But nevertheless I would eventually like to get an external SWR/Power meter/Analyzer so I can at least match up the two readings. Far, far future project for this TK810 is a "VFO" for it as well (so I no longer need to pull the EEPROM to program it), though I really have no real desire as it already has a PLL unlike my 2m radio which doesn't have a PLL at all.

However, having the microcontroller monitor the protection circuit as it steps through the frequency band, and suddenly I have an antenna analyzer for 70cm (granted the TK810 sucks so bad that I can't get over ~20MHz spread before the PLL conks out)?

Actually I should add this feature on my 2m radio if I can figure out its protection circuitry, as it needs the PLL/VFO anyway... ooh... more features to add and bloat the microcontroller...
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Old 06-18-2018, 2:54 AM
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Some CB meters are rated as far as 150MHz or so - not all by a long way, but I have one that is reliable up to marine band. Totally useless at UHF, and to be honest, the internal construction also means they don't even pass the signal that well - not remotely 50 Ohm impedance. The diodes start to behave in a very non-linear manner and the readings as you say, make no sense whatsoever!
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Old 06-18-2018, 6:52 AM
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Here is a link to a guy who used a HF (CB frequency part of the meter) swr meter for 2 meter amateur experiment. It worked; and I have tried it also. The CB meter compared with my VHF meter almost exactly.

https://www.kb6nu.com/no-vhf-swr-meter-no-problem/

The best thing to do is experiment.
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