Author Topic: Starter antenna  (Read 8008 times)

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Tevin

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Re: Starter antenna
« Reply #15 on: July 17, 2013, 05:50:46 PM »
Experience teaches, so they say, but sometimes the lessons are obscured by irrelevancies.  Spent some time trying to figure this multi band stick full of whiskers out.  The instructions give typical SWR for the out of the box assembly.  Thing is, the frequencies they cite are not usable with my xceiver; they are locked out.  So I bumped the freqs into range (top and bottom of the ranges) and gave the auto tuner its head, then ran the manual tuner and compared the results. 



You're not real clear on how you did your testing: Are the SWR results you give straight off the radio, without a tuner? That is the only way to really know where the antenna works and where it doesn't.

Some hams, particularly newbs, somehow have it in their head that all they need is an antenna tuner and their matching issues will be magically solved. As I mentioned in a previous post, antenna tuners do not turn a crappy antenna into a good one. All the antenna's flaws and inefficiencies still exist; the tuner in essence "lies" to the radio by artificially creating a nice 50 ohm resonant load for the transmitter to dump into.

Properly designed antennas should have a respectable SWR on their own and at most need a tuner to smooth out rough band edges or to make corrections for less than ideal installations. I stand by my belief that "Swiss Army Knife" antennas that actually work well on all bands, all the time, simply do not exist.

As for the issue with the weather effecting SWR, it's possible but weather alone should not make an antenna go that far off the map. If the change is big enough to notice, then something is jacked up beyond mere weather.

The MFJ antenna should give a good SWR on at least one and probably two bands without the need for a tuner. You are headed in the right direction but I think you still have some tweaking to do.


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Re: Starter antenna
« Reply #16 on: July 18, 2013, 07:33:29 PM »
Please keep on keeping us updated about your progress with this antenna, ghrit.  I don't have anything to add, certainly no advice to give, but I'm learning a lot from your posts and the replies other give.

BTW, my MFJ-17758 goes up this weekend.  Will post about any progress or results.
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ghrit

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Re: Starter antenna
« Reply #17 on: July 19, 2013, 08:12:00 PM »
You're not real clear on how you did your testing: Are the SWR results you give straight off the radio, without a tuner? That is the only way to really know where the antenna works and where it doesn't.
Two different ways, once with the internal tuner, once with the manual tuner.  When using the at, the manual was in bypass.

Some hams, particularly newbs, somehow have it in their head that all they need is an antenna tuner and their matching issues will be magically solved.
I've managed to avoid that fondest of wishes.
As I mentioned in a previous post, antenna tuners do not turn a crappy antenna into a good one. All the antenna's flaws and inefficiencies still exist; the tuner in essence "lies" to the radio by artificially creating a nice 50 ohm resonant load for the transmitter to dump into.
Yep, had that figured out.  Tuners simply save the xmitter from itself.

Properly designed antennas should have a respectable SWR on their own and at most need a tuner to smooth out rough band edges or to make corrections for less than ideal installations. I stand by my belief that "Swiss Army Knife" antennas that actually work well on all bands, all the time, simply do not exist.
Acknowledged, again.  We will differ to an extent, the swiss army knife will tell you a lot about what type of fine cutlery you need.

As for the issue with the weather effecting SWR, it's possible but weather alone should not make an antenna go that far off the map. If the change is big enough to notice, then something is jacked up beyond mere weather.
Might have had a loop in the feed line, don't remember.  I've thrown loops before.  This temporary arrangement has me taking it down and putting it up as I learn things and make changes.  The big change has been from a one inch pipe to an inch and a quarter conduit and raise it another foot.  As expected, that had no effect on SWR.

The MFJ antenna should give a good SWR on at least one and probably two bands without the need for a tuner. You are headed in the right direction but I think you still have some tweaking to do.
There is zero question that more tweaking (if not major surgery) needs done.  20 and 15 M both show functional SWRs (under 1.5) after at gets a shot at it, and fiddling with the manual tuner gets similar results.  The others are as yet mysterious, but not what I would (in my ignorance) call usable, yet.  The xceiver tells me it cannot tune to a match less than 10, and shuts off outgoing rf on some bands, and cuts back power on others where the SWRs are between 3 and 10  (That tells me I ain't gonna be heard very far away.)

One of the difficulties is that the at does not hold a reading long enough to really see it.  Tuning starts, gets done as the meter drops (or not) and changes back from CW to whatever mode was previously selected.  The manual tuner will hold the reading as long as the "key" is down.  I am quickly coming to the conclusion that I want an antenna analyzer for the whisker trimming.  I've been told just once too often that I don't need one.  Sure do think it will save a lot of up and down, ether of me or the antenna.
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Tevin

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Re: Starter antenna
« Reply #18 on: July 19, 2013, 10:50:29 PM »
There is zero question that more tweaking (if not major surgery) needs done.  20 and 15 M both show functional SWRs (under 1.5) after at gets a shot at it, and fiddling with the manual tuner gets similar results.  The others are as yet mysterious, but not what I would (in my ignorance) call usable, yet.  The xceiver tells me it cannot tune to a match less than 10, and shuts off outgoing rf on some bands, and cuts back power on others where the SWRs are between 3 and 10  (That tells me I ain't gonna be heard very far away.)

I am quickly coming to the conclusion that I want an antenna analyzer for the whisker trimming.  I've been told just once too often that I don't need one.  Sure do think it will save a lot of up and down, ether of me or the antenna.

I admire your fortitude! I suggest you pick a favorite band and get the antenna below 1.5 without help from an antenna tuner. You can always readjust for a different band later. Don't beat yourself up trying to get it to work on all bands, all the time, because it's likely not possible.

It's important to point out that SWR reflection losses are not linear. Example: A 1.5 SWR equates to approximately a 4% effective power loss. That's not bad. However, a 3.0 SWR does not mean an 8% loss. It actually comes out to about 20%. These examples are a bit oversimplified because they do not take frequency and feedline into consideration, but you get the idea. Google is your friend: There are tons of on line SWR calculators you can use to play with the numbers.

An antenna analyzer is an amazing tool if you can get one. I have an MFJ 269 and it has saved me countless hours of trial-and-error and all the headaches that go with it. As you probably know they are quite expensive. For this reason they are not a good investment unless you plan on doing a lot of antenna work. You might ask around on the repeaters if anyone has an analyzer you can borrow.

« Last Edit: July 19, 2013, 10:54:20 PM by Tevin »

ghrit

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Re: Starter antenna
« Reply #19 on: July 26, 2013, 04:51:53 AM »
I admire your fortitude! I suggest you pick a favorite band and get the antenna below 1.5 without help from an antenna tuner. You can always readjust for a different band later. Don't beat yourself up trying to get it to work on all bands, all the time, because it's likely not possible.

It's important to point out that SWR reflection losses are not linear. Example: A 1.5 SWR equates to approximately a 4% effective power loss. That's not bad. However, a 3.0 SWR does not mean an 8% loss. It actually comes out to about 20%. These examples are a bit oversimplified because they do not take frequency and feedline into consideration, but you get the idea. Google is your friend: There are tons of on line SWR calculators you can use to play with the numbers.

An antenna analyzer is an amazing tool if you can get one. I have an MFJ 269 and it has saved me countless hours of trial-and-error and all the headaches that go with it. As you probably know they are quite expensive. For this reason they are not a good investment unless you plan on doing a lot of antenna work. You might ask around on the repeaters if anyone has an analyzer you can borrow.
So far, I've been playing on 15 and 20 meters with the occasional foray into 6.  No significant contacts, but have added to a couple contest counts.

I'm well aware that SWRs do not give linear losses as the numbers go up.  Logarithmic, no?  In any case, driving SWR down is always a good thing unless you have power to burn.  They say if you can't hear them, you can't work them.  Well, I can hear a lot more than I can talk to at this point.  Seems to me that high SWR has no effect on reception, but I won't swear to it.  I'm also aware that this antenna is a compromise, it can't do it all well, but can do some within reason.  (I like the Swiss Army knife analogy.)

Anyway, one of the club Elmers came thru last night and lent me an analyzer.  I downloaded the manual; will be hacking around with that this coming week.
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ghrit

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Re: Starter antenna
« Reply #20 on: August 06, 2013, 08:25:13 AM »
Update on the learning curve.  Sheesh, not so sure how much of the lessons will be retained long enough to use.
(The borrowed analyzer is the same MFJ 269 you have, Tev.)  It is a 259B, not 269.  Ooops.

Took a while, full of fiddling and meddling, but got the analyzer to start giving me consistent readings with the twitchiness of the freq setting knob.  But they are consistent, a good place to start.  Side note, the antenna O&M gives some "typical" SWRs for an untuned, out of the box antenna at one freq in or near a ham band.  WILDLY  optimistic, gotta say, but not particularly surprising.

So, went into data collection mode, picked a spread of freqs in each band and collected points to plot.  (Didn't plot them, the trends are as obvious as my nose.)
40M - Resonance is below the band, should be easy to raise it with trimming.
20M - I don't think I can improve this at all, it shows close to resonance (2:1 or better) between 14.225 and 14.300.  Outside of those freqs, the curve goes steeply up.
15M - Probably not much to be gained here, either.  Using the arbitrary 2:1, the range is from 21.000 thru 21.375.  Going away higher from that range, the curve rises slowly to 2.4:1 at the general ticket upper limit.
10M - Out of the box resonance is WAY below the ham range.  This will take some hacking, maybe serious hacking.
6M - Also way below.  Also to be trimmed.  Worth noting, maybe, the 6M portion of the antenna is a separate pair of whiskers that act as a dipole, they are not on the trapped portion of the beam.
2M - Also a separate pair of whiskers.  Since the xceiver won't do 2M and the 269 won't either, I'll probably never use it for that band.  (Have an ht that does, so no need for the base antenna to function there.)

Next step is finish the mast, put the antenna up a bit higher than it is now, then run another set of readings to confirm no significant change, then get out the nippers and make small bits of the big ones. 
« Last Edit: August 07, 2013, 09:20:33 AM by ghrit »
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Re: Starter antenna
« Reply #21 on: August 06, 2013, 08:32:45 PM »
Thanks for the update sir.
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Tevin

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Re: Starter antenna
« Reply #22 on: August 07, 2013, 05:49:24 PM »

The easiest way to use the 259B is plug it into an antenna and adjust for lowest SWR. For better or for worse, wherever it lands is your resonant frequency. You can tweak it from there. Using my 269, I set up all four of my dipoles using this technique and got them all below 1.3. It sounds like this is pretty much what you are already doing.

As you are finding out, adjusting on one band will often mess up another. There's no real way around this. I still think your best option is to pick one or two bands, get it working there, and don't worry too much about the rest. An SWR of  2:1 is totally unacceptable. I'm sorry, but if that's the best you can squeeze out of this thing on any band, then something is jacked up and/or the antenna design is inherently flawed. I would need to see 1.5 or less over most of at least one band to consider it an effective antenna.

ghrit

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Re: Starter antenna
« Reply #23 on: August 07, 2013, 06:29:22 PM »
An SWR of  2:1 is totally unacceptable. I'm sorry, but if that's the best you can squeeze out of this thing on any band, then something is jacked up and/or the antenna design is inherently flawed. I would need to see 1.5 or less over most of at least one band to consider it an effective antenna.
I think I didn't get my point across very well.  Arbitrarily, I assigned a max of 2:1 to define the useable range within any given band, not as a minimum within the range.  As it happens, 20 and 15 already have a range that meets that criteria.  The rest need adjustment, and yes, those will be tested along the way to be sure any effects are found before troubles arise.  As you have said and I know well enough, multi band antennas have limitations.  Case extant, I'm not at all surprised with the lousy performance so far, and expect it will be improved quite a bit when I finally get around to trimming.  (Damn rain will NOT quit.)
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ghrit

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Re: Starter antenna
« Reply #24 on: August 20, 2013, 11:15:15 AM »
Life got in the way of progress since the last post.  But now, it's time to ask questions before actually taking a nipper to the whiskers to avoid blundering along blindly.
So, Elmers, lead me down the path of knowing more than I do now -
-I've been operating on the assumption that "resonance" defined means achieving a 1:1 SWR at a unique and specific frequency.  I no longer believe that is a good working assumption.  (1:1 means that the antenna radiates 100% of input power with no losses, that is, 100% efficient conversion from wire to radiated rf.  True?)
-Since 100% conversion seems impossible (with this antenna in particular) I've revised my working definition to take "resonant frequency" as the frequency that has the lowest SWR within a band.  (Does that sound reasonable?)

With that working definition in mind, I took a set of SWR readings on the 40M portion of the antenna, they sucked with a half life, about as close as I found was about 2:1 at 7.125MHz.  Something clicked in the void between my ears, so I rotated the antenna about 45deg, and LO! what to my wondering mind appeared, but differences!  Being all of a sudden curious, I spun it another 45deg, then another, and found that it likes a particular orientation; got the SWR down to 1.3 at 7.125, the best on that band.  (Still not trimmed.)  As expected, the resonant range (2:1 or better) will be very narrow if the data taken is indicative.  In any case, I'm blaming the house for the effect, it's within 12 feet or so of the antenna, and no ready way to move it further away.  (The in service location will be higher, but not further.)

I have no current intention to use code or data modes, so it seems best to trim things to resonate somewhere around 7.2MHz, roughly the middle of the phone zone.  In effect, trimming the whiskers will not alter the SWR, but will move the resonant frequency up.  Or maybe both, dunno yet, but will be surprised if the SWR is more than a tenth different after trimming the 40M whiskers.  After some of you chime in on the questions above, I think I'll align the boom to the most favorable orientation and trim accordingly over the following day or so and report.
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ghrit

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Re: Starter antenna
« Reply #25 on: September 01, 2013, 11:11:48 AM »
As often happens around this domicile, some projects go on the back burner for a while.  Back on the front burner, so here's a rundown on the current status of this antenna exercise.  With possible future minor tweaks, I think I've got things as good as they are going to get, there should not be any more updates barring something of a revolutionary nature popping up.  Please excuse the repeating of some of the info, this is a summary type post.  Note also that this is a unique installation, as are ALL antennae.

Being a complete ignoramus at the git go, I thought I needed to have a better feel of what was going on with this fuzzy stick.  Using the borrowed 259B, I took a set of readings, all at an elevation I could reach readily.  As noted, a brain fart told me to swing the antenna and see if horizontal orientation made a difference in SWR readings.  It did, which, in retrospect, I should have expected at the gitgo since the setup for testing is pretty close to the house.  Seriously glad I farted, or I could have blundered into some sort of problem.  In all cases, orientation affected the range of frequencies when my arbitrary "useable range" is defined by a SWR of 2:1 or better.  Anyway, at 40M, the initial readings showed an extremely narrow range of useable frequencies centered about 7.125MHz.  At 20M, the useable range was in the middle of the band, again orientation dependent.  At 15M, the entire range of frequencies met the criteria except on the high end at two orientations.  At 10M, it showed a trend toward resonance at way lower frequencies, but nothing useful at all within the band.  At 6M, the same tendency as at 10M showed itself.  I did not bother with 2M because my xceiver won't do that, and I have 2M (and 70cm) covered with an h/t.

I had to make some sort of decision, to trim and check at all orientations, or pick one direction and trim to suit.  Picked one, got out the nippers and had at it.   Also, since I have no current interest in CW, I chose to bring resonance up from the lower ends of the bands to where phone is more prevalent.  As directed, started with 40M and worked up in frequency, got to as good as I could, and raised the antenna to operating height, and ran another full set of readings at the same orientations as the pretrim readings.  I'll tell you what, a quarter of an inch at a time is tedious, even with the meter to work with, there is NO substitute for the time spent. 

Results, then. 
At 40M, I have moved the defined range to a higher frequency, centered more or less at 7.175MHz, and the auto tuner will likely make the rest of the band (except the extreme high and low ends) useable, but power restricted (as expected.)  This is not a condemnation of the Swiss Army knife scheme, it's a reflection of the reality with multi-band antennas, and this one in particular.  Here, it's worth the note that the auto tuner in the xceiver will deal with some of that mismatching and broaden the useful range a bit, but NOT to the whole range of freqs in all orientations; at the extreme high and low ends, the SWRs are off scale of the meter and range of the auto tuner.  (The xceiver will protect itself by refusing to operate.)
At 20M, the useable range is top to bottom, all except the extreme top and bottom is well within auto tuner range, and the majority will have no power cutbacks.  The trimming of the 40M spokes very definitely did affect this, no trimming was done to the 20M spokes.
At 15M, there was no detectable change, good from top to bottom regardless of orientation.
At 10M, major trimming took place, the entire band is now useable without needing to call on the auto tuner, again without considering orientation.
At 6M, I screwed the legs to the minumum length, and for now will accept that for use.  It is resonant to less than 2:1 at 50.500, and the rest of the band is within the range of the auto tuner.  At some point, I'll hack off a couple inches and see some improvements, but that's back burner, I have a couple more things to do to set up the shack properly (like lightning protection, rf gounding, and a way to get the coax outside without passing it thru an open door or window.)
« Last Edit: September 01, 2013, 11:17:33 AM by ghrit »
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spacecase0

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Re: Starter antenna
« Reply #26 on: September 01, 2013, 11:13:06 PM »
So, Elmers, lead me down the path of knowing more than I do now -
-I've been operating on the assumption that "resonance" defined means achieving a 1:1 SWR at a unique and specific frequency.  I no longer believe that is a good working assumption.  (1:1 means that the antenna radiates 100% of input power with no losses, that is, 100% efficient conversion from wire to radiated rf.  True?)
I think you are correct,
if you take an antenna that has an inherent impedance of 100 ohms and tune it to 1:1 at 50ohms, it will not be resonant.
you always have losses so your antenna will never be 100% efficient,
I have seen antenna systems where most of the power is lost in the feed line, even thought I still have 1:1
and I have seen everything in between,
1:1 just means that nothing is reflected back into the transmitter,
it does not tell you where the power is going.
remember a dummy load (50 ohm resistor) has 1:1 SWR and no signal out.

as to the rest of the questions, my mind is not working well enough to get them (at least at the moment)

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Re: Starter antenna
« Reply #27 on: September 02, 2013, 03:01:12 PM »
remember a dummy load (50 ohm resistor) has 1:1 SWR and no signal out.

Think of  RF Power as Heat.... Your Transmitter produces RF Heat, and you pipe that heat to the Antenna, where it gets Radiated to the outside world....
Now some of that RF Heat is lost in the FeedLine, on the way out to the antenna, and if the Antenna is NOT Resonant, some gets Reflected back down the FeedLine to the Transmitter. while doing so some of that Reflected RF Heat is again lost in the FeedLine, and what makes it back to the Transmitter is lost in Heating up the Final Amplifier elements. Ok, then what RF Heat is NOT reflected back, is pumped into the Antenna, where some is again lost in Heating the Antenna, and what is left is then actually Radiated into the Ether.... Add up all the Losses, and subtract that from the Output, and you will know just how much RF is actually doing something.... Just say'en.....
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