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Author Topic: 2m antenna tuning  (Read 11336 times)

Geek

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2m antenna tuning
« on: July 01, 2013, 08:18:17 AM »
I've been able to communicate okay on my 2m radio, but in trying to get email to work over the local packet network we've traced the problem to the antenna.  I do not have an antenna tuner, which seems to be one alternative and have read recommendations to trim the antenna based on SWR measurements, which strikes me as a quick way to ruin a perfectly good antenna.  When looking for tuners I seem to find plenty of HF alternatives, but not much for 2m.

What do those of you who use 2m do?

KK0G

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Re: 2m antenna tuning
« Reply #1 on: July 01, 2013, 08:49:22 AM »
First of all I'd forget about a 2 meter tuner, they're not only completely unnecessary but also counter productive. At VHF frequencies the loss in the coax from an SWR high enough to require a tuner would be horrendous. I wouldn't worry about ruining your antenna by trimming it, that's how it's done. Just take your time, pay close attention to what you're doing and you'll be fine.

Are you sure the problem is in the antenna? It seems very odd that it works fine on phone but not packet. 2 meter antennas are usually broad banded enough to easily cover the entire band with no problems, hell many times I've used business band antennas for use around 154 MHz on 2 meters without adjusting them at all.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

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Re: 2m antenna tuning
« Reply #2 on: July 01, 2013, 09:12:18 AM »
First of all I'd forget about a 2 meter tuner, they're not only completely unnecessary but also counter productive. At VHF frequencies the loss in the coax from an SWR high enough to require a tuner would be horrendous. I wouldn't worry about ruining your antenna by trimming it, that's how it's done. Just take your time, pay close attention to what you're doing and you'll be fine.

Are you sure the problem is in the antenna? It seems very odd that it works fine on phone but not packet. 2 meter antennas are usually broad banded enough to easily cover the entire band with no problems, hell many times I've used business band antennas for use around 154 MHz on 2 meters without adjusting them at all.

At the moment the antenna is the only thing that we haven't gone over multiple times.  The indicators on the TNC and the radio indicate transmission is occurring, but receiving seems to be sporadic.

KK0G

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Re: 2m antenna tuning
« Reply #3 on: July 01, 2013, 10:37:51 AM »
I'm far from a packet expert but I did mess with it a little bit many years ago. I do remember that it requires a good strong signal in order to reliably decode, more so than phone. How strong of a signal are you receiving? What's the antenna? How high is the antenna? Is it fed with good low loss feedline?
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

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Re: 2m antenna tuning
« Reply #4 on: July 01, 2013, 12:32:35 PM »
I'm far from a packet expert but I did mess with it a little bit many years ago. I do remember that it requires a good strong signal in order to reliably decode, more so than phone. How strong of a signal are you receiving? What's the antenna? How high is the antenna? Is it fed with good low loss feedline?

I've used a couple different antennas.  All the equipment is on the second floor of the house in a room with very large windows facing in the right direction.  I am on the side of a hill and the ground drops away in that direction.  The only feed lines are the cables that came incorporated into the antennas.  Everything is right together.

KK0G

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Re: 2m antenna tuning
« Reply #5 on: July 01, 2013, 01:00:58 PM »
I'm far from a packet expert but I did mess with it a little bit many years ago. I do remember that it requires a good strong signal in order to reliably decode, more so than phone. How strong of a signal are you receiving? What's the antenna? How high is the antenna? Is it fed with good low loss feedline?

I've used a couple different antennas.  All the equipment is on the second floor of the house in a room with very large windows facing in the right direction.  I am on the side of a hill and the ground drops away in that direction.  The only feed lines are the cables that came incorporated into the antennas.  Everything is right together.

I'm assuming this is an indoor antenna, what kind is it? Can you try an outside antenna mounted high in the clear?
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

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Re: 2m antenna tuning
« Reply #6 on: July 01, 2013, 02:17:16 PM »
The antenna is a wire hanging in the window facing in the direction of where the packet site is.  Putting an outside antenna on the roof is an option, the attic would get me a few feet higher, and a better quality antenna could be purchased.  I would prefer to stay inside as anything outside would be vulnerable to the next Hurricane Sandy, which is exactly when I want to be able to use this capability.  In addition, I am not a young guy and I am disinclined toward going up on the rather high roof.  I would need to hire someone to get the thing up there.

The window the wire is in is at least 25' above ground level now and the ground drops away in the required direction.

I have also tried a Larsen mobile antenna, but that didn't work too well without a large metal ground plane.  I intend to try that again with a decent ground plane, but have no assurance of a better result until I try it.  I'm really looking for additional ideas of things I can do without the hassle of getting on the roof.

KK0G

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Re: 2m antenna tuning
« Reply #7 on: July 01, 2013, 03:16:05 PM »
Remember the old adage; "A crappy antenna up high beats a great antenna down low". Of course that's generalizing and ignoring lots of possible variables but it holds a lot of truth. Ditto outdoor vs. indoor antennas. This is one of those situations where it's very difficult to visualize and diagnose remotely, If we were neighbors I'd just come give you a hand but alas............ Is there a ham club, Elmer, or ham experienced with VHF and or packet experience near you that could take a look at your set up?
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

gil

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Re: 2m antenna tuning
« Reply #8 on: July 01, 2013, 04:02:06 PM »
It might be that the antenna is too close to, well, anything, especially any metal. For a directional transmision, given the small size of 2m antennas, I'd try a Yagi or a quad, even a Moxon, which would be the simplest...

Gil.

KK0G

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Re: 2m antenna tuning
« Reply #9 on: July 01, 2013, 04:15:41 PM »
It might be that the antenna is too close to, well, anything, especially any metal. For a directional transmision, given the small size of 2m antennas, I'd try a Yagi or a quad, even a Moxon, which would be the simplest...

Gil.

Good idea Gil. It wouldn't take much at all to slap together a 3 or 4 element Yagi, especially for 2 meters, they can literally be built from a broom handle and coat hangers.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

gil

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Re: 2m antenna tuning
« Reply #10 on: July 01, 2013, 04:18:04 PM »
Yep, the Moxon is even easier! Or, see the Slim Jim I built in the antennas board, though that isn't directional...

Gil.

WA4STO

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Re: 2m antenna tuning
« Reply #11 on: July 01, 2013, 04:47:10 PM »
  When looking for tuners I seem to find plenty of HF alternatives, but not much for 2m.

What do those of you who use 2m do?

I agree with those here who have said that an antenna tuner for VHF and/or UHF is a step in the wrong direction.    The reason they are so plentiful on HF is because on some of the designs of 40 meter antennas (as an example) are such that a tuner is required, due to the balun inside.  Plus, if you take my 270' long wire as an example, you have to 'tune' the system any time you change frequencies.

But on VHF, the first step should be to determine what the SWR is.  I think (but I can't be sure) that the radio that Dave let you borrow has an SWR meter incorporated into it.  By keying the mike and setting the meter appropriately, you should be able to determine if you're getting a huge amount of power blasted back at you -- a bad thing.  The SWR in that case would be very high and the meter would show that. 

That situation you can fix.  Tell Dave what your findings are (SWR-wise) and that will tell him that there's something very wrong with either the antenna connectors, the coax, or the antenna itself -- usually a simple thing to rectify.

Best 73

Luck, WA4STO


Quietguy

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Re: 2m antenna tuning
« Reply #12 on: July 01, 2013, 05:08:41 PM »
I do remember that it requires a good strong signal in order to reliably decode, more so than phone.

I ran into this not long ago trying to do 2 meter WL2K via a digipeater - some days it worked and some days it didn't.  I thought it was the WL2K station being up and down until I tried some peer to peer via the same digipeater with a friend who lives beyond simplex distance from me.  The bottom line is my connection to the digipeater station is marginal and sometimes it works - other times it works poorly or not at all.

One problem with packet is it is hard to judge the quality of the signal by listening to it because packet bursts inherently sound like they have static noise mixed in.  You really need a full quieting signal; any noise mixed in will degrade the connection.

What kind of wire antenna do you have - is it a commercial unit or did you build it yourself? 

Wally

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Re: 2m antenna tuning
« Reply #13 on: July 02, 2013, 01:05:08 AM »
  When looking for tuners I seem to find plenty of HF alternatives, but not much for 2m.

What do those of you who use 2m do?

I agree with those here who have said that an antenna tuner for VHF and/or UHF is a step in the wrong direction.    The reason they are so plentiful on HF is because on some of the designs of 40 meter antennas (as an example) are such that a tuner is required, due to the balun inside.  Plus, if you take my 270' long wire as an example, you have to 'tune' the system any time you change frequencies.

But on VHF, the first step should be to determine what the SWR is.  I think (but I can't be sure) that the radio that Dave let you borrow has an SWR meter incorporated into it.  By keying the mike and setting the meter appropriately, you should be able to determine if you're getting a huge amount of power blasted back at you -- a bad thing.  The SWR in that case would be very high and the meter would show that. 

That situation you can fix.  Tell Dave what your findings are (SWR-wise) and that will tell him that there's something very wrong with either the antenna connectors, the coax, or the antenna itself -- usually a simple thing to rectify.

Best 73

Luck, WA4STO


I believe the HF radio he has offered has that capability.  The 2m he provided for testing is more powerful than mine, but doesn't have an SWR.  I'm going to get a meter from either Dave or some other source to check the SWR on each of the antennas.  The plan has been to get this working before spending much time on HF issues.  He's given me several ideas but I figured I'd collect alternatives as well.

Among the ideas are testing from inside the attic and mounting a pretty serious roof mount antenna, though with the height of my roof, that is beyond anything I want to attempt myself.

From the way the conversation with Dave has gone and the suggestions here, it seems the first thing to do is get an SWR meter and see how much of an issue I have with the existing antennas.

WA4STO

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Re: 2m antenna tuning
« Reply #14 on: July 02, 2013, 07:58:06 AM »
what bugs me the most about your difficulty is something that likely bothers you a LOT more -  :(

And that's the idea that you can do ok on 2 meter voice, just not on packet. 

KK0G correctly points out that decoding  blurrrrrrrrrrrrrrps of packet is somewhat more iffy than voice.

My guess is you'll find that the SWR is within reason and that there's some tweaking of the antenna itself necessary to ensure the very best signal possible.

One day soon, you'll find that these difficulties are behind you and then you'll move on to WINMOR/WINLINK and NTS(D) PACTOR, each of which will give you national capability.  Still, I remember when I was first doing packet (on Cape Cod) -- what a marvelous thing to experience.  It really was slick.

73

LH

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Re: 2m antenna tuning
« Reply #14 on: July 02, 2013, 07:58:06 AM »