QRZ Logbook

Friday, January 30, 2015

The last month

Starting Off The Year


In the first ten days or so since my last post, I spent some time hanging out on 15 meters and 40 meters.  I made a few American contacts, along with contacts in nine other countries.  These included my first two contacts with Japan - at almost 6,500 miles away, they were my furthest contacts to that point.  I even made a contact with Belgium on 10 meters - that was a pleasant surprise!

40 Meters and RF


I tried making a contact on 40 meters on the 9th.  I answered a call from K0DK - his reply was (and I quote) "You've got a hell of a lot of RF on your audio!"  I thanked him and told him I'd look into it.  I know that I still need grounding - for electrical safety, for lightning projection, and for keeping RF from coming back into the radio.

Ten Days Away


Between the 10th and the 16th, I was away for work, so the radio sat idle just waiting for me.  Once I got back, I was pretty worn out and had plenty to do around the house.  I didn't make any contacts until the 19th, when I had a short conversation with Ted, XE1YQQ, in Mexico.  This was on 15 meters, and he remarked that my audio was very clean and strong.

Knowing that I seem to have RF issues on 40 meters, I decided to stay away from it until I get some grounding.  I had already laid out some plans for a couple ground rods and some #6 wire to bond them to the service entrance ground.  I told my wife about these plans, as well as their cost (about $100).  She said, "Of course, it sounds like you need that.  But on one condition - you finish the paint and the tile in the bathroom first."

You see, the bathroom improvements got put on hold back in November when I got this big heavy box in the mail with a Kenwood radio in it... :)  So, I did do a little more work in the bathroom, stripping off the last remnants of wallpaper border above the light bar over the vanity.  Now the rest of the room can be painted, and the rest of the self-adhesive vinyl tile can be laid (after I strip the old adhesive off of the floor).  And then - grounding!

Winter Field Day


I mainly continued to stay away from the radio for a few more days.  But then, on the 24th, I made a couple contacts with people working Winter Field Day - N5HR on 15 meters and N3FJP on 20 meters.  I heard a number of people who reported that they were working outdoors - which makes sense, considering that it was Field Day and all.

The Past Week on 15 Meters


You know what?  It's a shame that I have to work a full-time job - because 15 meters is a lot of fun between about 10:30 AM and 3:00 PM.  Starting on Monday, I started taking a couple breaks here and there to listen to what was happening on 15 meters.  I was just spinning the knob, not even looking at the DX cluster on dxsummit.fi for the first couple days.  I kept running across country after country and answering CQ calls, often on the first attempt.

During the past 5 days near lunchtime, I made 26 contacts in 23 countries.  (One contact was with KL7YK in Alaska, who reported his country as "Alaska".)  I do admit that I started looking at the DX cluster a couple days ago in an attempt to catch as many countries as possible.

I tried working South Africa and Namibia, but I couldn't punch through the pileups.  I heard Japan starting in the late afternoon when I was too busy to try to work them.  But that's OK.  I've now talked to the USA and 34 other countries, and I've talked as far away as 6,500 miles - all with a 40-year old radio (with possibly all original parts except for one transistor replaced in the late '70s) and 65 feet of wire strung between a couple trees.

Looking Ahead


I've been thinking about antennas; here are a couple things I'm considering in addition to the HexBeam that I mentioned last time:

  • Replacing the coax feedline on my dipole with either ladder line or 300 ohm twinlead.  My tuner has balanced inputs and an internal 300 watt balun, so this would be fairly easy to do.
  • Stringing a random wire through the trees around the yard so that I can start working 80 meters.
  • Building a Swiss Quad for 10 meters.  VK4JU has six homemade Swiss Quads on two masts.  I figure I could start with the smallest one fairly cheaply, and at least give it a shot.  They are reported to have excellent gain and an excellent front-to-back ratio.

 I've been thinking about other things as well... 

  • Use of Arduino and/or Raspberry Pi hardware for things like Morse Code keying, frequency display, DDS VFO replacement, etc.  Although since my radio doesn't have the DK-520 adapter for the external DG-5 display, it might be rather difficult to get an external frequency display together.
  • Continuing learning Morse Code - although I think I want to start with a straight key rather than a paddle keyer.
  • Different logging software.  Currently, I am using QRZ.com for my master logbook and occasionally transferring updates to LoTW and eQSL.  I also just started uploading logs to ClubLog as well.  I trled LOG4OM on my old Acer laptop - it seemed to work OK, although it took a little setup and a little getting used to.  And then the wires to the charging jack broke yet again.  It's time for a more permanent fix there.
  • Grounding, shack arrangement, power distribution, mast/tower, test equipment... :)

I'll try not to be such a stranger.  But I think I'm settling in to a more appropriate focus (as in "less obsession") regarding the radio.  A little less time working it than back in December, and probably a little less frequent updates here on the blog.  But hopefully not a month apart!

73 - Mike

Monday, December 29, 2014

Thinking Ahead to Next Year

The Past Ten Days


I haven't made many contacts over the past ten days.  For most of that time, we've spent the Christmas Season at my mother-in-law's house.  It might go without saying, but I only have one radio and one antenna - and they're not at her house.

I have had to go by the house just about every day, though:  to water the cats, to feed the fish, to scoop the litter pans, to get the mail - and to try to track down a bad odor that's either "something moldy" or "something dead".  I still haven't found it, but it does seem to be abating.

However:  when I got the chance, I would sit down at the radio for 10 minutes or so, fire up the DXSummit website, and see who was calling on which bands.  I'd tune nearby, tune up the transmitter and the antenna, and then try to answer the calls.

And, on each of my last 8 or so attempts, my callsign transmission was answered on the first call.  I've rarely had this happen, but especially not 8 times in a row.  Even with the pileups with people trying to reach the roaming W1AW stations, I got through on my first attempt - even the Hawaii station on 15 meters, and even the busy Iowa stations on both 40 meters and 20 meters.  Even the busy "ARRL Centennial QSO Party" stations.  I just couldn't believe it!

This just goes to show me that there is plenty that I accomplish with my modest station consisting of a 40-year old transceiver and a couple pieces of wire strung between a couple trees.

What About Antennas?


I still need to post pictures of my "shack" and my antenna.  But I'm thinking ahead to two antennas that I would like to put up.

I'd like to put up a longwire antenna and use it to get on 80 meters.  I know very little about this, but it definitely sounds like something I want to do.

I'd also like to put up a Hexbeam antenna.  They look like this:


(This picture is linked directly from the website of KJ6YVT; I hope that's OK.)

Hexbeams are lightweight multi-band beams, with each band's pair of elements bent into a hexagonal shape.  They have good gain, a great front-to-back ratio, and they are very lightweight.  They work on 6, 10, 12, 15, 17, and 20 meters with a single feedline, and they can be put on a lightweight mast and turned with a TV antenna rotor.

These antennas are available from a few different sites; perhaps the two most well-regarded suppliers are K4KIO and NA4RR.  They're still considerably more pricey than a wire dipole; we'll get new furniture for the living room before I get a several-hundred-dollar antenna.

I need a better way to get my antenna cables into the house; I'm interested in something like the MFJ-4602 feedthrough panel if I can get a good deal on one:


What about other modes?


I've been working on my Morse Code skills by hanging out at the Learn CW Online website.  I need a lot more practice at hearing and copying Morse Code; I've still got a few letters confused (F versus L, B versus V) and it takes me too long to figure out each letter.

For real-world practice at listening, I fired up the Michigan WebSDR website (because, after all, my radio is at home and I'm not) and listened to CW conversations on 40 meters.  I found a couple people working under 10 words per minute; I found it quite hard to copy.  I'll keep at it.  I'll make sure I can copy Morse Code before I pick up a keyer.  I've seen some good deals on Heathkit HD-1410 keyers and MFJ-422 keyer/paddle combos; when the time comes, I should be able to find something reasonably cheaply.

I'm also interested in some of the digital modes, particularly JT65 and JT9-HF.  Their ability to pick signals out of the noise is really interesting.

So - maybe next year I will put up a longwire and get into CW and maybe one of the digital modes.  And then, sometime after that, maybe it will be time for the next step - the Hexbeam.

73 de Mike, K9MJA



Wednesday, December 17, 2014

The Past Ten Days

Lemme 'splain.  No, is too much.  Lemme sum up.


I've been quiet on the blog, but I've been having fun with the radio.  Since my last blog post I've made 38 more contacts:

  • By band:
    • 40 meters: 18 contacts
    • 20 meters: 8 contacts
    • 15 meters: 10 contacts
    • 10 meters: 2 contacts
  • Foreign countries by band:
    • 40 meters: Costa Rica and the Czech Republic
    • 20 meters: Canada and Aruba
    • 15 meters: Spain (2 contacts), Cuba, Nicaragua, and Guadalupe

I mostly stayed away from 10 meters, as there was a contest going on last weekend and it was just a series of pileups that I didn't feel like trying to punch through.

Over the last week or so, I haven't been spending as much time as I was previously.  I was frankly getting a little obsessed with it, I guess.  But it's just so interesting!

WebSDR


On the Yahoo Kenwood group, some people were talking about "web SDR" sites where people had put up web-based Software Defined Radio stations for listening.  This seemed like a good way to hear what you actually sound like on the air.

So, I checked out websdr.org where the list of these sites is kept.  I found a nearby site in Michigan that listens on 40 meters, and I used it tonight to listen to myself on 7.139 MHz.  It has a handy recording feature so that I could record what I actually sound like on the air.  I transmitted a short sequence saying that I was testing, but I couldn't really hear if my audio was clear or not.

Shortly after that, I heard W3AMF calling on that frequency.  I was still watching the WebSDR site, but it was delayed from the radio by about a second.  So, I hit the "Mute" button on the website, turned up the volume on the radio so I could listen in real time, and hit Record on the website.  I had a nice 30-minute conversation with W3AMF (Duff), during which he said that he could not discern any RF on my audio signal.  After he faded into the noise, I signed off with him and then downloaded the recording.

And guess what?  I should have muted the computer instead of muting the audio on the WebSDR site.  I downloaded 30 minutes of silence. :)

I won't make that mistake again.  I'll try recording myself next time I check in to the MIDCARS Net on 7258.

I turned off the radio and got back to work; making up some time in the evening due to being a zombie in the afternoon.

Until next time -

73, Mike

Saturday, December 6, 2014

It Was The Microphone!

So I Rewired It!


I had some lingering suspicions regarding the microphone.  After all, when I bypassed the amplifier in it, I just left all the other wiring intact.  I started to wonder if perhaps the amplifier in the mic was acting strange since I unhooked it.  So, I decided to do an experiment:  I moved the mic audio wire back from contact #2 to contact #8 (the amplifier output), put in a 9V battery for the amplifier, and turned the volume down most of the way.

Also, I had taken the stalk apart to take a look at the relay contacts.  Tonight, I cleaned them by pulling an eyeglass cleaner patch through the contacts - a little alcohol and the slightest bit of abrasion.  I also straightened a couple of the springs, and I bent the spring at the bottom of the stalk so that the PTT grip bar would spring back out like it should.  I've read horror stories about trying to get these things back togethers; I expected it to take about 20 minutes, a microscope, four hands, tweezers, and a dose of Divine Intervention.  But somehow I managed to get the relay and the mic plug back into the stalk in only a couple minutes.

I hooked it back up to the radio and talked into the dummy load.  I had good ALC and power meter movement.  I ended up with the Mic control on the radio set to about 4 and the (unlabeled) volume control on the mic base to about 1 or 2.

So, it was time for a test.  40 meters was wide open tonight, so I hunted around for an empty portion of the band.  I tuned up the transmitter into the dummy load, and then tuned the antenna.  And then I started talking to myself (saying "K9MJA testing").  And the radio was behaving like it should!  At least as far as I could tell by looking at the meters.

So, it was time for the real deal.  I heard VE4VT in Winnipeg calling CQ on 7.264, so I answered him.

And he repeated my call back to me, without confusing "Juliet" and "India"!  He gave me a solid 59 report.

Time for another test.  I saw on DX Summit that WK9U from Wisconsin was calling on 7.272, so I went there and answered his CQ.  He was coming in at well over S9.  I asked him for a quick audio check due to my lingering audio issues.  He said that my audio was strong and clear, and that I didn't have any sign of RF on my signal at all.  I thanked him and looked around for another contact.

I found Milt, AD5XD near Dallas, on 7.239 and had a quick little conversation with him.  He was being very polite and patient, as the conversation was interrupted by my phone ringing in the background.  I could heard Barb asking me to answer it, while I was also trying to listen to Milt.  I jumped up to find a phone, and I finally found one - just after it quit ringing.  I jumped back to Milt and apologized for missing his last comments because I was running around trying to find a ringing phone.  He just laughed and repeated his comments, which were well wishes for my family for the Christmas season.  I returned the well wishes, thanked him for his patience, and bid him 73 so that he could make more contacts (hopefully more coherent than me).

Epilogue


I wrote this and left it sitting for a half hour while I spun around the 40 meter band.  It was just so noisy.  I heard a few guys speaking Italian and lots of pileups.  I decided to try somewhere else.

I peeked at DX Summit again, and saw that RX0AK was calling CQ on 20 meters.  So I found him.  He was just above the noise level; I had to strain to hear him.  But I could hear him.  I tuned a little off-frequency and tuned up the radio and the antenna tuner.  I tuned back to him and heard him answer a couple calls.  So, after a couple minutes, I answered his CQ.

And he heard me, with the correct call on the first try.  I gave him a 57 report; he gave me a 55 in return.

From Asiatic Russia, over 5,800 miles away.

Two minutes later, the path to him had faded and he was gone.

This is one happy guy sitting right here!

Tonight's contacts from my QRZ logbook:


73 - and look for me on the air, where I will now be able to be understood!

- Mike, K9MJA

Friday, December 5, 2014

Audio Issues - The Saga Continues

RF and Grounding?


I'm really trying to track this down, because it's getting a little frustrating.  Here are the things that I'm seeing and hearing.

  • When I tune the radio up in CW mode and flip the Transmit switch (either while connected to the tuner's dummy load or while connected to the antenna thru the tuner), I get a steady 100+ watts out.
  • When I talk into the dummy load in the tuner, the ALC meter and the watt meter behave as expected.
  • When I talk into the antenna, the ALC meter doesn't move as far and I don't send out as much power.  The ALC goes up to about 1/3 of the way and then immediately drops back down; the wattmeter maybe hits 30 watts and then drops back down as well.
  • What few contacts I have made lately have a hard time hearing me; they report some sort of clipping and/or distortion.
  • I do know that I seem to have a problem with the relay contacts in my microphone.  If I squeeze the grip bar too tightly, it causes the ALC meter to jump really high.

Given the first four points above, it sounds like RF getting back into the radio.  My first thought is that I do need to ground my station.  This is one thing that I know I need to do.  I need to run a ground wire around from my service entrance to outside the shack (about 75 feet around the perimeter of the house) and bond it to another ground rod; then I need to run a wire to the back of the radio and to the tuner.

It has rained for 2 days straight here; I don't think I have any water in my coax on my homemade dipole, because I can tune to 1:1 SWR on various parts of the 40, 20, 15, and 10 meter bands.

Another thing I need to do is shorten my feed coax.  I have a hundred feet of it, most of which is just lying outside the house on the ground (the center of my dipole is about 15 feet off the ground, and only about 10 feet from the shack - but I have 100 feet of coax).  I've felt bad about cutting this coax, as my friend Keith WA9DRO passed it along to me.  But, I think it's time to cut this one to length; I'll just buy him a replacement.

My friend Fred, W0FMS, suggested using ladder line to feed the dipole - less loss, but takes a little more care getting it into the house.

Microphone Issues


I also need to clean the relay contacts in the microphone and adjust the spring for the grip bar, so that I don't have to constantly worry about squeezing it too hard.  So, I took it apart:


I can already see that I need to straighten a couple of the relay springs in addition to cleaning the contacts.  Here is a closer view:


I'm chalking all of this up to being part of the learning experience.  But it's a little aggravating to not even be able to reliably check in to a semi-local net (MidCARS on 7.258) - and it's even more aggravating to hear Japan and Venezuela and Brasil and Spain coming in at S9 and not be able to contact them.

I'll get it figured out.  In the meantime, I keep practicing tuning the radio.  I also need to adjust my S-meter; it doesn't go all the way down to zero.  I can't really adjust it for S9 without a signal generator (which I don't have).

I hope to be back on the air soon!

73 de Mike K9MJA

Monday, December 1, 2014

Audio issues tracked down?

More audio issues?


This morning, I checked into the MIDCARS net on 7.258, at around 9:30 AM.  The reply I got from the service control operator at the time, George K8HLJ, was that my audio was quite distorted.

What?  On 40 meters?  I had not had such a report on that band before.

Another guy chimed it to say it sounded like I perhaps had RF coming back into the radio; maybe I didn't have a good enough match to the antenna, or maybe I had a ground problem.

I turned down the Mic gain a bit (I noticed while talking that I was overdriving it a bit).  George said that it did get better, but was still somewhat distorted.

I was a little disheartened.  But, I tuned aside a bit, tossed out a carrier, and checked my meter.  Sure enough, although I had tuned 7.258 to a 1:1 match the night before, I was now at about a 1.2:1 SWR.  It seemed that I couldn't get quite a good match at my "normal" antenna tuner setting (capacitors at about 2, and the inductor on H).

So, I consulted the manual for the tuner.  It said that if you're unable to get a match, turn the inductor down one letter and try again.  So, I turned the inductor down to G and the capacitors up to about 6.  I was still not able to get a really good match.

So, down once more for the inductor, down to F.  I had never tried tuning 40 meters with the inductor on F.  But, with the capacitors at about 8, I got a perfect match.  Hey!  Here is picture of the rig putting out over 130 watts on 7.258 with practically zero reflected power:


(Note: the tuner manual does say "Always use the lowest alphabetical setting possible."   Also, the manual suggests both 6/G/6 and 8/F/8 as potential matches for 40 meters.)

In the early afternoon, I checked in to the net again and asked for an audio report.  This time, I was met with glowing audio reports from Jim KT4FQ as well as one other perosn.

I said "Well, the main thing seems that I need to get better grounding in my shack.  Which means that I need to get some grounding in my shack."  I was met with laughs of agreement.

When I posted a similar story on Facebook, my friend Jeff (W0ODS) also agreed that the grounding is important.  Currently, I have none at all.  I need to route my cables into my house a little better (currently they come in through open windows) and I need to ground the shields at that point and run a ground wire around to the service entrance ground.  Then I need a ground bus on the desk so that I can also ground all my equipment.  Also, I'm planning on wiring up the TS-520 for DC power (I have an Astron 35amp power supply sitting mostly unused) and also providing DC to light up the lamp on the tuner meter.

Working QSX


In my previous blog post, I mentioned trying to "work split" with a station that was "listening up 5".  Well, today, I found one again.  This time, it was C6ADX calling CQ from the Bahamas.  He was transmitting on 21.285 and listening on 21.290.  I tuned about 10KHz away from him to tune up my transmitter and antenna, ensuring I had as good of a match as I could get.  I tuned back to him and practiced my "RIT button / knob twist" technique for a couple minutes to hear the calls and replies, and then tried calling him.  This meant that I needed to turn the knob 2KHz to the right, key up the mic and talk, and then turn the knob 2KHz to the left so I could hear his reply.

On the fourth try, he heard me.  He gave me a 59 signal report; I gave him a 58 in return.

I tried making a few other contacts on 10 and 15 and 20 meters, but to no avail.  That's OK - I think I ran my audio problem to ground, so to speak.

Hopefully soon I'll actually get a "shack" picture and an "antenna farm" picture posted.

73 de Mike K9MJA

Sunday, November 30, 2014

A quiet week - and then...

Busy Busy Busy


I had a busy week since last weekend.  With a shortened work week, I had lots to do in three days - so I worked a little extra and left the radio off during the mornings (instead of listening to the MidCARS net on 7.258).  Then, the family spent most of the Thanksgiving holiday at my mother-in-law's house.

I did have to do some water heater maintenance at home on Friday and Saturday, so I did have a little excuse to play with the radio (since I was home and all).

Friday


While waiting for the water heater to drain (which took hours, thanks to the 2 inches of lime sludge in the bottom of the tank), I tuned up on 15 and listened around the band.  I heard EA8YB call CQ from the Canary Islands, so I answered his call.  He said that I was very faint, and he kept asking me to repeat my call suffix.  He never could pick out the "Alpha" at the end.  On one hand, it was a little disheartening because I never completed the contact.  On the other hand, though, I was being heard at 4,000 miles away.

I spent a little time on 10 meters as well; I heard PU2YZP and PP5EI from Brasil, but neither of them could hear me as I replied to their CQ calls.

Listening Up 5


I heard several people Friday on 10 meters calling CQ and "Listening Up 5".  This was my first experience with this type of operation; I had read about it, but had not experienced it yet.  So, when I heard one particular station coming in at S9 and Listening Up 5, I decided to give it a shot.

First, I spun my VFO knob up 5 KHz and listened to all the people trying to answer his CQ calls.  Then I spun back and forth between his transmitting and listening frequencies.  This proved to be error prone and somewhat annoying.

Then I remembered the RIT (Receiver Incremental Tuning) feature.  So, I played with the RIT control until I figured out how it works.


By turning it all the way to the left, the receiver was listening 3KHz below the VFO frequency.  This wasn't quite enough to cover the 5KHz split between his transmit and receive.  However, it was much easier to move the VFO only 2KHz than to try to manage a 5KHz swing without using RIT.

So, I practiced a few times:  Spin the VFO knob about an inch to the left (2KHz down) while pressing the RIT button to turn it on, and I could hear him calling CQ; spin the knob back about an inch to the right and turn off RIT, and I could hear all the responses.

I tried answering a couple times, but to no avail.  But at least I was transmitting on the right frequency.  Several times, I heard a distant station ending in "Sugar David" trying to answer on his calling frequency instead of answering up 5KHz.  After about the 4th instance of this, someone else replied quickly "Sugar David, you're stupid!"  That's not particularly helpful, is it?  I would have answered "Sugar David, he is listening up 5 kilohertz.  Please move up 5 kilohertz to transmit and then listen on this frequency."

Saturday


I only really had a little time to listen.  I heard OD5ZZ from Lebanon booming in at 59+10 on 21.335.  But, once again, no contacts.  I started to wonder if I still had my audio problems on 15 meters.  So, late Saturday evening, I posted a question to the Kenwood Hybrid Yahoo group about how to track down this issue.

Sunday


In the morning, I got a response in the Yahoo group from K9TW, who worried about the driver tube and suggested checking the RF drive - providing steps for doing that:
Can also test the RF drive level on all the bands by turning off  the SG switch and with rig in CW mode flip to send and adjust Car Level for max ALC and watch the key down level to see if it holds where you set it. Do this on all bands and let it rest a little between bands. 12BY7A cathode emission begins to fall off on the higher bands as the tube ages.
So, this evening I performed those checks; all bands held the meter steady for 10 seconds.  It even held steady on 10 meters, which is the first band to go "soft" when the driver tube starts to wear out.  I hope that 10 seconds was a reasonable duration for each test; I was driving the 12BY7A at full output each time, and I didn't want to overheat it.

I haven't seemed to have much luck on 15 meters except in the early afternoon, but I decided to give it a shot anyway in the evening.  I spun around the band, and I heard PY2VI (Brasil) calling CQ on 21.318.  I tuned about 8 Khz away, tuned up the transmitter and the antenna tuner, came back to his frequency, and waited for him to call.  I answered him.

And he heard me.  He kept thinking that my "Juliet" was an "India" (the same problem that EA8YB had on Friday).  I think I might have to substitute "Japan" into my call when repeating it, or maybe I was overdriving the audio a little bit.  But, he gave me a 58 report once he got my call right; I gave him a 57 and thanked him for the DX contact.  He is 17 miles shy of being 5,000 miles away from me - my farthest contact so far.

I jumped up, ran into the bedroom, and told my wife and my daughter that I just talked to Brasil.  They looked at me kinda funny.  (This would be a good time to point out that I live just outside of the town of Brazil, Indiana).

So, I spun around the band some more.  It seemed like the band was closing, but I heard KB7MBI near Seattle calling CQ on 21.285, so I answered him.  We chatted for a bit and then I described the problems I had been having and asked him for an audio report.  He said that I was coming though with a strong 58, with no audio issues at all.  And he remarked that my D-104 and the TS-520 were remarkably well matched and that my audio was very strong and clear.  I thanked him, chatted for about 20 seconds, and waited for him to respond.  But the band had faded away, just that quick.  I threw him a 73 without knowing if he could hear me or not; then I spun around the band for a couple minutes.  Where I had previously heard a reasonable amount of chatter, now I heard nothing.

But the good news is that it seems that I actually did fix my audio issues when I fixed the two bad solder joints last weekend.  I'm going to keep an eye on it, but the big picture is that my sub-$300 eBay special is actually in pretty good shape and working pretty darn well.

73 de Mike, K9MJA