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160 and Small Antennas




 
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kf6pqt
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« Reply #100 on: April 18, 2007, 10:57:20 PM »

Nah, this one:

http://www.amwindow.org/tech/htm/160smallants.htm

Wink  The same, but with pictures...
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W6IEE, formerly KF6PQT
AB1GX
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« Reply #101 on: April 19, 2007, 11:08:33 AM »

I have a question about this antenna tuner.  Two things need to happen.  You need to add inductance to the 'short' antenna, and you need to match the impedance of the resulting tuned antenna to 50 Ohms of the transmitter/transmission line. So I would think that both the primary and secondary of the tuner coils need taps.  Is this right.

The cap on the antenna coil is clearly for in-band fine tuning (or for shortening the antenna for higher frequency bands).

I think for a 'New England AM KiloWatt', 1/2" or larger copper pipe should be used on the antenna side, particularily if the antenna is quite a bit less than 80M.

73, AB1GX
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Steve - WB3HUZ
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« Reply #102 on: April 20, 2007, 03:09:34 PM »

Only one thing needs to happen - the SYSTEM (that's the antenna and the feedline) need be brought to resonance. That's it. Whether it's a capacitive or inductive situation will depend on the complex impedance at the end of the feedline and that is determined by feedline length.

The cap on the antenna side of the link is varied to bring the system into resonance. You could use fixed caps and roller inductors to do the same thing, although this would be a more difficult mechanical problem with a link topology.

The matching to 50 Ohms does not require taps on the transmitter side of link (although it could be done this way). The transformation ratio is easily varied by the taps on the antenna side (look at it as an autotransformer) and by varying the coefficient of coupling between the antenna and transmitter sides of the link (either through physically varying their relative positions as in a swinging link, or by putting a cap in series with the transmit side link).

Half inch copper pipe is probably overkill for anything under 1 kW into the tuner, but if you already have it on hand, it should make for a high efficiency tuner.

I have a question about this antenna tuner.  Two things need to happen.  You need to add inductance to the 'short' antenna, and you need to match the impedance of the resulting tuned antenna to 50 Ohms of the transmitter/transmission line. So I would think that both the primary and secondary of the tuner coils need taps.  Is this right.

The cap on the antenna coil is clearly for in-band fine tuning (or for shortening the antenna for higher frequency bands).

I think for a 'New England AM KiloWatt', 1/2" or larger copper pipe should be used on the antenna side, particularily if the antenna is quite a bit less than 80M.

73, AB1GX
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AB1GX
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« Reply #103 on: April 20, 2007, 07:10:21 PM »

Steve Thanks!

I'm using a 80 meter dipole antenna for 160 meters, and I added a large inductor at the feed point to resonate the antenna.  There's an air variable cap across this inductor for tuning down the 160 meter band.

Obviously, the output is balanced, but I really like you autotransformer approach.  I think I'll try attaching the transmission line coax ground to the center of the inductor and attach the unbalanced feed to a tap for impedance matching.

Now, if I want to measure antenna current does it make sense to put a current sensing loop around a center tap wire but in the direction away from the autotransformer taps.  That way the feed current is not sensed and the voltage near the loop is low (as opposed to where the inductor attaches to the antenna).

73
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Steve - WB3HUZ
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« Reply #104 on: April 21, 2007, 12:40:20 PM »

There are actual antenna tuners using the design you describe. Other than the lack of magnetic isolation provided by the link, the electrical action is identical. Go for it!


Steve Thanks!

I'm using a 80 meter dipole antenna for 160 meters, and I added a large inductor at the feed point to resonate the antenna.  There's an air variable cap across this inductor for tuning down the 160 meter band.

Obviously, the output is balanced, but I really like you autotransformer approach.  I think I'll try attaching the transmission line coax ground to the center of the inductor and attach the unbalanced feed to a tap for impedance matching.

Now, if I want to measure antenna current does it make sense to put a current sensing loop around a center tap wire but in the direction away from the autotransformer taps.  That way the feed current is not sensed and the voltage near the loop is low (as opposed to where the inductor attaches to the antenna).

73
Steve Thanks!

I'm using a 80 meter dipole antenna for 160 meters, and I added a large inductor at the feed point to resonate the antenna.  There's an air variable cap across this inductor for tuning down the 160 meter band.

Obviously, the output is balanced, but I really like you autotransformer approach.  I think I'll try attaching the transmission line coax ground to the center of the inductor and attach the unbalanced feed to a tap for impedance matching.

Now, if I want to measure antenna current does it make sense to put a current sensing loop around a center tap wire but in the direction away from the autotransformer taps.  That way the feed current is not sensed and the voltage near the loop is low (as opposed to where the inductor attaches to the antenna).

73
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kf6pqt
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« Reply #105 on: April 25, 2007, 06:38:44 PM »

So, I have a coax-fed 75m inverted V that is a near-perfect 50-ohm match at 3870. Yeah, I'm that good. Wink
-jk.

So, after gathering the parts and supplies that I need for this tuner, (50-foot coil of 1/4" tube, yowtch, $39!!) and a 100ft roll of new 450-ohm line, its hit me...

If I want to keep using this antenna on 3870, with the new feed an tuner, I had better alter the length, right? Lengthen it, to get away from 3870 so that I can use the tuner to effectively bring it BACK to 3870?

Or am I overthinkin' this?

Thanks,
Jason kf6pqt
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W6IEE, formerly KF6PQT
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Are FETs supposed to glow like that?


« Reply #106 on: April 25, 2007, 07:06:37 PM »

Yep, you're overthinking it....

Actually overlooking something.....

The antenna is STILL going to be resonant at the same frequency it was with coax feed......

The feedline won't change that....


You MAY want to lenghten the antenna anyway though....

If you intend 160 meter operation.... get all the length you can on the dipole....

If you're restricted....
try to get at least long enough to cover the 80 meter band.... ( 130 feet + ...overall length)


You'll be ok either way.... don't sweat it...





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kf6pqt
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« Reply #107 on: April 25, 2007, 08:09:47 PM »

I am at just under 130', 124' as I recall, as the vee is a tad shorter than the regular dipole.

So this sort of tuner isnt the sort that will care about about loading up an exact 1/4 and 1/2 wave of the input frequency? I understand this to be a problem with other tuner types.

I'm just hoping to get 160/75/40m functionality out of the one 75m antenna, with the emphasis only on slightly improving the efficiency on 75 by getting away from coax feed. 160 and 40 will just be a big bonus!

Thanks,
Jason kf6pqt
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W6IEE, formerly KF6PQT
W1GFH
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« Reply #108 on: April 25, 2007, 08:17:26 PM »

You might want to add some extra wire to the ends of the antenna legs if you have room to "droop down" these in a dangling fashion.

If you can give up 160M: Some AM ops have successfully employed an EDZ antenna for 80-10M using 44' per side making it 88' in total length - a very compact size that has the advantage of getting ALL your WIRE IN THE CLEAR. Check out K6ESE's Mar Vista shack pix for the antenna setup: http://www.k6ese.com/Mar_Vista_Shack.htm

And....I hope you are changing out that coass for balanced line.  Never mind, I see you are. Grin
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KF1Z
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Are FETs supposed to glow like that?


« Reply #109 on: April 25, 2007, 08:51:34 PM »


So this sort of tuner isnt the sort that will care about about loading up an exact 1/4 and 1/2 wave of the input frequency? I understand this to be a problem with other tuner types.




I believe you're refering to FEEDLINE length there.... NOT antenna length....

With any tuner, matcher whatever you choose to call it....
IF you run into trouble obtaining the match you desire....

Trimming or adding FEEDLINE will usually do the trick....

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Steve - WB3HUZ
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« Reply #110 on: April 25, 2007, 09:56:06 PM »

Trimming the feedline is usually easier, but changing the length of the antenna will too. You have to look at the wire in the air, the feedline and the tuner as the antenna SYSTEM. The entire system must be brought to resonance for it to accept power and work. Changing any of the physical parameters of the system will have an effect.
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kf6pqt
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« Reply #111 on: April 25, 2007, 10:07:09 PM »

Ohhhkay... I think I gots it now.

Joe, I actually went in to HRO today... believe it or not, they actually had the best deal running on a 100ft roll of 450 ohm, 18ga solid ladder line, even cheaper than MFJ!

Making my own out of bare 14ga and home-made spreaders would have been cool and oldbuzzardly and all, but would have co$t a lot more when you add the copper+time together.

Trevor's kite-pole sounds great, but I'm wondering about a plain 'ol painters pole from home despot...

"In the clear" will never describe my antenna, until I move!  I did buy one of those combination lopper/saw on-a-pole thingys from Harbor freight, should allow my to get one end of the antenna ten feet higher in the Mulberry tree from hell...

Thanks,
Jason kf6pqt
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W6IEE, formerly KF6PQT
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« Reply #112 on: April 25, 2007, 10:30:58 PM »

Sounds like you did good. As others have said, don't overthink the deal. Antenna challenges build character. And if the neighbors see you fooling with coils of copper tubing, just tell them you're making a still.
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kf6pqt
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« Reply #113 on: April 26, 2007, 01:30:01 AM »

Noooo! Then they'll be pestering me non-stop... "Is ready yet? No? How 'bout now? Are you sure? Well, how 'bout now, maybe?
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W6IEE, formerly KF6PQT
W4EWH
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« Reply #114 on: April 26, 2007, 01:59:19 AM »

Hey, for the tuner design in the amwindow tech page... Do I really, really need to run 3/8" tubing for the main coil? Sad that its the most painfully expensive part these days! I have a 4kv ef johnson varable cap as big as my thigh that I picked up at a swap for cheap, but the thought of shelling out for all that copper makes me cringe.

I'm not going to be running a "New England Kilowatt," just a pair of 811a's.  Wink

Thanks,
Jason kf6pqt

Jason,

Chat up your local electricians: they routinely deal with scrap lengths of wire, often in larger diameters, and will likely sell it to you for the scrap price.

HTH.

Bill
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AB1GX
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« Reply #115 on: April 26, 2007, 11:10:02 AM »

So, I have a coax-fed 75m inverted V that is a near-perfect 50-ohm match at 3870. Yeah, I'm that good. Wink
-jk.

So, after gathering the parts and supplies that I need for this tuner, (50-foot coil of 1/4" tube, yowtch, $39!!) and a 100ft roll of new 450-ohm line, its hit me...

If I want to keep using this antenna on 3870, with the new feed an tuner, I had better alter the length, right? Lengthen it, to get away from 3870 so that I can use the tuner to effectively bring it BACK to 3870?


Or am I overthinkin' this?

Thanks,
Jason kf6pqt


No you may not be overthinking this!  First you need to recognize the term 'antenna tuner' doesn't necessarily mean antenna tuner (making the antenna resonate).

If the antenna tuner is actually an antenna tuner and is located at the feed point of the antenna, you are correct in that the length of the antenna may need to change if you just add inductance from that expensive copper tubing you bought.  But you can also add capactance to keep your 'perfect' dipole in tune.  In fact, with this approach any length of wire can be brought into resonance (but the impedance may be too high or low to be useable).  This is the best performing placement of an antenna tuner, but it's also a pain to climb up 50' to tune the damn thing and adjust VSWR.  How do I know?  I fell out of the tree.

Next, you can move the antenna tuner to the ground by using ladder line from the feed point to the tuner.  This also works well because you're still mainly tuning the antenna which is now the combo of the transmission line and dipole.

And finally there's the antenna tuner in the shack which is just a glorified VSWR adjuster and doesn't tune anything.


Another choice is to go with a vertical 1/4 wave.  Now the tuner is on the ground!  This is the approach the US Navy has been using for the last 50 years (even though big dipoles can be installed on most ships).

Steve's input was key to my getting a handle on this antenna tuner jazz.



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Steve - WB3HUZ
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« Reply #116 on: April 26, 2007, 11:22:03 AM »

Quote
And finally there's the antenna tuner in the shack which is just a glorified VSWR adjuster and doesn't tune anything.

Au contraire my friend. The tuner performs the same function, no matter where it is located. And yes it does tune, just as one tunes their transmitter by adjusting the tune and load controls. In each case, the system is being brought to resonance - be it the feedline and wire in the air as part of the antenna system, or the tube and the output network in the transmitter.

Suggest you get a copy of the book Reflections by Walt Maxwell. All this antenna, feedline, impedance, tuning, resonance stuff is very clearly explained. I've read the book twice and learned new things each time. I'll probably read it again, since my brain can only absorb so much, and I've probably forgotten what I learned the first time around. Smiley
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AB1GX
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« Reply #117 on: April 26, 2007, 12:09:50 PM »

Quote
And finally there's the antenna tuner in the shack which is just a glorified VSWR adjuster and doesn't tune anything.

Au contraire my friend. The tuner performs the same function, no matter where it is located. And yes it does tune, just as one tunes their transmitter by adjusting the tune and load controls. In each case, the system is being brought to resonance - be it the feedline and wire in the air as part of the antenna system, or the tube and the output network in the transmitter.

Suggest you get a copy of the book Reflections by Walt Maxwell. All this antenna, feedline, impedance, tuning, resonance stuff is very clearly explained. I've read the book twice and learned new things each time. I'll probably read it again, since my brain can only absorb so much, and I've probably forgotten what I learned the first time around. Smiley

What!!!  Say it isn't so. Shocked
When the antenna tuner is connected to the output of the transmitter, and it's output is connected to the transmission line (of non trivial length), I claim (as I previously learned from your posts) that the antenna tuner can only (I repeat only) adjust VSWR!!!  Yes the LCs in the antenna tuner are tuned, but so are the LCs in the transmitter.  The antenna (it's resonance) is 100% uneffected by any knob twisting at the antenna tuner (actually in this configuration the antenna tuner should be called a transmatch).

All this is much clearer if you just sketch the equivalent circuit.

 
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Steve - WB3HUZ
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« Reply #118 on: April 26, 2007, 02:46:04 PM »

Look at it this way - The transmission line is nothing more than an impedance transformer when there is anything other than a perfect match at the far end (i.e. flat line). So there is a Z at the feedpoint of the antenna (R +/- jX). Let's call this Impedance A. That impedance is transformed to some other impedance at the other end of the feedline. Let's call this Impedance B. The exact impedance is dependent on the length of the line. So, if you place a tuner right at the antenna (the wires in the air) feedpoint, it will have to deal with Impedance A. If you place the tuner at the end of the feedline, it will have to deal with Impedance B. Either way, it's dealing with impedance. There is no difference. Sure the impedances will be different (higher or lower resistive component and inductive or capacitive reactance at varying amounts), but both are complex impedances and the tuner will deal with each in the exact same way.

The only reason to be concerned with the length of the feedline is if Impedance B is one that cannot be properly dealt with by the tuner (doesn't have enough capacitance, inductance or some combination thereof). So, changing the feedline length to a point were Impedance B is better dealt with by the tuner is a good option.

Go to the URL below and play around with the feedline length to see for yourself.


http://fermi.la.asu.edu/w9cf/tran/index.html
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AB1GX
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« Reply #119 on: April 26, 2007, 04:06:14 PM »

Steve,

I know exactly what you're suggesting, but... It's well known that losses from this type of transmission line impedance transformation are almost always excessive in multi-frequency applications.  It's bad enought to put a fan on the tuner, but having a red hot coax is over the limit. Wink  Things are much better with 600 Ohm ladder line, but at 1.5KW the losses are still around a few hundred Watts.

Just to avoid terrible losses - using that calculator applet, I'd have to install my dipole at least 80 ft in the air (at 40 or 50 feet the resistance of my antenna is way too low for reasonable efficiently at 75 and 160 meters).

I don't think there is any way to come close to the efficiency of a tuner at the base of a vertical antenna.

Does anyone know what '51W Fred' uses? (other than a 2KW linear)

tom
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Steve - WB3HUZ
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« Reply #120 on: April 26, 2007, 06:03:57 PM »

Quote
It's well known that losses from this type of transmission line impedance transformation are almost always excessive in multi-frequency applications.

It's well known where? Losses with open-wire line are minimal. Here's an extreme example using the W9CF calculator.

Freq: 1.9 MHz
Feedline: 600 Ohm Ladder
Length: 100 feet
Input Resistance (that at the antenna feedpoint): 10
Input Reactance: -500 (these two numbers are about what a short dipole, like only 120 feet long, will yield on 160 meters)
Z at end of the feedline (in the shack): 11.95 +j373.11
Load SWR: 92.16 (pretty extreme)
Input SWR: 74.43

Now let's look at the losses.

Match Loss dB/100 ft: 0.02 (this is the loss of the feedline when SWR is 1:1, perfect match, you can see ladder line is rather low loss at this freq).

Total Loss (additional loss due to the mismatch/SWR): 1.64 dB (yes this amounts to several hundred watts if you are running 1500 watts into the feedline) but no one is going to notice a 1.64 dB drop in your signal. And this is a extreme case. Generally, the loss is well less than 1 dB (e.g. a slightly longer antenna and a 70 foot feedline yielded only 0.5 dB loss).

 In an absolute sense, you are correct. You can't beat a tuner right at the antenna feedpoint. But in practical application, it makes little or no difference where the tuner is located. And it's a heck of a lot simpler with the tuner in the shack. Grin

Unless you want to use an exceedingly short antenna on 160 meters, losses will be a fraction of a dB with the right feedline. What sort of antenna are you proposing?


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AB1GX
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« Reply #121 on: April 26, 2007, 07:19:36 PM »

Steve,

First, thanks - great discussion.  I had planned to use a dipole that's 40 meters long for 160 meter operation.

I used coax for my calculations which yielded much, much worse results (3x worse).  Not everyone can string 600 Ohm ladder line through their home and walls. 

What's worrying is that the current in the transmission line peaks in one physical location along the line.  Even 14 gauge copper ladder line will not support the 100+ Amps at a single position that we see in the calculator's plot.  That's several hundred Watts being dissipated in a couple of feet.  Clearly for KW+ operation requires 10 or 12 gauge copper!

Assuming ladder line, ff I combine the insertion loss of the antenna tuner (which gets HOT) of about 1.5 dB with the 1.5 dB from the transmission line impedance converter, I see 3 dB of loss.  That means my 90% efficient 1.5KW rig which I worked so had to build has the effective power of a 750W transmitter.

And -3dB may not seem like not much is lost.  In absolute terms that's right, but there's a noise floor in signal reception and the higher the noise floor, the more important 3 dB is.

Since I'm tired of falling out of trees, I think I'm gonna go for a 50' vertical antenna with the tuner at the base. Burying radials is much lower risk!

Tom

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Steve - WB3HUZ
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« Reply #122 on: April 26, 2007, 07:29:32 PM »

Good discussion indeed. Yes, coax is a no go for sure. A 130 foot long antenna feed with open-wire line will work. I've done it. So have others, some putting out very good signals on 160 meters. It's not easy though. Open wire line with #12 or larger conductors is needed and a strapping tuner. Whether this is more it less work than a vertical and a bunch of radials (probably at least 30 to keep the losses down) is up to the individual. If you go the vertical route, take a look at the losses involved with a short radiator and the associated ground losses.

Have fun!
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« Reply #123 on: April 26, 2007, 09:59:00 PM »

coax on a short antenna with high VSWR is a waste of time.
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KF1Z
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Are FETs supposed to glow like that?


« Reply #124 on: April 26, 2007, 10:55:27 PM »

One feedline type that gets overlooked may come in handy for this type of application...

I use 2 runs of RG8-U coax, (#9 awg solid center) as 'balanced line'....


The two braids are connected at the shack end, and to ground.

The two center conductors go directly to the link-coupled (k1jj type) tuner.


Now... It works... the impedance was measured at 115ohms

I have not done ANY efficiency calculations, or measurements...

I just see it as an easy way to run a balanced line through the attic to the back of the house....

The braids tied together to ground reduced the noise (computers, etc.) by at least 15-20db (2+ 'S-units on reciever).



The only other "test" I did... was to sling the 2 pcs of coax about 20 feet apart in the middle of the run... There was NO difference in the measured vswr...
To me, that only means that the lines probably stayed balanced, unaffected by their proximity to each other and other objects.


As I said, no idea if I'm losing scads of power or not...
Might be worth looking into....

Thoughts?

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