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Author Topic: Passband tuning or better BDR / IMD ?  (Read 1638 times)
INPROGRESS
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« on: May 09, 2005, 08:57:59 PM »

I need some practical advice from hams who have used different sorts of gear. Firstly, I am not a contester but do hate to loose a QSO to QRM. What is more helpful to a successful QSO on say 40 cw, dual passband tuning or higher Blocking Dynamic and IMD ratings? My budget allows $1,000 for a upgraded transceiver and I have two rigs picked out that I like. One rig has the nice dual Passband feature with 2 filters but poorer BDR/IMD specifications. The other rig has much better BDR/IMD specifications but cuts QRM with a single filter, IF Shift and a DSP filter. What direction would experience say is more beneficial?
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WB2WIK
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« Reply #1 on: May 10, 2005, 10:09:26 AM »

These aren't well related issues.

BDR is only important if your antenna's big enough to allow strong signals to overload your receiver.  Many hams don't have big enough antennas for this to ever be a factor.  (Well, living next door to another ham using the same band could also be a problem -- but separation of even a few city blocks knocks the problem way down.)

I don't think one can accurately assess performance by reading data, whether that data be published specifications or lab test results -- doesn't matter.

Sit with both receivers and try them out, preferably using big antennas under crowded band conditions.  I do this at the local HRO store all the time (and they do have beams on a tower, etc -- so signals are strong) to "A/B" compare rigs and it's much more telling than reading test data.

WB2WIK/6

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INPROGRESS
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« Reply #2 on: May 10, 2005, 10:28:18 AM »

My goal is to be able to single out a cw signal for a QSO without having the radio overloaded by strong QRM. 100 Hz selectivity means zero if the radio is quieted by agc or IMD is all over my contact. I have no hands on experience with these things. For example, a DX394 swl receiver is nearly useless on 40 cw if the band is even close to open. I hope to select a transceiver that can handle those conditions as much as $1000 can buy.
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KE4MOB
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« Reply #3 on: May 11, 2005, 07:23:05 AM »

Whoa...calm down.  I agree with WIK..you're waaay to concerned with raw numbers here.  There is no perfect radio, just compromises.

And while I agree that BDR/IMD is important (for some), are you willing to give up other important numbers such as sensitivity or other specs?

"Gee, my radio is completely IMD free.  It's also deaf as a post!!"

The answer to your question (and one I don't think you'll like): I'd choose the two filters and dual passbands.  Every receiver has its limits. And when those limits are passed (and they will be) it's much better to have a lot of versatile tools rather than a few fixed ones.

A lot depends on your operating habits as well.  Do you work a lot of contests?  If so, then the IMD number is something of a concern.  However, if most of your time is spent ragchewing, then it really shouldn't matter, as the receiver will probably never be pushed hard enough to show its shortcomings.

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WB2WIK
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« Reply #4 on: May 11, 2005, 10:06:40 AM »

>RE: Passband tuning or better BDR / IMD ?  Reply  
by INPROGRESS on May 10, 2005  Mail this to a friend!  
My goal is to be able to single out a cw signal for a QSO without having the radio overloaded by strong QRM. 100 Hz selectivity means zero if the radio is quieted by agc or IMD is all over my contact. I have no hands on experience with these things. For example, a DX394 swl receiver is nearly useless on 40 cw if the band is even close to open. I hope to select a transceiver that can handle those conditions as much as $1000 can buy.<  

The DX394 isn't a serious receiver and it's a rare ham who would use one to operate the HF ham bands.  I wouldn't compare it to "anything" hams regularly use.

What you need is reasonable BDR, of course (the difference between the "best" and the "worst" ham receivers, in terms of BDR, is probably 20 dB, if measured the same way), along with interference-fighting tools such as selectable IF filtering, passband tuning or IF shift, a notch filter (either manual or automatic), stuff like that.  BDR alone doesn't tell you much.

For $1000, there isn't a single "new" piece of ham gear on the market with stellar receiving performance.  However, for the same $1000, there's a lot of used gear with far better performance.

Having owned about 90% of everything ever sold into the ham market for HF transceivers over the past 30 years or so, I get rid of a lot of stuff because I simply don't care for it.  Two rigs I've held on to through all of the "selection" process have stellar HF receive performance under tough conditions: The Kenwood TS-850S and the Drake TR-7.  They're both old (TS-850S/AT I purchased new in 1990; TR-7 I purchased new in 1978) and I've held on to them all this time because they simply outperform the other stuff I've bought (and then sold).

I've never seen a signal overload the TR-7, including my neighbor Neil K6SMF who runs a kilowatt to huge beams on tall towers a few blocks away from me.  I can operate within 1-2 kHz of Neil on any band, with any beam heading, and not even know he's there with the TR-7 receiver.  Most other receivers can only get within 20-30 kHz of him, under the same conditions.

WB2WIK/6
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K3QS
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« Reply #5 on: May 12, 2005, 06:00:11 PM »

INPROGRESS,
I operate CW almost all the time, both at the home station and in the mobile, and I would definitely go with the receiver with dual filters, and I think you mentioned passband tuning, even if it has slightly worse BDR/IMD. My radio with IF shift and DSP doesn't stand a chance against QRM compared with my other radio with PBT and several IF filters.

73, K3QS
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K5DVW
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« Reply #6 on: May 13, 2005, 11:29:03 AM »

I disagree somewhat on the bigger antenna theory, and here's why. I don't have a big antenna system. What I have is a ground mounted vertical. It's modest for sure but maybe typical.

Now, I'll compare my two radios. One is an IC756pro2 and one is an IC706MkIIG. I have them both and can switch the antenna back and forth between them. My test case is 40m novice portion at night and using a 500 Hz CW filter.

What I notice is a much higher noise level on the 706 vs the 756. The noise is due to IMD being generated by the ultra strong short wave broadcasters. The 706 isnt very good with IMD but the 756 is much better.

A test is I find a CW QSO spaced near one of the BC stations and see how the radios compare. Yep, the 756 can pull it out when the 706 is nothing but noise. I dont even hear the CW!!! I hit the attenuator on the 706 and I now hear the CW, but I dont have as much signal to noise as with the 756.

Practically this matters a lot to me because I've been in the middle of a CW QSO on 40m when one of these broadcast monsters powered up. Ugh. With the 756 I can narrow up the CW filter and keep going. With the 706, I have to quit. I like to think it matters in other situations too, like if another local powers up in band, which does happen.

My suggestion to you is to not go for one of the DC/Daylight rigs, but find an all around decent HF only rig. If you like building things, those elecraft K2 radios are awesome performers and cost less than your budget.

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N5XM
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Posts: 242




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« Reply #7 on: May 15, 2005, 08:38:31 AM »

My experience with DSP is that its best use is to reduce the QRN around a decent CW signal.  I don't do much SSB, so I can't comment on that.  I've had 4 Kenwoods, 2 Yaesus, and 11 different Ten Tecs, so I've used a LOT of different rigs.  It shouldn't take long to figure out I like Ten Tecs!

There are great buys on very good used rigs.  After getting on HF, it took me 6 or 7 years to save, trade, buy, trade and add money, trade two for one, until I had a couple of used top of the line rigs.  Some wouldn't agree, but I would consider calling Ten Tec and getting a refurbished Omni V full of filters (about 900.00) or for a little more an Omni VI, which has a keyer in it.  You could even consider one of their re-done Corsair II's full of filters, with remote VFO and PS for around 750.00.  Go to the review section and read the reviews on these radios.  The C II has a built-in keyer.  

Other rigs to consider are the Kenwood 850, and even an 830S Gold full of filters.  The Yaesu 990 is a great radio, but Yaesu filters are expensive.  I agree about the Drake TR-7.  I would also consider the K2/100.  I have little experience with Icom, but their controls are too small for my taste.  They are good radios.  You have a lot of options, but look at all the reviews you can look at first.
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N0TONE
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« Reply #8 on: May 15, 2005, 10:22:17 PM »

K5DVW, your assessment is incorrect.  On the IC-706, you are not hearing IMD/BDR problems.  You're hearing horrible phase noise.  The 706, an offshoot of the 735, has about the worse phase noise of any radio made.

An IMD problem would require TWO close undesired signals, that mix and generate a very audible mess on your intended frequency.  Being merely close to another station, and hearing what sounds like "hiss", is phase noise.

WIK made an exceptionally good observation above.  Get a Kenwood TS-850SAT - $700 and you get IMD/BDR about as good as a modern FT-1000, and dual-slope IF tuning as well.  The Icom IC-765 is another under-$1000 radio with specs as good as the Kenwood, and better reliability (the TS-850 has a known $300 repair - it has either had it, or it's going to).

73,

AM
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K5DVW
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« Reply #9 on: May 18, 2005, 06:24:53 AM »

N0TONE> Yes, phase noise is horrible on this radio too and I'm sure that is part of the bigger issue. I should have been more clear in my post since I was talking about IMD. I have definately verified that IMD is an issue on the 706 by seeing it generated by two BC stations mixing on 40m. I know the frequencies of the BC stations, so I know where the IMD should lie... and there it was, plain as day (or nite in this case). I did the same test with two local AM stations which put IMD in 160m. This was with an antenna that wasnt anywhere near optimum on the AM BC frequencies.

I dont know how the other DC/Daylight rigs compare, but I think they probably all stink when it comes to IMD/BRD/phase noise.

My broader point is that all this stuff matters, but most hams ignore it and dont push the manufacturers to do better.

Tnx,

DVW

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INPROGRESS
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« Reply #10 on: May 18, 2005, 09:06:23 PM »

Thanks to you guys for adding your comments. I learned some stuff.
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