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Author Topic: IC-7000 cant hear MW broadcast but cheap transistor can?  (Read 13362 times)
CHRISDX
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Posts: 244




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« on: October 09, 2012, 11:00:08 PM »

Hi all,  I have an IC-7000 transceiver with a G5RV and it works well on the ham bands, 80m, 40m, etc but it can only get one or two very strong local AM stations, yet the cheap portable radio can hear them.  Also I have a Kenwood HT TH-F6A which when switched to the internal bar antenna, can hear some AM stations but not as well as the transistor.

Is this an antenna type issue, or is the Icom just deaf down low? I can do better with a crystal set!  lol

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KB5ZSM
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Posts: 70




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« Reply #1 on: October 09, 2012, 11:56:59 PM »

My guess on the 7000 is antenna. I have the same radio and have no problem receiving the AM MV band. Just the other night I had nothing but a test lead draped over a curtain rod and was receiving a football game from over a 100 miles away with no problem.
I also have the TH-F6 and agree that it leaves a lot to be desired on any of the HF but the better the antennas the better the results. Just don't expect a lot with the tiny handheld on HF & below.
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TANAKASAN
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« Reply #2 on: October 10, 2012, 12:51:03 AM »

You'll probably find that there's a high-pass filter at the start of the receiver chain that attenuates everything below 160m. Without it Top Band would be unusable because of IMD from the strong broadcast station signals.

Tanakasan
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N4NYY
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Posts: 4758




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« Reply #3 on: October 10, 2012, 04:46:37 AM »

I have an FT-950 and on my 80-40-20 trap dipole, I hear MW quite well. I would almost bet that it is the antenna.
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SWMAN
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Posts: 562




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« Reply #4 on: October 10, 2012, 06:56:31 AM »

 My TS-570 has an easy mod to bring AM broadcast band back to normal. It involves removing a small pin jumper on the board, most HF radios have this capability, I know the the newer 590 does this also. Real easy mod, just pull the jumper pin out. I think the others like Icom and Yaesu have the same. Check mods.dk to find out for shure.
  73 Jim  W5JJG
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WA3SKN
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Posts: 5475




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« Reply #5 on: October 10, 2012, 08:52:13 AM »

I haven't used the ic7000, but would assume that the sensitive front end is WAY overloaded.  Disabling any RF amplifiers and adding some attenuation might help get the input levels tamed.
73s.

-Mike.
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KA1MDA
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Posts: 543




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« Reply #6 on: October 10, 2012, 08:56:25 AM »

It's an Icom thing. I had an IC-706MK2G and an IC-746, and neither one of them would hear much below 80m, and pretty much nothing below the AM BCB. The Kenwood TS-430S I had before the 746 heard fine down there, as does the TS-2000X that replaced the 746 (all using the same antenna).

For example, in the 4 years I had the 746, I could barely hear 1 local NDB, and only if I disconnected the antenna shield. The first night I had the TS-2000X, I logged about 20-30 NDB with no problem! I've also received numerous LF SW broadcasters in the 130-190 Khz range (Iceland, Morocco, and France) with the TS-2000, all with a ground mounted Hustler 6BTV vertical. Perhaps it's just different front end design philosophies..
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AC5UP
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« Reply #7 on: October 10, 2012, 09:02:49 AM »

When the front end of a receiver overloads the most obvious symptom is hearing the strongest station(s) at several different places on the band. Usually distorted, but not necessarily. As WA3SKN mentioned, an overload will de-sense the receiver to the point of near deafness.

Turning off the pre-amp, reducing the RF gain, and using the RX attenuator can help. It's also very common for a receiver to get 'ugly' in the presence of a strong signal when the noise blanker is ON. Make sure it's off.

OTOH... In case the G5RV really is deaf on the BCB there's an easy way to check that. Unscrew the PL-259 antenna connector. Unscrew the outer shell until the center pin is fully exposed, then plug only the center pin into the antenna jack. Do not allow the coax shield to make contact with the SO-239.

If the radio is operating normally you should hear plenty of stations on the AM BC band.

And remember to normal up the antenna connection before you try transmitting..........  Wink
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CHRISDX
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Posts: 244




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« Reply #8 on: October 10, 2012, 09:09:47 AM »

OK guys thanks for replies.  Found out it is at least partly/mostly antenna. I draped a wire over the curtain rod and connected to the center on radio, voila got BCB stations up to 50 miles away, and faintly NYC 100mi. Same results with the center of the G5RV directly, not connecting the outer (shield). Interesting how the G5RV gets stone deaf with both but works for BCB with just the ctr.

Here's new question though, my outside antennas (G5RV and a 10m Vertical) go straight to a Alpha Delta 4 way switch, the switch feeds a MFJ tuner, and the tuner feeds Ant1 on radio.

When I connect the little test wire antenna to the center of an input on the switch, with the switc-tuner-radio hooked up normally, its deaf. Weird. Tuning the tuner doesnt help.  I would like to have a BCB switch position on the AlphaDelta and not have to unscrew the outer on the radio to listen to MW... the patch from the switch to tuner and tuner to radio are only 12" each.
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AC5UP
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Posts: 3864




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« Reply #9 on: October 10, 2012, 10:02:36 AM »

...sure would have been useful if you'd have mentioned the antenna tuner initially.    Wink

The tuner is an impedance matching device that's adjustable over a range of frequencies and mismatch conditions. AM BCB is well beyond its range because it's assumed you'll never TX on the band and the variable inductors needed to tune that low get very large. And expensive. No point in designing for a situation that's irrelevant to an amateur station........

Just for grins try the G5RV plugged directly into the Icom. Should prove the point about the tuner being a liability at 770 kHz.

BTW: The G5RV is an interesting compromise on a 20 Meter dipole that plays well on a few other bands. Also does well for SWL use. But when you consider that 1000 kHz on the AM band is 1/14th the frequency of the bottom end of 20 ( or a 14,000% error ), expecting the antenna to perform well on AM broadcast is very optimistic.
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CHRISDX
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Posts: 244




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« Reply #10 on: October 10, 2012, 10:11:46 AM »

Understood. I did plug the G5RV center directly to the radio and it performed slightly better than the drapery wire method. Both got stations from 50mi well, and weakly from 100mi in NYC. Looks like I need to either get a tuner that has a bypass switch (or add one to mine?) or another switch at the radio that can swap in a long wire, loop (which I plan to build and try), or the tuner and its antennas for ham work. I do have 2 free slots on the Alpha Delta though so maybe a bypass inside the tuner is the cheap and best solution actualy. 

Any thoughts on adding a bypass switch on my MFJ-971? I dont want to make a design mistake that would degrade the tuner in any way.







...sure would have been useful if you'd have mentioned the antenna tuner initially.    Wink

The tuner is an impedance matching device that's adjustable over a range of frequencies and mismatch conditions. AM BCB is well beyond its range because it's assumed you'll never TX on the band and the variable inductors needed to tune that low get very large. And expensive. No point in designing for a situation that's irrelevant to an amateur station........

Just for grins try the G5RV plugged directly into the Icom. Should prove the point about the tuner being a liability at 770 kHz.

BTW: The G5RV is an interesting compromise on a 20 Meter dipole that plays well on a few other bands. Also does well for SWL use. But when you consider that 1000 kHz on the AM band is 1/14th the frequency of the bottom end of 20 ( or a 14,000% error ), expecting the antenna to perform well on AM broadcast is very optimistic.

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G3RZP
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Posts: 4552




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« Reply #11 on: October 10, 2012, 10:21:11 AM »

Originally, the 5RV was used on 160 by shorting the two feeder wires together and feeding against ground with a tuner. This made it a top loaded vertical, depending on feeder length. With a long enough vertical feeder (say 60 or 70 feet) and a decent ground, it has DX potential.

For MW DXing, short the inner and outer of the coax and bypass the tuner.
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AC5UP
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Posts: 3864




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« Reply #12 on: October 10, 2012, 10:23:30 AM »

Any thoughts on adding a bypass switch on my MFJ-971?

I like to keep things as simple as possible. Makes the station easier to operate and there is such a thing as hanging too much schizz on a radio......... Besides, if you've proven a cheap transistor radio works better on AM BCB, why not go with the flow?

Use the Icom where it can perform.

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CHRISDX
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Posts: 244




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« Reply #13 on: October 10, 2012, 11:11:59 AM »

We have a VERY small house and my station is comprised of the IC7000 and pwr supply, tuner, switch next to the recliner. I also am restoring an old Hallicrafters which the modern rig will sit on top of. Point being, space is at a premium here.

This thread helped me figure out something that should have been obvious   Embarrassed   and now it's time to put the new knowlege to use.

How to add a bypass switch to the manual tuner that wont hurt its performance is the logical step. Way better than moving the radio out and unscrewing the coax everytime I want to catch my talk show or weather and traffic report or do some MW DX at night. Also can have a long wire and try a homebrew loop on switch positions 3/4

Any issues or advice for adding a bypass to the tuner? Can I just jump the capacitors etc with a toggle? Should the switch leads be shielded with the shields grounded?
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AA4PB
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Posts: 12832




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« Reply #14 on: October 10, 2012, 11:33:28 AM »

If it were me, I'd build an external switch rather than trying to modify the tuner. Remember that any switches you use must be able to handle the 100W on transmit and that anything connected to the antenna side of the tuner may be at a higher than 50 Ohm impedance (high voltage) on some bands.
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