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3rd harmonic on 2M. Any fixes?

 

I'm using a 817ND as the uplink rig in my satellite station and am getting a large spur at?446.264 when I transmit on 145.952 (for example). I'm still working on measurement/testing but I wanted to see if anyone has this problem and if/how they might have solved it. I've run 2M low-pass and bandpass filters and it is still pretty severe. The problem is that it interferes with the downlink rig tuned about 1MHz down, causing hash noise. Thanks and 73 N1QDQ
 

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The third harmonic of 145.952 mhz would be at 145.952 x 3 = 437.856 mhz, not 446.264 mhz as you indicate.? Typo error, I assume?? Also-does the undesired ("third harmonic") signal show up when you transmit into a dummy load?

Mike Dinolfo N4MWP

On 11/10/22 13:10, N1QDQ wrote:

I'm using a 817ND as the uplink rig in my satellite station and am getting a large spur at?446.264 when I transmit on 145.952 (for example). I'm still working on measurement/testing but I wanted to see if anyone has this problem and if/how they might have solved it. I've run 2M low-pass and bandpass filters and it is still pretty severe. The problem is that it interferes with the downlink rig tuned about 1MHz down, causing hash noise. Thanks and 73 N1QDQ
 

I might have transcribed those number incorrectly. I took some notes and had to leave the project for a few hours. I just got back home and put it on the bench, into a good dummy load, and used my IC705 as the test receiver. Transmitting on 145.950, seeing a clear signal on 437.850 (the calculated third harmonic). Put in a Micro-Circuits BLP-300+ which should have -30dB attenuation at 450MHZ and not much of a difference at all. Certainly not 30dB. I'll keep working on it. Thanks for the reply. N1QDQ
 

N1QDQ, when you are seeing interference that is not a harmonic and is not attenuated by a low pass filter at the transmitter, then the interference is outside of the radio and is likely a mixing product with something else.? The difference is around 300 MHz which could be a local military installation or it could be a sub harmonic of a UHF TV station near you.
--
Al Skierkiewicz
WB9UVJ
 

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You are in the near field and possibly picking up radiation from within the radio.
What does the harmonic look like in the far field?

73
Glenn
WB4UIV

On 11/10/2022 4:24 PM, N1QDQ wrote:
I might have transcribed those number incorrectly. I took some notes and had to leave the project for a few hours. I just got back home and put it on the bench, into a good dummy load, and used my IC705 as the test receiver. Transmitting on 145.950, seeing a clear signal on 437.850 (the calculated third harmonic). Put in a Micro-Circuits BLP-300+ which should have -30dB attenuation at 450MHZ and not much of a difference at all. Certainly not 30dB. I'll keep working on it. Thanks for the reply. N1QDQ 

-- 
-----------------------------------------------------------------------
Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv@...    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI, FRA, NRA-LM    ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
 

Hi Al,
Thanks for the feedback, and I get that, but the issue seems too consistent. I can transmit into an antenna or dummy load, 1/2 watt or 5 watt, and it still happens. I can tune the 2M side up and the 70cm product goes up with it. Also, I don't have any local source like that. It's actually pretty quiet here. I use an SDR on a attic dipole and I don't have interference issues with that. But your point about how a 2M LPF isn't knocking it down is well taken. Could it be coming from the 817 mixer?

Here is a pic of the 817 in transmit and and the display on the 705. Both rigs in FM. 817 fed with sine wave audio and triggering the VOX.



Pete N1QDQ

 

Glenn, Thanks for the assistance. I just tried that again to make sure I was covering my bases and with the 817 transmitting a tone in FM at 1/2 watt I can walk around the house with the 705 and a rubber duck and the 437 signal stays strong. Not as strong as when they are in the bag together, but what you would expect in "walkie talkie" mode.

73 Pete N1QDQ


On Thu, Nov 10, 2022 at 05:04 PM, Glenn Little wrote:
You are in the near field and possibly picking up radiation from within the radio.
What does the harmonic look like in the far field?

73
Glenn
WB4UIV
 

Usually when doing harmonic test with a spectrum analyzer or spectrum scope in the IC705 if one adds attenuation to the signal and it does not change means one is overloading the scope and the harmonic is being generated inside the scope.

to get a good test keep adding attenuation until if add a xdb then the signal on the scope changes by xdb.? But for the IC705 the scope is not exactly calibrated.

73, ron, n9ee/r

Ron Wright, N9EE/R
Hernando Co ARES EC
Spring Hill, Florida also n9ee55@...


On Thursday, November 10, 2022 at 04:25:03 PM EST, N1QDQ <n1qdq@...> wrote:


I might have transcribed those number incorrectly. I took some notes and had to leave the project for a few hours. I just got back home and put it on the bench, into a good dummy load, and used my IC705 as the test receiver. Transmitting on 145.950, seeing a clear signal on 437.850 (the calculated third harmonic). Put in a Micro-Circuits BLP-300+ which should have -30dB attenuation at 450MHZ and not much of a difference at all. Certainly not 30dB. I'll keep working on it. Thanks for the reply. N1QDQ

 

HI Pete,

Do you have any HP? or BPF?filtering in the input of the 705?? I am wondering if the 705 is doing some mixing with out of band signals?
The spectrum seen on the 705 is certainly not clean, just like there is some intermodulation going on ?

73
Barry VE4MA?

On Thu, Nov 10, 2022 at 3:25 PM N1QDQ <n1qdq@...> wrote:
I might have transcribed those number incorrectly. I took some notes and had to leave the project for a few hours. I just got back home and put it on the bench, into a good dummy load, and used my IC705 as the test receiver. Transmitting on 145.950, seeing a clear signal on 437.850 (the calculated third harmonic). Put in a Micro-Circuits BLP-300+ which should have -30dB attenuation at 450MHZ and not much of a difference at all. Certainly not 30dB. I'll keep working on it. Thanks for the reply. N1QDQ

 

Hi,
please? can you test without "Preamp" on the IC705?
73
Parice.


Le?ven. 11 nov. 2022 ¨¤?01:24, Barry VE4MA <barryve4ma@...> a ¨¦crit?:
HI Pete,

Do you have any HP? or BPF?filtering in the input of the 705?? I am wondering if the 705 is doing some mixing with out of band signals?
The spectrum seen on the 705 is certainly not clean, just like there is some intermodulation going on ?

73
Barry VE4MA?

On Thu, Nov 10, 2022 at 3:25 PM N1QDQ <n1qdq@...> wrote:
I might have transcribed those number incorrectly. I took some notes and had to leave the project for a few hours. I just got back home and put it on the bench, into a good dummy load, and used my IC705 as the test receiver. Transmitting on 145.950, seeing a clear signal on 437.850 (the calculated third harmonic). Put in a Micro-Circuits BLP-300+ which should have -30dB attenuation at 450MHZ and not much of a difference at all. Certainly not 30dB. I'll keep working on it. Thanks for the reply. N1QDQ

 

First of all I would like to thank everyone for their feedback and suggestions. It has been very helpful.

The signal is real, and it is coming in the antenna on the 705. I used the RF Gain and ATT on the 705 (with the preamp off, thanks!) to get the received signal down in a useful range, then I compare with and without the BLP-300+ LPF on the 817 and it is definitely having an effect on the spur. That's good. For evaluation purposes I also put the LPF on the input of the 705 and it knocks it down almost completely, which is another good sign (filter working, RF coming in the front door). I am going to set up for a mock sat pass and see how it plays, but I feel a lot better about the situation. As I said (I think) it hasn't stopped me from working V/U mode into RS-44, but it is annoying. I have a BLP-200+ coming and that should be an upgrade.?

This rig is the fourth FT-817 unit I have owned, and it has been about 5 years since my last one. I had forgotten what a fun rig it is!?

72, Pete N1QDQ
 

I decided to perform the most basic alignment on the 817ND and set the bias on the PA block. I started from scratch and got them on spec, at 37.5mA each, 75mA total. The harmonic is still present but the interference on the IC705 when I am 2MHz below it on the RS-44 downlink seems to be gone/negligible (knock wood). Other than this spur issue the rig is solid and works as it should. It's possible it is working as it should right now :) It looks like I will have a chance to A/B my current 817ND against another 817ND in early December so that should be interesting.?

One thing I did was to set up my FT991A on the bench with the 705 on top of it (my initial LEO setup) and run some of the same tests. The 3rd harmonic is still present on both the 705 and the TinySA analyzer but at a much lower power level. I'm OK with thing as they stand especially if I can consistently work V/U on my current setup without the front end of the IC705 getting overloaded.

Thanks again, Pete N1QDQ
 

The analog fix is to put a 70cm trap/s onto your 2m transmit line.
Here, there are stacks of UHF cavities available from old repeaters. Asking around would find them.
If you have an LPF, you might be able to make it into an epileptic style filter by putting C across the series L's to make them parallel resonant at 440MHz, and thus create deep nulls. These might be gimmick caps (i.e twisted wire)

The problem is that it interferes with the downlink rig tuned about 1MHz down, causing hash noise.

The root cause might be that you are using a '705.
It is a fundamental limitation of a "digitise the universe" type front end, that it will be overloaded at some point.
An indication that this is the case is that if you put an attenuator in the '705 rx line, and the interference stops at some attenuation, then you have overloading.(or drops much faster than the attenuation)??
Not sure if the '705 has internal attenuator on UHF?

If you swap the two radios i.e. RX with the '817 is it still an issue?
The harmonic output of the '705 is probably not much different than the '817
 

On Thu, 08 Dec 2022 12:57:50 -0800
"simon" <simon@...> wrote:


to make it into an epileptic style filter
ITYM "elliptic"

--

Brian Morrison G8SEZ

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