RX signal comparison: 710 vs. 6a

[Need2Know]

I had both radios receiving the same net on the same frequency at my base station last night to compare their reception audio.

The S meters on both indicated a strong signal. The signal is from a repeater a little over a mile away. The 710's signal was crystal clear hooked up to a J-pole inside my shack. The 6a HT had a very noticiable amount of background noise (white noise).

Why wouldn't the 6a HT have a background-noise-free audio as well as the 710 mobile? Is there something that might not be set right? They were both on the precise same frequency, 146.925 MHz.

Does this represent the typical performance difference between a mobile unit and an HT?

 

[Overmodulated]

.925? On 2m? Unless they are narrow-banding or weird splits, try .920.

73/k5zia

 

[Need2Know]

.925? On 2m? Unless they are narrow-banding or weird splits, try .920.

73/k5zia

Whatever thing they were doing, both radios were on that frequency, both were receiving a strong signal, the HT had a lot of noise along with it and the mobile didn't.

 

[Mudfoot]

Did you try tuning to

.920?

 

[Overmodulated]

Whatever thing they were doing, both radios were on that frequency, both were receiving a strong signal, the HT had a lot of noise along with it and the mobile didn't.

Depending on the filtering (and other characteristics) of the radio front end, one radio may sound better than the other when receiving while off-frequency. Wide filtering vs. narrow...

Try comparing when receiving on the "center frequency" of a known repeater.

 

[Need2Know]

Did you try tuning to

.920?

Not yet, but I'll do that. I should probably ask the operator of that net and repeater since the feeling here is that it is an odd frequency.

 

[Need2Know]

Depending on the filtering (and other characteristics) of the radio front end, one radio may sound better than the other when receiving while off-frequency. Wide filtering vs. narrow...

Try comparing when receiving on the "center frequency" of a known repeater.

Yes, I don't know for sure if either radio was set to "narrow" or "wide". I since reset both readios, so I'm not sure how either was set. I'm assuming they were both set at their factory default settings, whatever that is likely to be.

Even thought the HT is 1/4 the size of the mobile unit, I wouldn't expect the RX spec would be that much inferior to the mobile on a solid signal. So I must have set something wrong. That is the premise I am checking on.

On the other hand, as you suggest, this may be a consequence of an inferior front end compared to the mobile when "off frequency." And why would the net folks adverstise an "off frequency" frequency?

 

[Overmodulated]

I just did a repeater search around your callsign, and the W4OE repeater is indeed at 146.925! Never ran into those out here in Texas. My bad assuming it should be .920!

Next step - were you using the same antenna to test both radios? Also, look for a 440 repeater to test against.

 

[Need2Know]

I just did a repeater search around your callsign, and the W4OE repeater is indeed at 146.925! Never ran into those out here in Texas. My bad assuming it should be .920!

Next step - were you using the same antenna to test both radios? Also, look for a 440 repeater to test against.

No, not the same antenna, but the S meters were both full. The 710 had a J-pole at eye level. The 6a had its rubber ducky at eye level.

 

[Need2Know]

OK. I walked around the house with the HT. The house is clearly full of RF signals of the sort that are not helpful to receiving clear signals. In most parts of the house, these unwanted signals (noise) are stronger than the squelch, even when it is turned all the way up. Some corners of the house are relatively quiet, noise wise, such as the garage.

Now with both radios in the same room, with the antenna for each being in the same room at the same level, the 710 mobile has much cleaner reception.

My new hypothesis is that the 710 is much better at rejecting unwanted frequencies. I believe this characteristic is called "image rejection." It is a specification that is NOT provided for either radio, but is an important spec.

Here is more on image rejection from Radio Electronics.com :

Effect of poor image rejection

A receiver with a poor level of image rejection will suffer from much higher levels of interference than one with a high level of image rejection. In view of this, radio receivers to be used in high performance radio communications applications need to have a good image rejection performance.

When a radio receiver has a poor level image rejection signals which should not be received as they are on the image will pass through the IF stages along with the required ones. This means that unwanted signals are received along with the wanted ones and this means that the levels of interference will be higher than those with a high level of image rejection.

In addition to this the image signals will "tune" in the opposite direction to the wanted ones. When they interfere heterodyne notes will be heard and as the receiver is tuned, the pitch of the signals will change. In view of this it is very important to reduce the image response to acceptable levels, particularly for exacting radio communications applications.


Image rejection

It is clearly important to specify the level of image rejection. The specification compares the levels of signals of equal strength on the wanted and image frequencies, quoting the level of rejection of the unwanted signal.

The image rejection of a receiver will be specified as the ratio between the wanted and image signals expressed in decibels (dB)at a certain operating frequency. For example it may be 60 dB at 30 MHz. This means that if signals of the same strength were present on the wanted frequency and the image frequency, then the image signal would be 60 dB lower than the wanted one, i.e. it would be 1/1000 lower in terms of voltage or 1/1000000 lower in terms of power.

The frequency at which the measurement is made also has to be included. This is because the level of rejection will vary according to the frequency in use. Typically it falls with increasing frequency because the percentage frequency difference between the wanted and image signals is smaller.

Image rejection is distinct from either Sensitivity or Selectivity. I suspect that the HT has significantly inferior image rejection compared to the 710.

What are your thoughts. And does anyone know why manufacturers don't bother publishing this specification?

Possible lessons learned subject to results from larger samples:

• HT's generally have poorer image rejection than larger units [probably] because of their compact size
• If an antenna needs to be installed indoors, find the quietest part of the house.
• Avoidance of RF interference from electrical circuits and transmissions in the house is right up there with "higher is better" as a reason to get the antenna OUTSIDE of the house
• Attics are generally a quieter part of the house and achieve both height and separation from RF interference.

 

[Mudfoot]

The antenna on your HT will struggle in a receive contest with your mobile attached to your J Pole.

Sounds like your HT is picking up locally generated noise sources mixing with your signal. Shut the mains off in your house and operate both devices on battery power, then repeat your experiment with the repeater.

A quality aftermarket antenna for your HT may be in order.

Only the Amish would have a house free from RF.

 

[Need2Know]

The antenna on your HT will struggle in a receive contest with your mobile attached to your J Pole.

Sounds like your HT is picking up locally generated noise sources mixing with your signal. Shut the mains off in your house and operate both devices on battery power, then repeat your experiment with the repeater.

A quality aftermarket antenna for your HT may be in order.

Only the Amish would have a house free from RF.

I accept the fact that my active household circuitry is the source of my interference w/o turning off the power. Besides, I don't want to have to reset all my clocks.:headbang

What antenna do you recommend for the 6a? I have an SMA to BNC adapter on order so I can change antennae w/o stressing the SMA on the radio. Any thoughts on this antenna for wide band reception on the 6a?:

Radio Shack: Center-Loaded Telescoping Whip Antenna (cat.# 20-006)
Aluminum Telescoping Whip with Center Coil - Receives 25 to 1300MHz - Adjustable 6-1/4" to 26-1/4" (nine sections) - BNC connector.

Also for transmitting on 144, 220, 440MHz Ham bands. Adjusted for each frequency based on Faxback Doc. # 37207 Using the Antenna For Receiving.

Here is a great HT antenna review.
So you don't buy the "inferior image rejection" hypothesis?

 

[Mudfoot]

I accept the fact that my active household circuitry is the source of my interference w/o turning off the power. Besides, I don't want to have to reset all my clocks.:headbang

So you don't buy the "inferior image rejection" hypothesis?

Yes, I buy it. It's to be expected.

I guess I am on a different page than you. Do you want better reception with your HT, or are you dissapointed with it's receive sensitivity?

lol

 

[Need2Know]

Here is where I am with my "hypothesis":

I was surprised at the amount of unsquelchable noise picked up by the HT compared to the 710.

Since I have the 710 in my shack that I will be using most of the time with the antenna in the attic that receives without the unsquelchable noise (RF interference), I won't be using the HT there and don't need to do anything different with the HT for that application. (Note: At the time I tested both of these radios together, the 710 's antenna was in the room at the same level as the HT.)

My question was probably more theoretical than practical. I wasn't aware that a "better" antenna would selectively enhance the desired frequency without also increasing the reception of the RF interference. If a radios' image rejection is mediocre, intuitively I didn't think ANY radio mounted antenna would help - the noise will be enhanced along with the desired frequency. And I question why "image rejection" is not a standard spec with all radios.

My use of the HT will be primarily out in the garage (where there is less interference) hooked up to a long wire around the perimeter of the garage ceiling, to take advantage of the wide band coverage of the 6a. I understand I may have to switch on the 20db attenuation in the HT to prevent overload by strong signals using the wire. It will also be used in the car, hooked up to an external 2 meter antenna to pick up public safety transmissions. An SMA to BNC adapter will be used to reduce the wear on the SMC of the HT when antennae are exchanged. I don't think the image rejection problem will be a problem for the applications I will use this radio for in the lower noise environments, thank goodness. For example, I used this radio out in the car today and there was no ignition or other noise on this radio using just the rubber duck. I guess the excellent reception was helped by the fact that the car is a convertible.

 

[W5LZ]

I think the big difference between the two radios is the antennas. If you could swap those antennas I think the results would be jsut the opposite of what you are seeing/hearing. There's just quite a difference in those two antennas, the 'duck' being the disadvantaged one in all cases unless it happens to be VERY near the transmitter. If I'm understanding you correctly, that's a fairly typical performance by a 'duck'. Most 'S' meters in an HT are not exactly the most reliable for measuring strength, they are all relative at best.
- 'Doc