Loop antennas, tunable vs fixed?

[iMONITOR]

A couple years back I started exploring the use of loop antennas for receive only (no transmit). I chose the MFJ 1886 loop. It was not tunable and claimed it didn't need to be tuned. However I notice several similar competitive loop antennas are tunable. How does MFJ get around this, or do they? Is it because they use fairly large diameter tubing for the loop make the loop wide band to some degree? Let's not get into knocking MFJ, we've all heard the joke. It actually performed very well. Do tunable loops perform that much better? How do the tunable loops hold up outside in the weather? I just read about Wellbrook having issues with corrosion.

MFJ-1886, LOOP, RECEIVING, .5 TO 30 MHz, 110VAC

MFJ-1886 Loop AntennaDig out previously unreadable signals and track down annoying sources of interference with the MFJ-1886 HF Receive Loop The MFJ-1886 Receive Loop gives you tremendous power to copy weak stations through impossibly strong levels of QRN or QRM. Covering AM broadcast through 30...

mfjenterprises.com

 

[ka3jjz]

You're kinda mixing apples and oranges here. The comparison between an untuned active loop and a passive loop stretches a bit.

The MFJ (and several others) use an amplifier to bring the signal up - remember that a small loop has a tiny capture area in relation to a specific wavelength on HF. No tuning needed here.

The tunables (like the Chameleon) use a matching network which needs to be retuned every so often. This is an advantage in that anything that's outside the tuning range will be heavily attenuated; however the loop will be tuned so it's resonant. It will heavily attenuate MW, FM or TV stations that might give you issues - but the downside is the constant retuning.

Things like the YouLoop are quite useful for SDRs that are already very sensitive, and radios like the venerable Kenwood R1000 with a super hot front end. They don't need a lot of signal to perform well; however I understand an amp is being developed for those radios that would need it.

Corrosion issues are not unique to the Wellbrooks per se - any outdoor loops should be periodically checked at the amp and loop element for water issues. It's just good sense

Mike

 

[prcguy]

It is comparing apples to oranges. A tunable loop like the Chameleon is really low impedance on the loop and the loop conductor should be very large to reduce resistive losses. You can transmit on a Chameleon and similar loop with some higher power versions needing 2" or 3" diameter tubing and very high voltage vacuum capacitors to handle the power.

An untuned loop around 3ft diameter +/- a foot will have a fairly consistent impedance around 200 ohms across a very wide frequency range, possibly 500KHz to 30Mhz and maybe more. You can make a passive untuned loop by using a 4:1 transformer to feed 50 ohm coax or its common to have a 200 ohm input, 50 ohm output, preferably balanced input preamplifier. Since the passive loop impedance is fairly high, conductor size is not important, you can use thin wire.

I have not compared a tuned vs untuned loop side by side but I think they will have similar performance or sensitivity for a given size. The tuned loop is very narrow band at resonance, like less than 10KHz at lower frequencies and that might be an advantage on receive if you need to avoid a high level signal nearby. A tuned loop will also be limited to the bands it can cover and they will vary in size depending on the frequency range. A tuned loop that covers 14 to 28MHz would have a diameter about 2.4ft and the efficiency would be marginal at 14MHz and pretty good at 28MHz. A tuned loop about 9.8ft diameter would cover about 3.5 to 7.3MHz with ok efficiency on 40m and poor efficiency on 80m.

Personally I would favor an untuned amplified loop for receive only. They can work very well and all you have to do is rotate them and sometimes you can just park it in a spot that gets everything you want to hear like I do with my W6LVP active loop.

 

[iMONITOR]

It is comparing apples to oranges. A tunable loop like the Chameleon is really low impedance on the loop and the loop conductor should be very large to reduce resistive losses. You can transmit on a Chameleon and similar loop with some higher power versions needing 2" or 3" diameter tubing and very high voltage vacuum capacitors to handle the power.

An untuned loop around 3ft diameter +/- a foot will have a fairly consistent impedance around 200 ohms across a very wide frequency range, possibly 500KHz to 30Mhz and maybe more. You can make a passive untuned loop by using a 4:1 transformer to feed 50 ohm coax or its common to have a 200 ohm input, 50 ohm output, preferably balanced input preamplifier. Since the passive loop impedance is fairly high, conductor size is not important, you can use thin wire.

I have not compared a tuned vs untuned loop side by side but I think they will have similar performance or sensitivity for a given size. The tuned loop is very narrow band at resonance, like less than 10KHz at lower frequencies and that might be an advantage on receive if you need to avoid a high level signal nearby. A tuned loop will also be limited to the bands it can cover and they will vary in size depending on the frequency range. A tuned loop that covers 14 to 28MHz would have a diameter about 2.4ft and the efficiency would be marginal at 14MHz and pretty good at 28MHz. A tuned loop about 9.8ft diameter would cover about 3.5 to 7.3MHz with ok efficiency on 40m and poor efficiency on 80m.

Personally I would favor an untuned amplified loop for receive only. They can work very well and all you have to do is rotate them and sometimes you can just park it in a spot that gets everything you want to hear like I do with my W6LVP active loop.

My new Kenwood TS-590SG has a internal antenna preamp with two levels of amplification for receive. Would that be adiquate a passive loop antenna or would it be better to have the preamp at the loop and not use the internal one in the radio?

 

[iMONITOR]

Back in the late 60's ~ early 80's I bought a lot of military surplus radios and test equipment. Almost all of it was "tropicalized". They treated/coated the circuits and many components with a light yellow varnish type substance to protect it from heat, moisture and fungi for use in tropic environments. I'm surprised we don't see that in use more today with outdoor antennas and preamps, filters, balums, ETC. Was it not effective?

 

[prcguy]

It will help but the 590 preamp might have 10-15dB gain where some active loops have 20dB or more gain. I made a 3ft passive loop with a home made 4:1 balun and it works but its very quiet and kind of numb compared to my W6LVP amplified loop. I tried it with a couple of low noise HF preamps with about 15dB gain and the passive loop starts to come alive but its still nothing like the W6LVP which has a purpose designed preamp right at the loop instead of the loop feeding a 4:1 balun then a preamp.

The voltage at the loop feed point will be higher feeding direct to a preamp where a loop and 4:1 balun will reduce the voltage before feeding the preamp. So a 15dB gain amp that is matched to the 200 ohm loop will have an advantage over a loop with 4:1 balun reducing the voltage then a 50 ohm input amplifier.

My new Kenwood TS-590SG has a internal antenna preamp with two levels of amplification for receive. Would that be adiquate a passive loop antenna or would it be better to have the preamp at the loop and not use the internal one in the radio?

 

[prcguy]

That was an anti fungal varnish in the older military radios. Today similar equipment is conformal coated with a clear rubbery sealant that does the same thing. I used to conformal coat circuit boards when I worked at Hughes Aircraft in the 80s and the stuff is almost like water until it cures, you have to make a dam around the circuit board to be able to fill up the dam for some thickness and if there are any holes in the board you have to fill them over with a special goo otherwise the conformal coating fluid will run out the hole. I had a gallon of the stuff until recently and its extremely toxic in liquid state. I took the gallon bottle to one of our cities household paint and toxic item dump sites and they got really bent out of shape that I had the stuff in my garage and they accused me of being a commercial user taking advantage of their program.

Back in the late 60's ~ early 80's I bought a lot of military surplus radios and test equipment. Almost all of it was "tropicalized". They treated/coated the circuits and many components with a light yellow varnish type substance to protect it from heat, moisture and fungi for use in tropic environments. I'm surprised we don't see that in use more today with outdoor antennas and preamps, filters, balums, ETC. Was it not effective?

 

[merlin]

That was an anti fungal varnish in the older military radios. Today similar equipment is conformal coated with a clear rubbery sealant that does the same thing. I used to conformal coat circuit boards when I worked at Hughes Aircraft in the 80s and the stuff is almost like water until it cures, you have to make a dam around the circuit board to be able to fill up the dam for some thickness and if there are any holes in the board you have to fill them over with a special goo otherwise the conformal coating fluid will run out the hole. I had a gallon of the stuff until recently and its extremely toxic in liquid state. I took the gallon bottle to one of our cities household paint and toxic item dump sites and they got really bent out of shape that I had the stuff in my garage and they accused me of being a commercial user taking advantage of their program.

That stuff was very effective, why 60 years later it still works like new. In the 70s, I saw a lot of clear varnish either OEM or DIY. Very needed living by the Paacific Ocean. Saw plenty of zinc chromate paint also.
That is my 'go to' for outdoor antennae. (Andrew used it). Water sealing, Pack a little Dow Corning #4 compound and cover with 'FlexSeal' or sealant tape.
PCB manufacturing has changed, used to be coating with varnish except solder points then wave soldered. After that a bath in Tri-Chlor vapor to clean the boards. To cut costs, that is not done today.
Life expectancy of an aluminum antenna at my old place was about 3 years tops. (1/4 mile from the beach)

 

[prcguy]

I can't say I've seen any varnish or conformal coating inside a military radio since about the mid 80s and I've been inside lots of modern mil stuff. I believe the change is because the newer radios are mostly waterproof, you can drown them and their O-ring seals keep the moisture out. Radios designed during the Korean war and used through Viet Nam and earlier radios were not sealed as well, plus many were designed for hot humid jungle use and that's not where we are going these days.

That stuff was very effective, why 60 years later it still works like new. In the 70s, I saw a lot of clear varnish either OEM or DIY. Very needed living by the Paacific Ocean. Saw plenty of zinc chromate paint also.
That is my 'go to' for outdoor antennae. (Andrew used it). Water sealing, Pack a little Dow Corning #4 compound and cover with 'FlexSeal' or sealant tape.
PCB manufacturing has changed, used to be coating with varnish except solder points then wave soldered. After that a bath in Tri-Chlor vapor to clean the boards. To cut costs, that is not done today.
Life expectancy of an aluminum antenna at my old place was about 3 years tops. (1/4 mile from the beach)

 

[merlin]

My new Kenwood TS-590SG has a internal antenna preamp with two levels of amplification for receive. Would that be adiquate a passive loop antenna or would it be better to have the preamp at the loop and not use the internal one in the radio?

Gain is not the point. Your setup is like a system with a total gain in the system. You get .1 microvolts to your antenna jack, that is strong enough to demodulate anything. 'Antenna/coax' loss is all you need to overcome.
A good pre selector will have attenuation for stronger signals.
Passive loops are often larger than the preamped counterparts, Lossy, but not much, in some cases even a little gain. Signal to noise is very important, preamps not only amplify in band noise but can add to it, so just enough gain for the weak stuff. Some front end filtering goes a long way also. A must for these new SDR radios.

 

[merlin]

I can't say I've seen any varnish or conformal coating inside a military radio since about the mid 80s and I've been inside lots of modern mil stuff. I believe the change is because the newer radios are mostly waterproof, you can drown them and their O-ring seals keep the moisture out. Radios designed during the Korean war and used through Viet Nam and earlier radios were not sealed as well, plus many were designed for hot humid jungle use and that's not where we are going these days.

It is there, just limited. the idea is preventing corrosion. Newer commercial gear is sealed up tight save for waterproof pressure venting, can't have a seal blowout going to 10,000 feet.
Remember the old Dumont portables from the 70s ? They work under water and take a fire truck running over them. The CDF battalion I was with had them.

 

[prcguy]

I've been inside much of the late model Harris, Thales and a few other brand radios and have never had to deal with any circuit board coatings when making repairs and mods. Most of these newer radios have a Gortex window that allows the case to breath without any moisture getting in.

It is there, just limited. the idea is preventing corrosion. Newer commercial gear is sealed up tight save for waterproof pressure venting, can't have a seal blowout going to 10,000 feet.
Remember the old Dumont portables from the 70s ? They work under water and take a fire truck running over them. The CDF battalion I was with had them.

 

[merlin]

Hey, I was just doing some work on a Ranger low band receiver board and it had the varnish/paint sort of coating. Took some MEK to soften enough to scrape to clean copper.