Improving performance/SWR on 2m using an MFJ-1432

[synthetik]

I've built a bit of a Macgyver setup (apartment living) on a bottom fed monopole attached to a painters pole as a mast (isolated from aluminum via electrical tape) at about 18' high plus the 5' of the antenna length. I'm running 35' of American made RG8u, Specs below:

Attenuation @ 100ft/Pwr/Efficiency
1.8dB @ 150MHz/1.22kW/65.4% E
3.3dB @ 450MHz/0.69kW/47.3% E

Supposed manufacture specs:
7/8 Wave 5.0dB Gain on 2m 5/8 Wave 7.6dB Gain on 70cm

I've put a set of ground plane radials on it (https://www.amazon.com/gp/product/B010N5GHEY/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1 ) I understand there are a lot of factors a play here but I'm trying to lower my SWR and pull in a bit more signal on RX for 2m, I'm also getting path noise on distant repeaters on 2m. It seems these radials are designed for a 1/4 wavelength antenna and the MFJ is 7/8 Wave on 2m 5/8 Wave on 70cm. My guess is the radials are a bit too short for 2m given its 7/8 wave.

SWR on 70cm is near zero, SWR on 2m is higher at the low end of the band and close to zero at the high end and increases slightly with power likely due to reflection from my roof and nearby buildings.

Any suggestions to improve my efficiency. Slightly lengthen antenna to move closer to the center of the band? Moving up to LMR400? Increasing radial length to match 7/8 wave on 2m? Non-RF conductive mast? Move to a Comet GP3 or Arrow Dual Band J-Pole?

What's going to give me the most bang for my buck?

All feedback and constructive criticisms are greatly appreciated. Thanks!
73

 

[mmckenna]

I've built a bit of a Macgyver setup (apartment living) on a bottom fed monopole attached to a painters pole as a mast (isolated from aluminum via electrical tape) at about 18' high plus the 5' of the antenna length. I'm running 35' of American made RG8u, Specs below:

Attenuation @ 100ft/Pwr/Efficiency
1.8dB @ 150MHz/1.22kW/65.4% E
3.3dB @ 450MHz/0.69kW/47.3% E

Supposed manufacture specs:
7/8 Wave 5.0dB Gain on 2m 5/8 Wave 7.6dB Gain on 70cm

I've put a set of ground plane radials on it (https://www.amazon.com/gp/product/B010N5GHEY/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1 ) I understand there are a lot of factors a play here but I'm trying to lower my SWR and pull in a bit more signal on RX for 2m, I'm also getting path noise on distant repeaters on 2m. It seems these radials are designed for a 1/4 wavelength antenna and the MFJ is 7/8 Wave on 2m 5/8 Wave on 70cm. My guess is the radials are a bit too short for 2m given its 7/8 wave.

SWR on 70cm is near zero, SWR on 2m is higher at the low end of the band and close to zero at the high end and increases slightly with power likely due to reflection from my roof and nearby buildings.

Any suggestions to improve my efficiency. Slightly lengthen antenna to move closer to the center of the band? Moving up to LMR400? Increasing radial length to match 7/8 wave on 2m? Non-RF conductive mast? Move to a Comet GP3 or Arrow Dual Band J-Pole?

What's going to give me the most bang for my buck?

All feedback and constructive criticisms are greatly appreciated. Thanks!
73

1/4 wave ground radials are fine.
Based on your 2 meter SWR values, it does sound like the antenna needs to be shortened a bit.

Changing coaxial cable probably isn't going to provide enough improvement to be noticeable. You are already in the sub-1dB range for loss with the cable you have. Switching to LMR-400 or something larger is only going to improve that by a fraction of a dB at the most. You usually cannot hear improvements like that.

Try changing the location a bit. See if moving it around on the balcony changes anything.

 

[synthetik]

Thanks, manual antenna tuning is the easiest fix. Am I wrong in my understanding that if SWR is higher at the low end of the 2m band I should lengthen (not shorten) the antenna slightly to move the resonant point closer to the center of the band?

 

[WA0CBW]

The lower the frequency the longer the antenna. The higher the frequency the shorter the antenna. So if you want to go higher in frequency you shorten the antenna. This assumes your antenna falls in the standard bell curve.
Bill

 

[synthetik]

Thank you, there are 3 chokes/traps on this particular antenna with 4 sections in total, all with hex screws connecting. Does it matter where (from what section) I trim the antenna and any ballpark on how much I would need to trim to shift from 148 to 146 for center resonance?

 

[WA0CBW]

You would change the the length of the 2 meter element. However that changes the length of the other two bands. It is very difficult to tune a multi band antenna. Did the antenna come with re-tuning instructions?
Bill

 

[synthetik]

Thanks for all your help.

Unfortunately, it did not come with any tuning instructions (and none online) and can not shorten w/o cutting. Which of these sections would be the 2-meter element? Its 5' total including the UHF mount tip to tip. I'm showing from a calculator that 7/8 wave on 2m would be 5ft. 7 - 11/32in. Something seems amiss here. I got it for free so if it is ruined it's not the end of the world. I'm willing to take a slight hit on 70cm to improve 2m but don't want to start cutting if it won't make much of a difference.

I'm considering a Comet GP3 or one of the arrow dual band j poles as a replacement.

 

[mmckenna]

It's probably not a 7/8th's wave antenna. It's probably a co-linear design.

If there is a set screw in the base where the whip attaches, you can try loosening that and moving the antenna down.

Don't cut the antenna unless lowering it in the base improves SWR. If you lower it all the way and it's good, leave it alone. If you lower it all the way and the SWR is still high at the bottom of the 2 meter band, then try cutting off 1/8" off the bottom and try again.

Big deal is that if you cut off too much and go to far, you're stuck. Make small incremental cuts.

But, before you do that, what exactly are the SWR readings on the two ends of the two meter band? Anything below 2:1 is considered OK. You can try for a bit better, but don't expect a huge improvement in signal strength.

 

[synthetik]

Thank you again for everyone who has offered their help. To clarify it is most resonant at 148 (swr near zero) than 146 (and worse at 144) so technically I'm trying to lower the frequency by about 2 MHz. I assume then I would need to lengthen (not sure how much).

There is a set screw at the base of the whip and as it came from the factory all the way in which leaves a tiny bit of room for extension. I've tried playing around with it but it has not had much effect. Another set screw for the last element with a little wiggle room for extension, the other set screws near the traps/chokes have zero room for adjustment.

My SWR is in an acceptable range up to about 30 watts but is increasing with power output (mostly on 2m not 70cm) which I'm guessing is mostly due to reflected signals back into the antenna (a lot of flat surfaces/buildings in the near vicinity of my antenna. Its also possible the aluminum painters pole mast is coupling with the antenna on 2m and causing this SWR flux (any way to fix this? So far I've isolated it from the mount with electrical tape but I understand that only effects electrical current and not RF). Fiberglass poles are not sturdy enough to carry the weight of the antenna, mount, and feedline.

I don't really have options for relocating it. I'm thinking I may want to switch from an 18' painters pole to a 24' to get a tad more height and switch over to the Comet GP3.

Thanks and 73

 

[mmckenna]

Thank you again for everyone who has offered their help. To clarify it is most resonant at 148 (swr near zero) than 146 (and worse at 144) so technically I'm trying to lower the frequency by about 2 MHz. I assume then I would need to lengthen (not sure how much).

Yes, lengthen to bring the sweet spot down lower on the spectrum. Take small steps, 1/8" at a time.

There is a set screw at the base of the whip and as it came from the factory all the way in which leaves a tiny bit of room for extension. I've tried playing around with it but it has not had much effect. Another set screw for the last element with a little wiggle room for extension, the other set screws near the traps/chokes have zero room for adjustment.

This is where the manufacturer should be supplying the information on how to tune the antenna. Since this is likely a multiband antenna, making changes to it is going to adjust the other bands, not just 2 meters.
It's trial and error at this point.

My SWR is in an acceptable range up to about 30 watts but is increasing with power output (mostly on 2m not 70cm) which I'm guessing is mostly due to reflected signals back into the antenna (a lot of flat surfaces/buildings in the near vicinity of my antenna. Its also possible the aluminum painters pole mast is coupling with the antenna on 2m and causing this SWR flux (any way to fix this? So far I've isolated it from the mount with electrical tape but I understand that only effects electrical current and not RF). Fiberglass poles are not sturdy enough to carry the weight of the antenna, mount, and feedline.

Well, that either means reflections are becoming an issue, or you are not recalibrating your SWR meter when you change power levels.

As long as the coil is above the conductive part of the pole, that shouldn't be an issue.

If the pole will not support the antenna, mount and feed line, then you are getting into a safety concern. Rethinking your setup would be in order.

I don't really have options for relocating it. I'm thinking I may want to switch from an 18' painters pole to a 24' to get a tad more height and switch over to the Comet GP3.

Thanks and 73

At some point you are either going to need guy wires, or a much stronger pole.

 

[W5lz]

SWR doesn't affect the efficiency of an antenna. It DOES affect the impedance match with the rest of the system. To improve your reception, raising that antenna would help! It's also going to mean several changes in your antenna system. You may not think those changes were worth it when you're through. one solution (not cheap) would be to increase the "size" of your antenna. Not the GP3, maybe the GP6?

 

[synthetik]

All good advice, I'll see if I can convince the property owner to let met up a GP6 on the roof

 

[wyShack]

As for the most part you will be on the high end (146-148) of the band, I would leave it alone. Anything below 2:1 SWR or so will make little difference on transmit and even less on receive. Getting the antenna up higher will help a great deal more. Also moving the antenna will likely change the SWR so any tuning should be done after you put it where you want it. make sure you are at least 5-6 feet away as anything in the 'field' of the antenna will affect SWR and operation.