Need to tune a vhf marine antenna to ham length... advice ???

[WSEM262]

I just added a Yaesu FTM200D to my boat... and due to the difficulty of mounting/changing the existing marine antenna, I decided to use it with this Yaesu. My meter is showing 1.8 to 2.5 SWR across both 2m/440 ham bands and believe it or not, even the GMRS frequencies... (which had the highest SWR@ 2.5)

The current antenna is mounted to my radar arch and there is very little room inside the arch to manipulate coax/fittings/etc... so I'd like to leave it as-is. I will add a 2nd antenna on the side of the arch and make that the main antenna for the marine radio.

My question is... should I live with the current SWR numbers ? or should I trim the antenna and try to tweak the SWR down more ?

This is the antenna in question:

https://www.amazon.com/gp/product/B000FCP1NO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[Hit_Factor]

The radio is probably reducing output power due to the SWR. Can you live with that?

 

[K6GBW]

The 2 meter ham bands are below the Marine bands so the antenna would have to be longer not shorter. I don’t know that there’s much you can do other than change the antenna completely and tune it for somewhere in the middle (154 MHz) and hope for the best?

 

[WSEM262]

The 2 meter ham bands are below the Marine bands so the antenna would have to be longer not shorter. I don’t know that there’s much you can do other than change the antenna completely and tune it for somewhere in the middle (154 MHz) and hope for the best?


I probably have spare stainless whips laying around so I could possibly swap them around... and yeah it occurred to me after making the post that trimming it would go the wrong way. lol
lower frequency = longer whip
higher frequency = shorter whip.

 

[lamarrsy]

Why not replace the whip with a longer one, and then trim it ?

From the Amazon pictures, the whip of the antenna doesn’t seem to be fiberglass, it seem to be a standard metal whip, and it even has an Allen screw on the side of the base to secure the whip.

Look for a 40 in. or longer whip, at any land mobile radio shop in your area. Or search online, and then replace the original whip and trim it according to your swr meter.

 

[WSEM262]

The radio is probably reducing output power due to the SWR. Can you live with that?

Yeah... i'd rather deal with lower output than having to pull the antenna off the radar arch.
I can only access the inner part of the arch through a dome light hole... and its only about 1" of open cavity inside. I had a heck of a time installing the existing antenna.
I could however... swap different stainless whips in/out of the load coil/base.

 

[WSEM262]

Why not replace the whip with a longer one, and then trim it ?

From the Amazon pictures, the whip of the antenna doesn’t seem to be fiberglass, it seem to be a standard metal whip, and it even has an Allen screw on the side of the base to secure the whip.

Look for a 40 in. or longer whip, at any land mobile radio shop in your area. Or search online, and then replace the original whip and trim it according to your swr meter.

Yeah this sounds like a viable idea... I will look into it for sure.

My only other option is to fabricate some kind of custom bulkhead so239/NMO fitting in order to standardize to NMO style antennas.
Problem is... there is just BARELY room enough to place a 90-degree PL259 connector inside the arch.

The Marine VHF radio/antenna situation is covered... as I have a Shakespeare 4' and stainless mount on order that I will bolt to the side of the arch. It will be easy to drill a single hole and fish the coax through it. It's the ham antenna on top of the arch that is a SOB to deal with.

I will upload some pics of the install later today after I get home from work.

 

[G7RUX]

As others have mentioned, the 2m band is lower in frequency than the VHF marine band so you would need to lengthen the elements…it will likely be easier to replace the antenna if you’re hoping to keep it watertight.

It is also worth checking the coax losses since the apparently good VSWR might be due to attenuation in the feeder; marine environments are notoriously hard on electrical, electronic and radio kit.

 

[mmckenna]

Even if you did a longer whip, it's probably never going to be right on 70cm or GMRS.

Options:
Live with it.
Add a longer whip and enjoy the 2 meter band.
Undo the PL259 connector at the bottom and get an appropriate antenna. Without a ground plane, getting an antenna that will stand up to the marine environment and works across all those frequencies with low SWR may be a challenge. There are companies that build marine antennas that cover different bands, but it sounds like that's not going to work out well.

 

[K6GBW]

Just something to think about...on a boat that VHF marine radio is kinda important. I'm not sure I'd want to compromise it much. As you make your decision, I'd bias it toward keeping that section of the band happy.

 

[WSEM262]

Even if you did a longer whip, it's probably never going to be right on 70cm or GMRS.

Options:
Live with it.
Add a longer whip and enjoy the 2 meter band.
Undo the PL259 connector at the bottom and get an appropriate antenna. Without a ground plane, getting an antenna that will stand up to the marine environment and works across all those frequencies with low SWR may be a challenge. There are companies that build marine antennas that cover different bands, but it sounds like that's not going to work out well.

This is my concern... By going longer with the whip, the GMRS frequencies will suffer. Right now with everything under 2.5, I shouldnt be hurting the radio at all. It's still measuring 48w @ 2.5 SWR.

Both antennas on the boat are marine-specific antennas so they are stainless construction. The boat is freshwater only, so salt is not an issue.

I think I may try to fabricate a custom NMO mount with good coax just so future antenna choices will be easier to accommodate.
I have a Comet 2x4SRNMO on my truck and it works great, with low SWR on any VHF/UHF frequency I throw at it.

Not sure how well the ground plane situation is working. The radar arch is heavy aluminum construction... but the boat is obviously fiberglass and not grounded. The Tram 1600 mounted on top of the arch is considered a "no ground-plane needed" antenna... so that works to my benefit.
I would consider another Comet 2x4SRNMO for the boat, but in my experience, the Comet doesnt work well without a good ground plane.

 

[WSEM262]

Just something to think about...on a boat that VHF marine radio is kinda important. I'm not sure I'd want to compromise it much. As you make your decision, I'd bias it toward keeping that section of the band happy.

I am keeping the marine radio and adding a 4' Shakespeare 5104 marine antenna to the side of the radar arch... so I will have two completely separate radio systems and antennas.
I was just curious if I could get lower SWR out of the Tram 1600 on VHF/UHF ham bands.

It seems I am somewhat lucky the SWR isn't worse, considering the range of frequencies I tested on. Even at a SWR of 2.5 it's still measuring 48w on the meter.

 

[prcguy]

Shakespeare has a wide band series like the 6500WB that will cover both 2m and marine. Just do it.

 

[WSEM262]

Shakespeare has a wide band series like the 6500WB that will cover both 2m and marine. Just do it.

Thanks for the suggestion. Looks like a great antenna but doesnt help me @ 440-470mhz.

 

[mmckenna]

This is my concern... By going longer with the whip, the GMRS frequencies will suffer. Right now with everything under 2.5, I shouldnt be hurting the radio at all. It's still measuring 48w @ 2.5 SWR.

I'd not trust that wattage reading into a poorly tuned antenna. 2.5 is getting a bit high and I'm not sure how well some of these Chinese radios will handle it. The name brand LMR stuff will throttle back power to prevent damage. Seeing 48 watts into a mismatch load like that is a bit concerning.

I think I may try to fabricate a custom NMO mount with good coax just so future antenna choices will be easier to accommodate.
I have a Comet 2x4SRNMO on my truck and it works great, with low SWR on any VHF/UHF frequency I throw at it.

A thick mount NMO would do the trick.

That antenna is half wave on VHF, which should work OK on a less than ideal mounting location. On UHF, it's 3 stacked 5/8's waves, which -do- require a ground plane. If there's at least 6" of aluminum radar arch in each direction under the mount, that would suffice for UHF.

Not sure how well the ground plane situation is working. The radar arch is heavy aluminum construction... but the boat is obviously fiberglass and not grounded.

RF ground and DC grounds can be different things. As long as there is at least 1/4 wavelength of metal in all directions under the mount, you'll be OK.

The Tram 1600 mounted on top of the arch is considered a "no ground-plane needed" antenna... so that works to my benefit.
I would consider another Comet 2x4SRNMO for the boat, but in my experience, the Comet doesnt work well without a good ground plane.

Yeah, and honestly, I'd not trust a hobby/ham grade antenna in an application like that. But since you are not using it for VHF marine, no biggie.

 

[w4bce]

I don't know how well the Tram is going to hold up in salt water. Since this is the most important addition to your radio- I would want a good marine antenna.

 

[ramal121]

Since you need more whip, before you replace it and start hacking on good stainless, try this. Get some stiff wire and wrap around the top of the antenna and let the end stick up a couple three inches. You can now prune this to see if you can get a better SWR and what length you'll need.

 

[WSEM262]

I'd not trust that wattage reading into a poorly tuned antenna. 2.5 is getting a bit high and I'm not sure how well some of these Chinese radios will handle it. The name brand LMR stuff will throttle back power to prevent damage. Seeing 48 watts into a mismatch load like that is a bit concerning.



A thick mount NMO would do the trick.

That antenna is half wave on VHF, which should work OK on a less than ideal mounting location. On UHF, it's 3 stacked 5/8's waves, which -do- require a ground plane. If there's at least 6" of aluminum radar arch in each direction under the mount, that would suffice for UHF.




RF ground and DC grounds can be different things. As long as there is at least 1/4 wavelength of metal in all directions under the mount, you'll be OK.



Yeah, and honestly, I'd not trust a hobby/ham grade antenna in an application like that. But since you are not using it for VHF marine, no biggie.

Im using a Yaesu FTM200D

I'd not trust that wattage reading into a poorly tuned antenna. 2.5 is getting a bit high and I'm not sure how well some of these Chinese radios will handle it. The name brand LMR stuff will throttle back power to prevent damage. Seeing 48 watts into a mismatch load like that is a bit concerning.



A thick mount NMO would do the trick.

That antenna is half wave on VHF, which should work OK on a less than ideal mounting location. On UHF, it's 3 stacked 5/8's waves, which -do- require a ground plane. If there's at least 6" of aluminum radar arch in each direction under the mount, that would suffice for UHF.




RF ground and DC grounds can be different things. As long as there is at least 1/4 wavelength of metal in all directions under the mount, you'll be OK.



Yeah, and honestly, I'd not trust a hobby/ham grade antenna in an application like that. But since you are not using it for VHF marine, no biggie.

I havent measured the arch but I believe it is approx 12-18" wide at the antenna... so that should suffice.

I will look at the thick mount NMO's. I completely forgot about those... and it should squeeze into the arch OK. The issue is that the arch also has a tow rope attachment on top... so there is extra thick aluminum reinforcement welded inside that adds to the thickness. Somuch so that it's super tight access inside.


The radio in question is a Yaesu FTM200D. It's been "tuned-up" a bit so it puts out more than the advertised 50w. I dont plan to run it on high power nor Tx on GMRS, but I always test it there to see worst case/emergency scenarios. Typically low-mid (5w-25w) power is plenty in my repeater-saturated area. I could be mistaken on the 2.5 SWR and 48 watt numbers as I tested several frequencies. It may have been 48w @ 146.520 and 1.8 SWR. I will need to do some more testing and actually write down the results.

 

[WSEM262]

Since you need more whip, before you replace it and start hacking on good stainless, try this. Get some stiff wire and wrap around the top of the antenna and let the end stick up a couple three inches. You can now prune this to see if you can get a better SWR and what length you'll need.

Thats a great idea... Ive got a bunch of scraps of 12g copper house wire from when I wired the garage. I can easily add a few inches to simulate a longer whip.

 

[G7RUX]

Still, please do check the feeder; water causing losses in the coax will give a misleadingly good VSWR.