Dual Band Amateur Antenna Side Mounted to GMRS Antenna Mast

[mbrun]

In a number of posts on this forum I have mentioned my intent to side mount my amateur radio antenna on the same mast that I use for the GMRS antenna. I can now say that it is done and has been working for almost a week. As promised elsewhere, here are some pics.

 

 

 

 

The side mount bracket is home crafted and painted a color similar to the main fiberglass mast. The amateur antenna is currently about 6-1/2ft below the bottom of the GMRS antenna. There is about 21” between the amateur antenna and the nearest metal (the GMRS feed line).

 

I am using LMR400 feed-line for both antennas and have ferrite chokes installed over the coax. I have not yet detected any material interference or objectionable desense in the GMRS radio while transmitting on the amateur radio. I do however observe one bar on the GMRS radio’s meter flutter while transmitting at 50w UHF on the amateur radio, if the GMRS radio is receiving a usable low-level signal at the point I key up the amateur radio, just not enough to be of concern presently.

 

I will be operating both antennas at the lower height you see in the picture until I am ready to guy the mast at full height, which is about 16’ higher.

 

One of the inquires I received elsewhere is listed below along with my original response. Posted here to prevent derailing another thread.

 

I'm very curious about your setup; partially because I'm thinking about something similar.    How much separation do you have between the antennas?  And how many watts does each radio put out?  Have you had any issues when running on Ham UHF bands causing issues between the two radios?

 

I will be running 50w UHF and VHF, 50w GMRS.

 

I will let you know once it is operational. I may not have it installed for two weeks.

 

I do expect some desensitization of the listening receiver while I am transmitting on the other service, but nothing that I expect will affect my operations. I will have ferrite chokes on the GMRS feed-line to cut down on the noise and reduce ingress from the 2m/70cm since that line will be running parallel to the amateur antenna.

 

 

Michael

WRHS965

KE8PLM

[axorlov]

Looks nice. Why not to use one antenna and a switch for Ham and GMRS? Do you need both radios on at the same time, or you're trying to avoid complexity with family users?

[mbrun]

Looks nice. Why not to use one antenna and a switch for Ham and GMRS? Do you need both radios on at the same time, or you're trying to avoid complexity with family users?

Two radios, two services, two antennas. Family is licensed for GMRS only while I am licensed for both. Family knows which radio they are licensed for and permitted to transmit on. Neither radio in the shack is capable of transmitting outside its approved frequency range. While I don’t “need” to operate both at the same time (as I can talk on only one radio at a time), I do regularly monitor both services. So I can still monitor one service, albeit at a lower level, while operating on the other.

[maddogrecurve]

On 7/12/2021 at 5:22 PM, mbrun said:

 

In a number of posts on this forum I have mentioned my intent to side mount my amateur radio antenna on the same mast that I use for the GMRS antenna. I can now say that it is done and has been working for almost a week. As promised elsewhere, here are some pics.

 

 

 

 

The side mount bracket is home crafted and painted a color similar to the main fiberglass mast. The amateur antenna is currently about 6-1/2ft below the bottom of the GMRS antenna. There is about 21” between the amateur antenna and the nearest metal (the GMRS feed line).

 

I am using LMR400 feed-line for both antennas and have ferrite chokes installed over the coax. I have not yet detected any material interference or objectionable desense in the GMRS radio while transmitting on the amateur radio. I do however observe one bar on the GMRS radio’s meter flutter while transmitting at 50w UHF on the amateur radio, if the GMRS radio is receiving a usable low-level signal at the point I key up the amateur radio, just not enough to be of concern presently.

 

I will be operating both antennas at the lower height you see in the picture until I am ready to guy the mast at full height, which is about 16’ higher.

 

One of the inquires I received elsewhere is listed below along with my original response. Posted here to prevent derailing another thread.

 

 

 

I will be running 50w UHF and VHF, 50w GMRS.

 

I will let you know once it is operational. I may not have it installed for two weeks.

 

I do expect some desensitization of the listening receiver while I am transmitting on the other service, but nothing that I expect will affect my operations. I will have ferrite chokes on the GMRS feed-line to cut down on the noise and reduce ingress from the 2m/70cm since that line will be running parallel to the amateur antenna.

 

 

Michael

WRHS965

KE8PLM

 

 

[maddogrecurve]

On 7/12/2021 at 5:22 PM, mbrun said:

 

In a number of posts on this forum I have mentioned my intent to side mount my amateur radio antenna on the same mast that I use for the GMRS antenna. I can now say that it is done and has been working for almost a week. As promised elsewhere, here are some pics.

 

 

 

 

The side mount bracket is home crafted and painted a color similar to the main fiberglass mast. The amateur antenna is currently about 6-1/2ft below the bottom of the GMRS antenna. There is about 21” between the amateur antenna and the nearest metal (the GMRS feed line).

 

I am using LMR400 feed-line for both antennas and have ferrite chokes installed over the coax. I have not yet detected any material interference or objectionable desense in the GMRS radio while transmitting on the amateur radio. I do however observe one bar on the GMRS radio’s meter flutter while transmitting at 50w UHF on the amateur radio, if the GMRS radio is receiving a usable low-level signal at the point I key up the amateur radio, just not enough to be of concern presently.

 

I will be operating both antennas at the lower height you see in the picture until I am ready to guy the mast at full height, which is about 16’ higher.

 

One of the inquires I received elsewhere is listed below along with my original response. Posted here to prevent derailing another thread.

 

 

 

I will be running 50w UHF and VHF, 50w GMRS.

 

I will let you know once it is operational. I may not have it installed for two weeks.

 

I do expect some desensitization of the listening receiver while I am transmitting on the other service, but nothing that I expect will affect my operations. I will have ferrite chokes on the GMRS feed-line to cut down on the noise and reduce ingress from the 2m/70cm since that line will be running parallel to the amateur antenna.

 

 

Michael

WRHS965

KE8PLM

 

Looks slick!  I hope that the PVC will hold up!  I live in a very windy area, but wish to try PVC as well, though I am fearful that the wind will splinter it like a darn toothpick!!

[mbrun]

Looks slick!  I hope that the PVC will hold up!  I live in a very windy area, but wish to try PVC as well, though I am fearful that the wind will splinter it like a darn toothpick!!

The mast is not PVC, although the side mount bracket is made is. The main mast is HD fiberglass mast and is very rigid; nearly straight as an arrow too at its full 56’ extension (with antenna). There are three sets of guy rings on the mast for use when it is fully extended.

PVC would be a limp noodle in this application and would definitely snap, probably under its own weight, and perhaps even without wind.

The venture to guess that the side mount bracket would likely outlive the antenna in high winds. It is pretty beefy and sees very little load.


Michael
WRHS965
KE8PLM

[WRNA236]

That's a really slick way to do it.  Very inventive!

On 7/12/2021 at 6:22 PM, mbrun said:

One of the inquires I received elsewhere is listed below along with my original response. Posted here to prevent derailing another thread.

Quote

I'm very curious about your setup; partially because I'm thinking about something similar.    How much separation do you have between the antennas?  And how many watts does each radio put out?  Have you had any issues when running on Ham UHF bands causing issues between the two radios?

 

 

Just a general FWIW statement, don't neglect the benefits of vertical separation.  Antennas don't radiate necessarily in perfect 180° spheres and the more gain they have the more flattened their patterns become.  There's a zone where the two might interact (which is the principle behind collinear elements) but it may not take much vertical distance to fairly effectively decouple them.

On 7/12/2021 at 6:22 PM, mbrun said:

I do expect some desensitization of the listening receiver while I am transmitting on the other service, but nothing that I expect will affect my operations. I will have ferrite chokes on the GMRS feed-line to cut down on the noise and reduce ingress from the 2m/70cm since that line will be running parallel to the amateur antenna.

Just ferrites chokes I wouldn't think will be enough to filter 70cm ham from GMRS.  They are effective for gross filtering, e.g. keeping a feedline from becoming an antenna or preventing a 20m station from affecting your 70cm.  They might be enough to keep VHF and UHF from interfering with each other, maybe. 

Ferrite chokes are several tens up to even hundreds of MHz in bandwidth.  So they can't substitute for true in-band notch and pass filters or at very close frequencies, cavity filters.  The good thing is it's doesn't have to be difficult or expensive to build and test.  Even simple coax 1/4λ stubs might be sufficient. 

You might find this approach interesting since it only requires coax to test: http://www.radagast.org/~dplatt/hamradio/K6OIK-filters.pdf

The fallback is to pull out some caps and get to winding coils.  It shouldn't be necessary to use cavity filters, although at 50 watts you might be looking at needing pretty hefty attenuation just the same.

[AdmiralCochrane]

I like it

[djxs]

How does one know high high of an antenna can be put up? Do local municipalities dictate these parameters? 

[MacJack]

You should check with your local folks but mostly how far are you from an airport...  My local authorities in the county do not care BUT the local county airport https://maconcountyairport.com is below me and I'm in the landing pattern, so my antenna is just equal to my roof line... so safe to use without special permits or lights.

Hope this helps.

MacJack  

[BoxCar]

1 hour ago, djxs said:

How does one know high high of an antenna can be put up? Do local municipalities dictate these parameters? 

The FCC has rules regarding antennas and their structure. If an antenna is not more than 20 feet above its supporting structure (chimney, tree water tower etc.) it's under the 6.2 meter rule. Antennas mounted to structures more than 20 feet in height should be registered. Antennas that are more than 200 feet above ground level (and that' the entire antenna to the tip) have to meet FAA requirements for lighting . As MacJack also stated, if you are within 5 miles of an airport there are additional rules that have to be followed.

[MacJack]

So the quick answer is keep it inch above roof/chimney height as you do not need a permit if you follow BoxCar reply.. FYI my trees are higher the the roof..  by 10 feet.

Go for it.

MacJack

[djxs]

Thanks for the responses. More than likely, if I do put one up, it will be a free standing structure. Perhaps a  telescoping pole in the corner of my property. I know my wife would not allow me to mount it on the house.

[mbrun]

It used to be the limit was a hard and fast max 20’ above the building or tree to which the antenna was attached, with further limits according to FAA restrictions when you are in proximity of an airport. In 2017 the FCC removed that 20’ limit for GMRS and instead replaced it with a redirect of the user to the 200’ AGL and FAA limits.

§ 95.1741 GMRS antenna height limits.

GMRS station antennas must meet the requirements in § 95.317 regarding menaces to air navigation. See § 95.317 and consult part 17 of the FCC's Rules for more information (47 CFR part 17).

§ 95.317 Registration of antenna structures that may constitute a menace to air navigation.

(a) Each antenna structure used for a Personal Radio Service station is subject to the antenna structure rules set forth in part 17 of this chapter. In particular, the owner of an antenna structure that is more than 60.96 m (200 ft) in height above ground level (see § 17.7 of this chapter for specific criteria) may be required to notify the FAA and register the antenna structure with the FCC.

Further, stations located on or near a military or public-use airport with an antenna structure that is more than 6.10 meters (20 feet) high may have to obey additional restrictions. The highest point of the antenna must not exceed one meter above the airport elevation for every hundred meters of distance from the nearest point of the nearest airport runway. Differences in ground elevation between the antenna and the airport runway may complicate this formula. For stations near an airport, see http:// appsint.fcc.gov/UlsApp/AsrSearch/ towairSearch.jsp to figure the maximum allowable height of the antenna. Consult part 17 of the FCC's Rules for more information (47 CFR part 17).


Michael
WRHS965
KE8PLM

Edit 1: I found the link below on FAA website that allows user to perform actions to help them determine if submission to FAA is requested. I got a kick out of the results. If I place an object the size of a basketball on my property, it requests I submit a request for approval on the basis it may interfere with aeronautical signal reception. https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp. BTW, if I place a tower 1000’ on my property it is still ‘requests’ that I file. Its message did not change to indicate I must file. Weird, considering 200’ is a hard and fast threshold for submission.

Edit 2: Try list link. Hopefully it works better.
https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp?action=showNoNoticeRequiredToolForm

[n4gix]

404 error on that link Michael... 

[BoxCar]

The FCC has a program for towers etc. TOWAIR on their website;

[mbrun]

I thought I would do a report out of some testing I conducted this weekend.

One of my original curiosities has been how much signal actually finds itself being picked up by the offset antenna when the other is used to transmit in the same band. To settle this I separately transmitted using my GMRS and amateur radios (70cm) and measured the signal level present on the non-transmitting antenna. While transmitting at 50w, the offset antenna showed about -15dBm (about 25 uW or .04V ) of signal present. This was nearly the same regardless of direction.

This tells me a couple of things:
1) The power levels being picked up by the opposing antenna is well with the safe range of the receiver connected to the opposing antenna.
2) Expect serious desense of the opposing receiver during those periods when transmitting on the other radio within the same band as the receiving radio.

In addition, I did do some operational tests to see if there was any material desense of the opposing radio when operating in different bands. So far, no desense of concern. This was checked while listening to weak VHF signals on the amateur radio while transmitting on GMRS, and transmitting VHF while listening to weak signals on the GMRS radio. This was purely a subjective test under conditions in which I know the desired incoming signal to both radios was -110dBm or less. So, not bad.


Michael
WRHS965
KE8PLM

[WRMH343]

That looks awesome, thanks for sharing! 

 

To add onto @axorlov: I tried to use a ham-tuned antenna for GMRS, and the results are not great (hence my interest in installing a dedicated GMRS antenna).  I currently have a triband j-pole, which can handle up to 75w.  Testing at 5w, I get a 1.1 SWR on UHF ham bands, but 1.8 SWR on GMRS bands.  Not horrible, but that's going to be a ton of reflected/lost energy if I use a 50w base GMRS radio on that ham antenna.

[WRNA236]

On 7/18/2021 at 8:40 AM, mbrun said:

One of my original curiosities has been how much signal actually finds itself being picked up by the offset antenna when the other is used to transmit in the same band. To settle this I separately transmitted using my GMRS and amateur radios (70cm) and measured the signal level present on the non-transmitting antenna.

If you're interested in going down the rabbit hole you can read about antenna isolation with respect to repeaters. 

The general rule of thumb is the safe separation for same-band unity gain antennas using 50 watts is four wavelengths.  How you arrive at that this is 50 watts = +47 dBm and you get about 34 dB of path loss at 4λ.  So +47 dBm - 34 dB = +13 dBm, which is right around the absolute max signal strength most radios can safely tolerate.  Most tests limit max signal strength at the antenna port to +10 dBm.  This is just to protect against damage.  You will desensitize the receiving radio, just probably won't blow the front end.

But this is impossible to achieve on a tower shared with other repeaters.  So they've done all kinds of study and experiments with how to best optimize physical separation because cavity filters are physically large and expensive, have insertion loss and keeping them tuned is a pain.

That's why vertical separation and radiation patterns are so important to repeaters.  You want to get one antenna physically located in the null of the other.

 

[wayoverthere]

One other anecdotal experience:

when I first set up my ham antenna, I had it clamped onto the side post of one of those metal wire racks, which gave a swr around 3:1...not good. Moved it from there to the top of an aluminum tripod, away from anything else...swr dropped to like...1.1:1 with no other changes.

[mbrun]

If you're interested in going down the rabbit hole you can read about antenna isolation with respect to repeaters. 
The general rule of thumb is the safe separation for same-band unity gain antennas using 50 watts is four wavelengths.  How you arrive at that this is 50 watts = +47 dBm and you get about 34 dB of path loss at 4λ.  So +47 dBm - 34 dB = +13 dBm, which is right around the absolute max signal strength most radios can safely tolerate.  Most tests limit max signal strength at the antenna port to +10 dBm.  This is just to protect against damage.  You will desensitize the receiving radio, just probably won't blow the front end.
But this is impossible to achieve on a tower shared with other repeaters.  So they've done all kinds of study and experiments with how to best optimize physical separation because cavity filters are physically large and expensive, have insertion loss and keeping them tuned is a pain.
That's why vertical separation and radiation patterns are so important to repeaters.  You want to get one antenna physically located in the null of the other.
 

You are most definitely on the right track. If I needed full simultaneous duplex comms all both radios then my current arrangement is definitely not appropriate. But for one man in shack doing voice on one service at a time it is very workable. Now, if I ever do decide to put up a repeater on my site then suddenly separation and serious filters will be needed. $$$$$$$$

Yep that separation in the near-field is a good thing.

Definitely appreciate the feedback.


Michael
WRHS965
KE8PLM

[mbrun]

One other anecdotal experience:
when I first set up my ham antenna, I had it clamped onto the side post of one of those metal wire racks, which gave a swr around 3:1...not good. Moved it from there to the top of an aluminum tripod, away from anything else...swr dropped to like...1.1:1 with no other changes.

One of the things I had experimented with too was proximity of the amateur antenna to the feed-line that goes higher up the mast to the GMRS antenna. There was a definite shift in SWR as the distance varied but in the end the shift was not enough to be of concern, merely recognition that it is playing a factor.


Michael
WRHS965
KE8PLM

[wayoverthere]

25 minutes ago, mbrun said:

One of the things I had experimented with too was proximity of the amateur antenna to the feed-line that goes higher up the mast to the GMRS antenna. There was a definite shift in SWR as the distance varied but in the end the shift was not enough to be of concern, merely recognition that it is playing a factor.


Michael
WRHS965
KE8PLM

Good information...I wonder if the effect would be different depending on the type of feedline.

[mbrun]

Good information...I wonder if the effect would be different depending on the type of feedline.

I theorize a similar same effect as reflector or director in a yagi with the diameter of the shield playing role.


Michael
WRHS965
KE8PLM