70 cm Antenna System

[N4GIX]

I have having some poor receive sensitivity with my new Bridgecom repeater system, and I'm trying to narrow down just where I might take some action to improve things. Beginning with the antenna system seems a good place to start.

The antenna is a monoband Comet CA-712EFC mounted on a roof tripod on a 10' mast. With the combined height of the roof the base of the 8' antenna is at 52' AGL. The feedline is 60' of new 1/2" Comscope heliax, which terminates in a new MFJ-270 Lightning Surge Protector (Gas) just outside my office/shack exterior wall. It is brought into the repeater with 6' of RG-213.

I've done a sweep of the entire antenna system with a Surecom SA-250 Antenna Analyzer. The lowest SWR is 1.09 @ 461.275 MHz. At the receive frequency of 467.675 MHz the SWR is 1.32 (see details in next image) :

Averages on a single frequency, 10 pass sweep are shown in the next image. The camera didn't focus too well for this image, but it should still be readable.
SWR 1.32
Zx 50.1 ohms
Rs 57.5
jX -28

The final image is at the transmit frequency of 462.675 MHz. This image is crystal clear so I won't transcribe the results. I cannot spot anything that stands out as a possible problem.

The last image is a composite trace of the duplexer. Note that the insertion loss at the receive frequency is -1.42 dB, which isn't all that bad! The reject of the transmit frequency isn't all that spectacular, but should be acceptable at -70.81 dB.

The transmitter is limited to 20 watts output in an attempt to better balance the system. It is capable of a full 50 watts continuous duty.

Range testing with a 4 watt Motorola XPR7550 shows full quieting at 1 mile, 70% quieting at 2 miles, but unreadable at 3 miles. With the antenna height and average 5' portable/mobile height, I should be seeing around 10 miles for 70% HT coverage by my calculations.

Does anyone have any suggestions as to where next to look?

 

[WA0CBW]

Did you do an ISO-"T" desense test?
BB

 

[kayn1n32008]

I have having some poor receive sensitivity with my new Bridgecom repeater system, and I'm trying to narrow down just where I might take some action to improve things. Beginning with the antenna system seems a good place to start.

The antenna is a monoband Comet CA-712EFC mounted on a roof tripod on a 10' mast. With the combined height of the roof the base of the 8' antenna is at 52' AGL. The feedline is 60' of new 1/2" Comscope heliax, which terminates in a new MFJ-270 Lightning Surge Protector (Gas) just outside my office/shack exterior wall. It is brought into the repeater with 6' of RG-213.

I've done a sweep of the entire antenna system with a Surecom SA-250 Antenna Analyzer. The lowest SWR is 1.09 @ 461.275 MHz. At the receive frequency of 467.675 MHz the SWR is 1.32 (see details in next image) :

Averages on a single frequency, 10 pass sweep are shown in the next image. The camera didn't focus too well for this image, but it should still be readable.
SWR 1.32
Zx 50.1 ohms
Rs 57.5
jX -28

The final image is at the transmit frequency of 462.675 MHz. This image is crystal clear so I won't transcribe the results. I cannot spot anything that stands out as a possible problem.

The last image is a composite trace of the duplexer. Note that the insertion loss at the receive frequency is -1.42 dB, which isn't all that bad! The reject of the transmit frequency isn't all that spectacular, but should be acceptable at -70.81 dB.

The transmitter is limited to 20 watts output in an attempt to better balance the system. It is capable of a full 50 watts continuous duty.

Range testing with a 4 watt Motorola XPR7550 shows full quieting at 1 mile, 70% quieting at 2 miles, but unreadable at 3 miles. With the antenna height and average 5' portable/mobile height, I should be seeing around 10 miles for 70% HT coverage by my calculations.

Does anyone have any suggestions as to where next to look?

Ditch the comet antenna. A 2 or 4 element exposed dipole array will run circles around the Comet.

 

[N4GIX]

Did you do an ISO-"T" desense test?
BB

No, I no longer have a service monitor at my disposal.

I should also add that Ron Kochanowicz tested my repeater before sending it to me and reports he got a solid decode down to less the 0.25uV with the duplexer installed. I've forwarded the same data I posted here for his inspection.

I've asked him to confirm, but I suspect that his results were from an ISO-"T" desense test.

 

[N4GIX]

Ditch the comet antenna. A 2 or 4 element exposed dipole array will run circles around the Comet.

I would except for two things. I already own the Comet, and the tripod and mast I am using won't physically support such a large antenna.

I do have a good friend with some "connections" who's trying to locate some vertical real estate for me, in which case I'll probably have to get a new Decibel 8 bay antenna and more bloody 1/2" heliax...

BTW, you really shouldn't quote an entire wall of text and pictures for a one line reply.

 

[kayn1n32008]

I would except for two things. I already own the Comet, and the tripod and mast I am using won't physically support such a large antenna.

I do have a good friend with some "connections" who's trying to locate some vertical real estate for me, in which case I'll probably have to get a new Decibel 8 bay antenna and more bloody 1/2" heliax...

BTW, you really shouldn't quote an entire wall of text and pictures for a one line reply.

Sorry on the long quote
Working off my phone... tapatalk sucks on a Rugby.

Sucks not having vertical realestate. Good luck, if you do get some height, you may want to think about 7/8" or 1-1/2" hard line, depending on how far up you go.

 

[N4GIX]

Sorry on the long quote
Working off my phone... tapatalk sucks on a Rugby.

Sucks not having vertical realestate. Good luck, if you do get some height, you may want to think about 7/8" or 1-1/2" hard line, depending on how far up you go.

I just had a very long QSO with a friend who's currently at work (he's a night guard) at a plant that's about 4 miles from me.

His 4watt HT with an external mag mount antenna was about 40% and a bit scratchy. I switched the Bridgecom from repeat to base mode several times and his signal shot up to a solid 95% copy with just a bit of very low volume 'hiss' on his audio.

So, I'm suspecting a de-sense problem.

 

[kayn1n32008]

Yea. What kind of duplexer are you using?

 

[N4GIX]

Yea. What kind of duplexer are you using?

It's a Cellwave mobile duplexer, according to Ron K @ Bridgecom.

 

[cmdrwill]

Junk the Comet antenna, they are bad on duplex. A known problem.

 

[N4GIX]

Junk the Comet antenna, they are bad on duplex. A known problem.

It wouldn't be the first time I've picked the wrong horse.

If it doesn't rain tomorrow, I'm going to try using a dual antenna and by-pass the duplexer entirely. I will use the Comet for receive, and another antenna about 10' lower and physically 42' apart from the Comet. That should be plenty of separation.

 

[Thunderknight]

His 4watt HT with an external mag mount antenna was about 40% and a bit scratchy. I switched the Bridgecom from repeat to base mode several times and his signal shot up to a solid 95% copy with just a bit of very low volume 'hiss' on his audio.

So, I'm suspecting a de-sense problem.

Yes,I would agree 100% given that example.

Can you temporarily bypass the RG-213 jumper and the MFJ-270? Just to eliminate those without too much work.

 

[Project25_MASTR]

RG-213 is not what I would use in a full duplex environment (RG-214 is).

Other than that your issue is most likely in the antenna. Try commercial grade fiberglass antenna made by Sinclair or others. They are about 4 ft tall and work as decent repeater antennas.


Sent from my iPhone using Tapatalk

 

[kayn1n32008]

RG-213 is not what I would use in a full duplex environment (RG-214 is).

Good catch! Missed that

Other than that your issue is most likely in the antenna. Try commercial grade fiberglass antenna made by Sinclair or others. They are about 4 ft tall and work as decent repeater antennas.

Yea. Anything but hammy fibreglass. Comet ect. Are cheap, crappy garbage.

A guy I know has a dual band 5/8 over 5/8(on VHF) and it is out performed by a single Sinclair 1/2wave spaced 210-C1.

 

[N4GIX]

Yes,I would agree 100% given that example.

Can you temporarily bypass the RG-213 jumper and the MFJ-270? Just to eliminate those without too much work.

If I put the repeater outside connected directly to the 1/2" heliax, yes. With the weather flip-flopping between rain and dry but cloudy that isn't going to happen though. :wink:

OTOH, I do seem to have about 10' of excess heliax and my workstation/shack is directly underneath the last window, so perhaps I could temporarily route the heliax into the office by opening the window and removing the screen.

BTW, ignore the two smaller coax cables. They are remnants of a deceased 6m antenna and an inverted V for HF that's likewise kaput. The steel water pipe mast is supporting a copper J-pole for experimental usage.

 

[N4GIX]

RG-213 is not what I would use in a full duplex environment (RG-214 is).

Other than that your issue is most likely in the antenna. Try commercial grade fiberglass antenna made by Sinclair or others. They are about 4 ft tall and work as decent repeater antennas.

I do plan to replace the 6' RG213 jumper with 1/4" Superflex soon.

The cost of the antenna really isn't the issue so much as having to call in my installer and pay him again. He just was here a few weeks ago and charged $250 to install the new Comet, tripod, mast and heliax.

The specs on the Comet cite 9.8dBi, which converts to ~7.6dBd gain.

 

[kayn1n32008]

The specs on the Comet cite 9.8dBi, which converts to ~7.6dBd gain.

dBi= dBimagined
dBd=dBdoubtful

Pretty much sums up hammy fibreglass antennas. You would probably see better results with an elevated feed 5/8 over 5/8 mobile antenna on a BSA kit.

 

[N4GIX]

dBi= dBimagined
dBd=dBdoubtful

Pretty much sums up hammy fibreglass antennas. You would probably see better results with an elevated feed 5/8 over 5/8 mobile antenna on a BSA kit.

An antenna tuned specifically for 460-470 MHz falls outside the 'hammy band' entirely. When placed in "Base" mode, the receiver has plenty of signal. Four watt HT at 5 miles = full quieting.

i = isotropic and d = unity as you undoubtedly do know... :lol:

Ron Kochanowicz replied this morning and confirmed that he did perform an iso-T test while he had it on the bench prior to shipment and was "getting solid decode down to less than 0.25 uV with duplexer installed."

Even so, he suspects it may well be a duplexer desense issue. BTW, I am definitely not in any kind of RF congested environment.

 

[kayn1n32008]

An antenna tuned specifically for 460-470 MHz falls outside the 'hammy band' entirely.

It is a Comet antenna... Regardless as to whether it is tuned for the ham band or the commercial band the antenna quality is of cheap hammy garbage. Comet is Comet is Comet. It's junk.

When placed in "Base" mode, the receiver has plenty of signal. Four watt HT at 5 miles = full quieting.

You are having desense problems. What kind of jumpers are between your duplexer and the repeater. I would start looking there.

i = isotropic and d = unity as you undoubtedly do know... :lol:

dBd = gain over a 'd'ipole.

Even so, he suspects it may well be a duplexer desense issue. BTW, I am definitely not in any kind of RF congested environment.

If I remember correctly, you were getting 70-ish dB isolation... In retrospect @5MHz separation you should be able to get more. First thing would be to get rid of the Rg-213 and use something 100% shield or double shielded.

 

[N4GIX]

You are having desense problems. What kind of jumpers are between your duplexer and the repeater. I would start looking there.
<snipped for brevity>
If I remember correctly, you were getting 70-ish dB isolation... In retrospect @5MHz separation you should be able to get more. First thing would be to get rid of the Rg-213 and use something 100% shield or double shielded.

The jumpers in the repeater case are double-shielded MIL-C-17 RG-214/U.

FWIW, I just received an email from Ron offering to replace the duplexer under warranty if I send the repeater back to them.

The RG-213 jumper from outdoors to the repeater will be replaced with 1/4" Superflex in the very near future.