Baffled... when more dB gain might not be the answer.

[gman1971]

So, I am still trying to understand why my RX range nearly tripled in range when going from a high end Diamond 7.8 dBd gain, a 5/8 over 5/8 over 5/8 (yes, a triple collinear) vertical to a single 5/8 vertical (which has virtually no gain over a 1/4 wave)

 

The rest of the system was identical in both cases, 15 feet 1/2 Heliax with N-connector (all trimetal), a preamp (which was OFF in both tests), two TX/RX Bird tuned cavities set with a 0.59 dB insertion loss each and 70+ dB attenuation at anything beyond 0.5 Mhz (to deal with the flamethrower uber-1400-foot-tower) and a Vertex Standar EVX-5300 radio.

 

So, initially I went the route of adding more dB gain to the system, first added a high gain antenna, then cavities, then the preamp, all that under the clear assumption that more dB will get me more RX/TX range, but so far it seems that the more gain I throw, the faster the RX range goes south. While the TX range is noticeably better with the 7.8dBd antenna, what I can't understand is why the RX decreases? shouldn't the RX range increase as well with a higher gain antenna?

 

Any help would be appreciated.

 

Thanks!

 

G.

[Lscott]

What does the local terrain look like around the site?

 

Another point to consider is very high gain vertical antennas have a narrow radiation pattern perpendicular to the antenna. General reciprocity antenna theory says the RX and TX patterns should be similar.  Anyone close in will have a hard time getting into a repeater like this since the antenna is deaf if you're not in the RX pattern. Commercial broadcasters and some repeater installations use antennas designed so that the narrow radiation pattern is not perpendicular to the antenna but has a small "down tilt" angle of several degrees to compensate some what for this condition.

 

A lower gain antenna has a wider RX and TX pattern perpendicular to the antenna. The gain fall off is less severe. That's why you read about people claiming that a simple 1/4 wave on UHF gave them better results in hilly terrain verses using a high gain antenna where other stations are at different elevations relative to each other. 

[gman1971]

Hey Scott, happy new year and thanks!

 

Terrain is fairly hilly around here, lots of altitude changes for sure.

 

The interesting fact is that with the 7.8 dBd antenna it was pretty much a solid coverage 5 miles out, literally, it didn't matter if you were inside the car, inside a building, nada, anywhere within that 5 mile magic range it was pretty much full quieting on the base; now, if you passed that 5 mile mark line, it went from full quieting to it wouldn't break the squelch at all. Now, with the single 5/8 atop I can hear degradation in the signal as distance increases, and while at 5 miles it doesn't full quiet the receiver, when you pass that distance it doesn't go dead anymore...

 

Interesting, so for hilly terrain its better to use a lower gain antenna then, I guess that would corroborate what other users are stating.

 

Looks like the single 5/8 is the antenna will remain in the mast. Potentially I could tilt the higher gain antenna a bit down towards the area of interest, see if that makes any difference, giving up 7.8 dBd is hard to swallow and I can certainly tell now when we are inside buildings that before would've been crystal clear, now its super noisy, or it simply won't even break the squelch at all, but you walk outside and its good to go.

 

What are your thoughts on using preamps for UHF/VHF, could the lower gain antenna benefit from a 10dB gain preamp? I already have two tuned cavities after the preamp (which is off ATM)

 

Thanks!

 

G.

 

 

What does the local terrain look like around the site?

 

Another point to consider is very high gain vertical antennas have a narrow radiation pattern perpendicular to the antenna. General reciprocity antenna theory says the RX and TX patterns should be similar.  Anyone close in will have a hard time getting into a repeater like this since the antenna is deaf if you're not in the RX pattern. Commercial broadcasters and some repeater installations use antennas designed so that the narrow radiation pattern is not perpendicular to the antenna but has a small "down tilt" angle of several degrees to compensate some what for this condition.

 

A lower gain antenna has a wider RX and TX pattern perpendicular to the antenna. The gain fall off is less severe. That's why you read about people claiming that a simple 1/4 wave on UHF gave them better results in hilly terrain verses using a high gain antenna where other stations are at different elevations relative to each other. 

[Jones]

I had good luck running a pre-amp on a repeater reciever until the VHF high-band TV station on the same tower went digital. ..so your mileage may vary.

 

Also, when you say: "I already have two tuned cavities after the preamp", I assume you mean before the pre-amp? All filtering on the RX side should be done before it hits any amplification, so as not to amplify the noise and out-of-band stuff also. 

 

                                                      additional cavities - pre-amp - receiver.

                                                    /

Thus: Antenna feed - Duplexer -

                                                    \

                                                      Power amp - exciter or Transmitter

[BoxCar]

With the high gain antenna and a pre-amp you may have been overloading the receiver front end which in itself contains amplifier circuits to boost signal levels

[gman1971]

Hey Jones, thanks!

 

Hmmm, that is an interesting fact there, I am pretty sure all the RF stuff coming from that 1400 foot tower is all digital by now... so I would be willing to bet that is the reason why the preamp didn't produce more range.

 

This is for a simplex setup, so there is no duplexer, the preamp has a switch inside that seems to work rather well.

 

I've tried both ways, before cavities and after cavities, both configurations produced reduced range... so it could be as @Boxcar stated, too much gain overloading the RX of the radio...

 

Now that I have the preamp, I will probably experiment further with it (just got it a few days ago), and potentially moving to a 1/4 wave vertical on the mast...  like @Lscott explained, it could be the elevation changes what was is killing the range... making any gain on the antenna not feasible.

 

Thanks and happy new year, guys.

 

G.

 

 

 

 

I had good luck running a pre-amp on a repeater reciever until the VHF high-band TV station on the same tower went digital. ..so your mileage may vary.

 

Also, when you say: "I already have two tuned cavities after the preamp", I assume you mean before the pre-amp? All filtering on the RX side should be done before it hits any amplification, so as not to amplify the noise and out-of-band stuff also. 

 

                                                      additional cavities - pre-amp - receiver.

                                                    /

Thus: Antenna feed - Duplexer -

                                                    \

                                                      Power amp - exciter or Transmitter

[marcspaz]

I've tried both ways, before cavities and after cavities, both configurations produced reduced range... so it could be as @Boxcar stated, too much gain overloading the RX of the radio...

 

Hey, G... Do you have a meter that can read 0.XX μV scale?  I thought you told me you have an analyzer and the voltage was low.  Is that correct? If so, what is the voltage reading going into the radio from the antenna transmission line, without the preamp? 

 

The filters help eliminate noise if you have poor selectivity, but the preamp is to help poor receive sensitivity or high line loss.  Most good receivers will have a receive sensitivity of 12 dB SINAD on a signal that is 0.25 μV.  Some are 12 dB at 0.20 μV or better.  If the voltage on the antenna line (at the radio input) already meets or exceeds the radio requirements, the preamp won't do anything good.  They are typically for signal loss due to high loss lines or signals filtered/trapped excessively.  

 

... and potentially moving to a 1/4 wave vertical on the mast...

 

 

You may be better off with a 5/8 wave.  Do some research on the antenna takeoff angles of specific antennas you are interested in.  In the same way that high gain antennas are more narrow with regard to their usable frequency, some antennas tend to radiate up and out instead of down and out.

 

Also, have you considered a vertical dipole?  Or a loop?  They are very easy and cheap to make... so you can experiment.  Full wave loop antennas are true omnidirectional and fantastic with regard to receive sensitivity.  I am about to build one for 80 meters after seeing a friend of mine a few miles away from me have full-quiet conversations with people I almost could hear at all.

[gman1971]

Always super helpful! Thanks Marc!!

 

So, as for the 0.XX uV scale, could I measure that using the nano VNA? I can read dB on the S21 side of the VNA... I've used that to determine noise floor measurements on the new antenna.

 

The EVX-5300 is rated at 0.25 uV 12 dB SINAD.

 

Certainly agree, the 5/8 will probably remain up for the foreseeable future. Perhaps those extra 4 feet of height I added to the mast did help...

 

When I took the triple 5/8 apart after bringing it down I noticed it had water inside, some of those foam inserts that keep the antenna straight up inside the radome were soaked, so perhaps that was part of the reason why performance was so weak. I won't be able to put the triple collinear 5/8 wave back up on the mast until the 10th, but it would be interesting to see if these extra 4 feet of height is what really made the difference, or if the radiation pattern is the culprit, or both...

 

I have considered a dipole. I've looked at those commercial folded dipoles as well, these probably seem the way to go. A loop has never crossed my mind, but it looks like I have some homework to do now! Thanks!!!

 

Wow, an 80 meter loop? that is going to be one heck of a giant antenna!! hahaha... 

 

G.

 

 

Hey, G... Do you have a meter that can read 0.XX μV scale?  I thought you told me you have an analyzer and the voltage was low.  Is that correct? If so, what is the voltage reading going into the radio from the antenna transmission line, without the preamp? 

 

The filters help eliminate noise if you have poor selectivity, but the preamp is to help poor receive sensitivity or high line loss.  Most good receivers will have a receive sensitivity of 12 dB SINAD on a signal that is 0.25 μV.  Some are 12 dB at 0.20 μV or better.  If the voltage on the antenna line (at the radio input) already meets or exceeds the radio requirements, the preamp won't do anything good.  They are typically for signal loss due to high loss lines or signals filtered/trapped excessively.  

 

 

 

You may be better off with a 5/8 wave.  Do some research on the antenna takeoff angles of specific antennas you are interested in.  In the same way that high gain antennas are more narrow with regard to their usable frequency, some antennas tend to radiate up and out instead of down and out.

 

Also, have you considered a vertical dipole?  Or a loop?  They are very easy and cheap to make... so you can experiment.  Full wave loop antennas are true omnidirectional and fantastic with regard to receive sensitivity.  I am about to build one for 80 meters after seeing a friend of mine a few miles away from me have full-quiet conversations with people I almost could hear at all.

[marcspaz]

Some makes/models VNA have digital volt meters built into them and some have directions on how to use an external digital volt meter while the VNA sweeps.  You would have to look at the directions for the VNA. The ones that use an external meter should have a clearly marked port.

[gman1971]

This one has just S11 and S21, I don't think it can measure the voltage...

 

G.

[gman1971]

Looks like the loop is a horizontally polarized antenna, so that probably won't work. Certainly easy to build. The folded dipole phased array might be the better alternative. Perhaps the double zepp is another one I've been eyeing, not sure about the radiation patter on those..

 

Also, there seems to be a way to alter radiation pattern with stacked/phased dipoles.

 

I live in one of the highest points of the area too, so perhaps that is why the 5/8 is working better. Things are starting to make sense.

 

G.

[marcspaz]

G, what dictates polarization is size in wavelength and physical orientation. A full wave is omnidirectional.

[marcspaz]

G, here is a good link you can use. It will help you understand how a horizontal quad loop works, why its omnidirectional, and why there is such good natural noise reduction in the design.

 

I am building this to scale for 80 meters, but you can easily build a VHF version. My friend has his fed with ladder line in the corner closest to his house and its 35 feet off the ground, due to the frequency its being used on.

 

You cut the ladder line to length for tuning the SWR, as the ladder line is part of the antenna design.

 

http://www.caarc.ca/articles/horizontal-quad-antenna

[gman1971]

Very nice!! I am not sure if those will play well with the vertically polarized 1/4 wave car antenna... I guess I can just build one and try

 

Thanks Marc!

 

G.

 

G, here is a good link you can use. It will help you understand how a horizontal quad loop works, why its omnidirectional, and why there is such good natural noise reduction in the design.

I am building this to scale for 80 meters, but you can easily build a VHF version. My friend has his fed with ladder line in the corner closest to his house and its 35 feet off the ground, due to the frequency its being used on.

You cut the ladder line to length for tuning the SWR, as the ladder line is part of the antenna design.

http://www.caarc.ca/articles/horizontal-quad-antenna