Silly question

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Re: Silly question

by G0DJA » Sat Jan 04, 2020 1:07 pm

RG213 is reasonable on 144MHz, loss per 100 metres about 6.9dB, so your run of 12m means a loss of about 0.828dB. So, if you put 10 watts in at the bottom, you should get 8.26 Watts out at the top. Assuming a good match to the antenna, and I don't just mean what you read on a typical Amateur Radio VSWR meter...

The thing is, that loss is the same on the signal coming down the cable as well, which is why most stations use some form of mast head pre-amplifier, to overcome the losses on the received signal.

I'd always say that any form of "White stick" antenna, especially one that says it will do three bands, is going to be a compromise.

For general FM simplex contacts, or using repeaters, with the occasional unexpected "DX", your set up is OK. However, if you ever get seduced to the dark side of VHF/UHF DXing, you will rapidly come to realize that, for a little extra effort, a better antenna and a pre-amp will pay dividends.

Re: Silly question

by sidspop » Sat Jan 04, 2020 12:17 pm

Current feeder is RG213, for a run of 12 metres max. Can't remember exactly. Through the wall, up the outside of house to chimney.
For the costs involved I will just change the lot, no point messing about.
I have never been one for the final tenth of a dB, but what is up I expect to work, and this has highlighted the issue.
Will replace antenna with X700, and new feeder.

Re: Silly question

by G0DJA » Sat Jan 04, 2020 12:04 pm

M0XXQ wrote:
Fri Jan 03, 2020 8:51 pm
My money is on faulty coax, the reason behind my theory is power.
lets say you have one or several micro-fractures in the coaxial cable, on RX this is significant why? Because there isn't enough voltage to breakdown the air in the micro-fractures, however on TX lets say you're sticking 100W into an ideal match of 50 Ohms that will give you around 200V PP, more than enough to breakdown micr-fractures in cable.
Check and/or replace the coax and report back, I might be wrong but my gut is telling me its your feeder
It's a possibility, but in that case I'd put money on either a faulty connection, or water in the coax.

I have heard of microfractures on Echoflex cable, where there's a solid outer sheath round the braid, but only when people wind it up and unwind it again, as in a contest station, but they cured that by using a different type of cable. I've not had any problems with Echoflex15 permanently installed, used on 23cm, for many years. I've not heard of it happening with the more commonly used cables.

As an aside, what type of cable are you using. Even for 144MHz at least UR67 would be a minimum (I am breaking my own rule at the moment, feeding a small 70cm antenna with the thinner RG213 but I know it's going to be lossy, I just have not got round to running a length of UR67 yet. When I do, I expect to notice a difference, or else I'll know that length of UR67 is duff (I've had some of my cable over 20 years and I've not exactly been kind to it...)

I did hear people complain about the connectors on Echoflex15 having the centre conductor pulling out of the connector, but again I've not suffered this. Maybe because I didn't leave the weight of the cable pulling on the connector.

Re: Silly question

by sidspop » Sat Jan 04, 2020 11:07 am

M0XXQ wrote:
Fri Jan 03, 2020 8:51 pm
Okay lads, gonna throw this one out for the craic - My money is on faulty coax, the reason behind my theory is power.
lets say you have one or several micro-fractures in the coaxial cable, on RX this is significant why? Because there isn't enough voltage to breakdown the air in the micro-fractures, however on TX lets say you're sticking 100W into an ideal match of 50 Ohms that will give you around 200V PP, more than enough to breakdown micr-fractures in cable.
Check and/or replace the coax and report back, I might be wrong but my gut is telling me its your feeder.

Cheers

Rob.
That seems to be a sensible suggestion.
I had already decided to replace that antenna anyway, I never use 70cms and 6 metres will be redundant as I have a 5 ele to go up. I also have a few rolls of new feeder, so in terms of safety going up the roof I would rather change the antenna and feeder at the same time.
Doesn't give a definitive answer to question, however I could use the 'faulty' feeder as a connecting lead between sig gen and radio, and check what the performance is like on known signals.

Re: Silly question

by VK5TM » Fri Jan 03, 2020 11:56 pm

Have you thought of checking some of the remote receiver sites to see if you are actually getting out?

There is the Reverse Beacon network for digital modes, but you can use WebSDR's for voice modes, you can either wear headphones to listen to your own signal or inject a test tone for a short period (followed by your callsign and "testing" of course).

Re: Silly question

by M0XXQ » Fri Jan 03, 2020 8:51 pm

Okay lads, gonna throw this one out for the craic - My money is on faulty coax, the reason behind my theory is power.
lets say you have one or several micro-fractures in the coaxial cable, on RX this is significant why? Because there isn't enough voltage to breakdown the air in the micro-fractures, however on TX lets say you're sticking 100W into an ideal match of 50 Ohms that will give you around 200V PP, more than enough to breakdown micr-fractures in cable.
Check and/or replace the coax and report back, I might be wrong but my gut is telling me its your feeder.

Cheers

Rob.

Re: Silly question

by G0DJA » Fri Jan 03, 2020 5:27 pm

Those multiband "White stick" antennas are a bit of a compromise. Ignore the claimed gain figures, they are often optimistic and are often quoted in meaningless units like "XdB gain", compared to what, a piece of wet string? When you dig deeper, you often find that they quote the gain over an isotropic radiator (in dBi) which is a theoretical radiator that transmits a signal over a uniform spherical pattern. That gives them 2.15dB more gain to claim than from something compared to a dipole (dBd).

Your mates 3 element Yagi will have a gain of about 7.5 dBd. Your white stick is throwing the RF in all directions and, on looking the specification up I see that they have, indeed, used dBi for their gain claims. So, your V2000 has a real world gain of about 4.05 dBd on 144MHz, if you believe the claims of the manufacturer. That's the same as your mate using a V2000 as well, but doubling his power output. (7.5dBd - 4.05dBd = 3.42dB and every 3dB extra is the same as doubling the power output).

I wouldn't trust a rigs "S-meter" reading either, they often don't keep to the "6dB per S point" standard and especially when the needle goes past the 'S9' reading. One manufacturer often sets their S-meter readings to a different value to another. The ARRL used to test new radios and the discrepancies where sometimes quite large.

Again, I assume your mate is using his Yagi in vertical polarization? If not, then you can take another 1dBd or so off your gain figure for the vertical, although there's some debate about whether this actually happens when signals are being reflected and bent by propagation effects.

At 1st I thought you were saying that your mate could hear you, but you couldn't hear him, but you mention a station on the Isle of White. If we are talking about your mate hearing the guy on the IoW and you can't then that just adds another level of complexity. How far away are you from each other? Propagation can be very good in one spot, I've heard of people working Spain on 144MHz FM using handhelds from Cornwall when people at home, a few miles away, hear nothing even though they have, technically, a better set up. It can be really frustrating to see DX Cluster spots from a neighbour, only a few miles away, working long distances but you can't hear a thing but, one day, the boot is on the other foot and you are working long distance stuff and your neighbour is hearing nothing this time. It gets 'worse' the higher you go in frequency. The guys near the coast, using 23cm and above, often hear and work DX that we poor people further inland don't hear due to sea paths. Then, another day, a good duct throws signals down in the middle of the country, going over the heads of the guys on the coast...

Re: Silly question

by sidspop » Fri Jan 03, 2020 4:43 pm

Thanks for the reply,
stations are similar in terms of feeder lengths, mine may be slightly longer. Both using RG213, mate does not have a pre-amp.
Mine is 3 band white stick (V2000), he is using a 3 element. His antenna is lower, so less feeder loss (if that could be a factor), but I have an advantage in height.
At one point Tom was receiving at 9 plus 60, and I could not hear the bloke in the I of W.
Using the assumption of 6dB per S point, that's well over 100dB difference, and I somehow doubt if that could be attributed to a 3 element.

Re: Silly question

by G0DJA » Fri Jan 03, 2020 11:10 am

There is a phenomenon where one station is being heard when the other isn't. Sometimes called 'one way propagation'. It is rare, but not unheard of.

Just a few questions. What antenna was your friend using? Was he using more power than you were? Were you using similar antennas, did his have more gain than yours? Were you both using vertical polarization? Was your friend using a pre-amp?

Unless you both were using exactly the same station, right down to the same feeder losses and front end sensitivities, you are not comparing like for like.

On 23cm I often hear well stations that have better locations, or are running more RF power to better antennas than I am and then struggle to get them to hear me. Although, it helps if they have really good receiving set ups as well.

The other day I worked a French station at over 1000kM on 23cm. I was running about 10 Watts, he was running a lot more but to a dish antenna with a low noise pre-amp and we worked each other easily. Later I struggled to get through with another, closer, station possibly due to them not hearing me as well as I was hearing them as I was too low a signal for them to hear easily with their set up.

Silly question

by sidspop » Fri Jan 03, 2020 2:35 am

his may appear to be a daft question, but all the best questions are......

I operate mainly on two metres, and for some time I have had doubts about my antenna system.
Using a tri-band vertical on the chimney (6/2/70) and good radios. I have checked radios on sig gen, including FT847, IC746, and FT221 with Mutek, and they are all sensitive, can hear signals down to -125dBm.

Bit of a lift on today, a local mate called in a break from the Isle of Wight (I am by West Brom), I could not even detect he was there, but he could apparently hear me, peaking 5&9.

Is it possible that an antenna can radiate well, but not receive well. SWR is good, for what that is worth. There is a possibility I have put excess power in to it, and may have popped caps, as the caps fitted tend to be low voltage.
It has me scratching my head

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