home brewing a gain master ???

[PsiDOC]

Well Done, PsiDOC.

I broke my own rule and paid attention to the speculations that this antenna could not be properly duplicated until the exact same type coax was employed. It appears you have done it otherwise, and beat me to it in the process.

Perhaps the one imperative question to be answered is whether this one puts the same gain on the horizon the original is said to be doing.

Thanks for posting up. Now I guess I'll have it to do.

Homer

There's a lot of naysayers and self proclaimed experts about Homer. However you never know till you try. It's quite a simple antenna. No fancy tuning circuits or coils to get wrong, just the matching stub which it seems I need to develop further.
I get a kick out of making antennas out of bits of wire and feedline - my first antenna was a home made end fed zepp made from ladder line and wire, so this was right up my street.
As a footnote. Using the maths derived from the stub point location with RG58 vs Stub point on the original I can safely say the VF of the coax used in the original to be approx 0.78 which falls in line with the foam dielectric, so we can even speculate what coax was used in the original with greater accuracy.

@ Shockwave.
Have a lump of westflex W103 here. I know it's an air spaced dielectric however it's about the lowest loss coax I know of thats readily available, and indeed lower loss than RG303. Would that be ok for the matching stub?

Cheers.

Psi

 

[Marconi]

Great job, Psi.

Thanks for sharing and answering my questions. Questions work great when you find a man with answers, <gotproof>, and not just words.

 

[Shockwave]

@ Shockwave.
Have a lump of westflex W103 here. I know it's an air spaced dielectric however it's about the lowest loss coax I know of thats readily available, and indeed lower loss than RG303. Would that be ok for the matching stub?

Cheers.

Psi

As long as it's good low loss cable it should be fine. However, as you might know the velocity factor may be different than Teflon and the physical length of the stub will have to compensate for this. By the way, I'm impressed! The fact you have nearly replicated the original SWR curve is a good indication you have all of the sections of the antenna tuned correctly.

 

[PsiDOC]

That's what I wanted to hear. This means I have the bits I need to hand so no waiting time for delivery.
Already calculated the stub length. W103 vf = 0.85 so it's 85*0.85 = 72.5 cm or there abouts. Will cut at 75cm and trim to suit.

I love it when a plan comes together!

Si

 

[PsiDOC]

Another follow up. After doing some research, using RG58 for the matching stub was a big no no - Thank you Shockwave for pointing this out - it's appreciated.
According to the calculations I have done approx 40 - 45% of the power being fed into the antenna was being lost in the stub. Not good at all! By using westflex 103 as the stub these losses have been significantly reduced.
So please... DON'T USE RG58 FOR THE STUB! Westflex 103 can be bought for about £1.70 / Metre and it'll be the best £1.70 you'll spend if you make this antenna.
The antenna is back on the test pole and working well. Height above ground is 2.5 meters to the coax choke (9.5 to the tip) and approx 75m above sea level so quite low in both respects. Yesteday (friday) on 28mhz 7 solid contacts into the East US (Staten Island NY, Upstate Ny and Pennsylvania) were made with the lowest signal being a 5/7. Switching to the wire made very little difference to signal rx or reported tx back.
Sadly cb / 10m local traffic are non existant here so I cannot test with any local stations.

Now I can finally finish answering the questions put by Marconi.

Before you modified your homebrew using Shockwaves stub coax idea, do you think the matching results, resonance, and bandwidth compared similarly to the published Gain Master results?

Yes the matching results resonance and bandwith were on a par with the original using the RG58, Using W103 for the matching stub has made a little difference on the lower end of the band as I am hitting 1.9:1 at 25.7 mhz instead of 25.5 mhz.

Have you recorded any contact results to compare if and when you modify the stub?

I did a field strength test before and after changing the stub and I did notice an increase in field strength in the region of 1/3 deflection of the full scale on the meter. Please note: the meter used was basically a last minute lash up of the field strength meter posted on this page and stuffed inside an old swr meter case that had been sitting in the junk box. The antenna used on the meter was an old 11 meter springer type 1/4 wave mobile antenna.
As explained above. There's no one local on the air to test with which is a pity
Psi

 

[PsiDOC]

And here for your viewing pleasure is an as built diagram with all the relevant numbers and materials used.

Psi

 

[RADIOOMAN]

I did a field strength test before and after changing the stub and I did notice an increase in field strength in the region of 1/3 deflection of the full scale on the meter. Please note: the meter used was basically a last minute lash up of the field strength meter posted on this page and stuffed inside an old swr meter case that had been sitting in the junk box.

I wanted to do the math on these numbers but am unsure as to how to quantify the '1/3 feflection of full scale on the meter".

I am trying to assume that it means an increase of 33 percent.
If that is correct then please reply accordingly.

Thank you

Please bear with me forum memebers as I have not figured out how to use some of the functions associated with posting in this forum.

 

[Shockwave]

PsiDoc, If you want the antenna to handle any power it would be a good idea to replace the RG/58 cable used to form the capacitor too. Your Westflex cut to the length that provides the equivalent capacitance will greatly increase power handling. The issue here is the coax cap tends to arc on the cut end. Care should be taken to cut the braid clean and a little shorter than the center insulation. You don't want the outside braid and inside conductor to have their cut ends line up. The center conductor and insulation should extend just past the braid to reduce the chances of arcing.

 

[PsiDOC]

I wanted to do the math on these numbers but am unsure as to how to quantify the '1/3 feflection of full scale on the meter".

I am trying to assume that it means an increase of 33 percent.
If that is correct then please reply accordingly.

Thank you

Hi Radiooman. That is exactly correct. I set the meter to 50% of full scale when testing with the RG58 stub at 25 watts. Then after changing the stub this reading increased to about 85%. I am sorry I cannot be more accurate than that. The field strength was a last minute idea so not calibrated. I was more intrested in seeing if I got an increase or decrease in field strength with the stub change than actual values.

PsiDoc, If you want the antenna to handle any power it would be a good idea to replace the RG/58 cable used to form the capacitor too. Your Westflex cut to the length that provides the equivalent capacitance will greatly increase power handling. The issue here is the coax cap tends to arc on the cut end. Care should be taken to cut the braid clean and a little shorter than the center insulation. You don't want the outside braid and inside conductor to have their cut ends line up. The center conductor and insulation should extend just past the braid to reduce the chances of arcing.

Shockwave. I have been over your high power thread a few times and actually adopted your method when making the capacitor.
To be honest I wouldn't use RG58 at all over 120 watts. That'd be going into meltdown territory IMHO. If I wanted a really high power version I'd use Westflex 103 for the radiator with something a lot more exotic for the choke, or even rework the choke using W103. I know I keep banging on about Westflex - sorry about that, it is however the best readily available off the shelf coax we get here in the UK and I just happen to have 1/2 a 300 foot drum in the garage.

Psi

 

[northern35s]

For most installations I wouldn't worry too much about the power handling capability of quality RG58 at 27MHz:

Belden Technical Info

 

[Mudfoot]

For most installations I wouldn't worry too much about the power handling capability of quality RG58 at 27MHz:

Belden Technical Info

They are talking a whole different ball game.

 

[Shockwave]

For most installations I wouldn't worry too much about the power handling capability of quality RG58 at 27MHz:

Belden Technical Info

The rated specifications for a given cable assume the load impedance matches the cable impedance. In a 50 ohm system RG-58 can easily handle its full rated power. Once the load impedance begins to deviate from the cables impedance, voltage and current spikes will be much larger than normal and will develop at various places along the cable related to the wavelength.

If the voltage spikes too high, the cable will arc between its conductors. When the current is too high, lots of heat will be dissipated in the cable and that directly corresponds to loss. In the case of the shorted tuning stub used in the Gain-Master, there are very high currents present on this cable. That causes the power handling of the cable used in this application to be significantly lower than normal with higher losses.

 

[W5LZ]

"The rated specifications for a given cable assume the load impedance matches the cable impedance. In a 50 ohm system RG-58 can easily handle its full rated power. Once the load impedance begins to deviate from the cables impedance, voltage and current spikes will be much larger than normal and will develop at various places along the cable related to the wavelength.

If the voltage spikes too high, the cable will arc between its conductors. When the current is too high, lots of heat will be dissipated in the cable and that directly corresponds to loss. In the case of the shorted tuning stub used in the Gain-Master, there are very high currents present on this cable. That causes the power handling of the cable used in this application to be significantly lower than normal with higher losses."

And there you have it! The reason why most coax cable is damaged electrically. It's also why SWR is significant at all, and why using feed line length as a means of adjusting SWR is not a very good idea in all cases. 'Break down' voltages used to be given for coaxial cable as a standard rating. That was changed some time ago to it's 'power rating'. Same basic information but that break-down voltage has to be 'found' by the user. 'Finding' that voltage means the user has to know -how-, and that's just not too likely in most cases.
Makes you wonder, don't it??
- 'Doc

 

[PsiDOC]

... and hence why I wouldn't run more than 120 watts into the antenna using RG58.

Psi

 

[northern35s]

I think the point is being somewhat lost, it has already been established that the capacitor and tuning stub benefit from a lower loss cable, that isn't being questioned, however the feed, and the rest of the antenna shouldn't suffer with most installations, IIRC the original Gainmaster was rated at 500W, but many failures of the capacitor were reported at powers less than this.