Inductive or Capacitive

[flintstone mop]

Happy Holidays
Here's a good mind bender.
I'm trying to troubleshoot a vertical antenna to lower the SWR for 75M. All the adjustments and laying of ground radials has been done, but the SWR is still 2:1.
My version of MFJ 259 cannot tell me why I cannot achieve a lower SWR.
What I'm trying to say is that it doesn't show me that there is excessive capacitive reactance or excessive inductive reactance. The  manual kinda hints that there is a way to find out which is what.
If you increase the frequency of the 259 and the impedance meter is rising then you have an inductive reactance. If you decrease the freqency and the impedance meter reads lower then you have capacitive reactance.
So, I'm thinking that I could overcome the inductive reactance by increasing the capacity hat that is on this particular antenna, by pulling the rods out more.

Am I on the right track here?? Am I getting nearer to my problem??

 A thought back in my crusty brain waves
I had this all figured out when I owned the Unihat vertical and I had to roll my own coil at the bottom of the mast. 1 KW of RF was melting the one provided by the manufacturer HA! Stretch the coil for less inductance and squeeze for more inductance. The capacitance was set by the size of the hat on the top. There was no adjustment for changing the capacity hat. The antenna I'm working with now, the inductance is set and the capacity hat is adjustable.

I hope I have not confused yous guys. I can always call someone by landline to talk more.
THanks

fred MOP radio

[Warren]

Fred,

    Did you sweep the MFJ 259 up and down the band to find which frequency has the lowest swr? Does your vertical have a loading coil or just a capacity hat? You say that you can adjust the capacity hat, if the lowest swr is at a frequency below your desired operating frequency then you need to reduce the capacitive top-loading, if the lowest swr is at a higher frequency than desired then you need to increase the capacitive top loading.

    It's also possible that the antenna is resonant and  you have a resistive load that is either 25 or 100 ohms. That is very possible - for example ground loss + loss in the loading coil and you might have 100 ohms input impedance at resonance. Conversely, if you have an excellent low loss ground (as in many radials in a salt marsh) and no loading coil (or a very high-Q coil) then you might have a 25 ohm input impedance for a top-loaded vertical. If either of these is the case then a 2:1 torroidal matching transformer would do the trick.

  Best of luck,
73 Warren K2ORS/WD2XGJ

[flintstone mop]

Thanks Warren.
The point of lowest SWR for this particular antenna is 2:1. I'll have to experiment more when the rain stops and try to go lower in freq. and see if the SWR gets better. I noticed that getting the correct balun is very critical for this antenna. OK I'll admit it....it's a miracle antenna from Cushcraft, The MA8040. A lot of good reviews on E-ham! And the price is great!!
With no balun, everything goes to hell. With my HB balun of 12 turns around a coffee can. (can removed) I have my present readings using RG213
Cuschraft recommends a 1:1 current balun. 12-14 turns of RG 303 around an FT240-73 torroid. (2 1/4 in OD)
Right now the ant is adjusted for the 75M portion of the band. The capacity hat is adjusted for minimum rod extension. I even removed a rod and the SWR increases slightly. There are 8 rods. The inductance is fixed. Above the rods is a stinger portion that can be extended to lower the resonance for 80M. The 40M is a fixed length of about 21 feet of mast before the 40M trap. I have a nice SWR 1.2:1 in the center of 6965-7265.
80M seems to be the tricky part of this aerial.
I'll play more with it. The system might be at it's best right now, who knows??? I'm not an SWR freek, but when you throw a solid state Tx on a 2:1 load, bye bye to full power and I don't want an antenna tuner in the line.

I'm reading more of your message here too.

Fred

[Bacon, WA3WDR]

Hi Fred,

Yes, for a given length vertical, generally you need more inductance and/or capacitance when you want to go lower in frequency, and less when you want to go higher.  But the impedance of a short vertical is very low - 25 ohms is easily believable, and it should probably be even lower than that.  A full quarter wave vertical is about 37 ohms plus ground resistance.  You may have a better ground system than the antenna designers expected.  You can tune out the reactance and still not have a 50 ohm load, and that seems to be what is happening to you now.

I agree that you can use a transformer, to step down from the 50 ohm cable to the antenna feedpoint.  A big toroid ferrite autotransformer with multiple taps might be the easiest thing.

You might try a coax matching setion.  Quarter wave matching sections of coax are tricky - you might get somewhere using two 1/4 wave (electrical) 75 ohm or maybe 50 ohm lengths in parallel as a matching section, and maybe you can get in between with one 75 ohm and one 50 ohm piece there, but you can't tweak the match easily.

Unfortunately you'll either have to run out to the antenna to switch bands, or put in a relay to switch remotely, once you get the settings right.

[Warren]

Fred,

   Sounds like you are on the right track. Just sweep the MFJ 259 and don't be afraid to go past the band edges. Possibly the antenna is resonant outside the ham band. From your comment about the swr going up further when removing a capacity hat spike makes me think that you may need to go in the other direction and increase the length of the capacity spikes.
  It's not surprising that 80 meter tuning  is more critical than 40 meters, there is more loading on 80.
  O.K. on the current balaun, your 12 turn coil of coax should be as good or better than the ferrite balun.
 
One other thought are you using in ground or insulated above ground radials? I've found that with ground plane antennas with above ground radials the length of the radials are fairly critical in determining the resonant frequency. For broader bandwidth with a ground plane using above ground insulated radials use a variety of radial length from a little shorter to a little longer than 1/4 wave

73 Warren K2ORS/WD2XGJ

[Warren]

  But the impedance of a short vertical is very low - 25 ohms is easily believable, and it should probably be even lower than that.  A full quarter wave vertical is about 37 ohms plus ground resistance.  You may have a better ground system than the antenna designers expected. 

   Fred,
          I don't think the impedance is likely to be so low since there is loss in the loading coil in addition to loss in the ground. I would try and make sure that the resonant frequency is where you want it before messing with transformers or matching stubs.
Best of luck,
   Warren K2ORS/WD2XGJ

[flintstone mop]

You guys on some big things.
The radials are just laying on the ground and the lengths were what Cuschcraft recommended. 7ea 66 feet long and 7ea 33 feet long. They warn that adding more may lower the impedance and raise the SWR of the antenna. The MFJ says that the impedance is 67 ohms presently. It was 78 ohms a day ago. Ground conditions are changing due to the WX.
The Cushcraft seems like it is better this morning. The ground is drying or maybe freezing and the SWR is getting lower down to 1.7:1 now. I remember reading an installation manual for an earlier attempt for a vertical back in the 90's that said, "If the SWR gets better when the ground is wet, then you need to lay more radials out under the antenna." I'm thinking reverse for the Cushcraft. It could be that the ground system is too good and the SWR will rise when wet. The antenna site is 100 feet from my house and it sits in a moist area of my property where a small stream runs through. I'm thinking I might have the ultimate antenna site for some DX.
I'll keep you posted. I think I have learned a lot more about these aluminum friends of ours. Especially the magic antennas.HA!
FRed

[Bacon, WA3WDR]

Fred, are you measuring the impedance at the shack end of the feedline?  If so, the feedline length can mess with the readings.  100 feet of RG213 is between 1/2 wave and 3/4 wave electrical at 75 meters (velocity factor 0.66 for solid polyethelene insulation), and this will give you confusing readings.  A resistive mismatch at the antenna will look like a different resistive and reactive mismatch in the shack, and likewise a reactive mismatch at the antenna can look like a resistive and/or a different reactive mismatch in the shack.

You could add another length of 50 ohm cable to make the feedline 1 electrical wavelength at 75 meters (about 160 feet at 3900 KHz for 0.66 velocity factor cable) - that would make the shack reading match the antenna feedpoint reading better.  Feedline loss will make a mismatch look a little bit smaller than it really is, but at least the resistive-reactive reading won't get rotated and be so confusing.

[flintstone mop]

hi Dave
The readings are at the antenna site. When I plug the MFJ  in the radio room the readings are the same, which says I have a transmission line, not a transformer. The balun is isolating the radio from the antenna field. When the balun is removed all readings go to hell !!
I think I have been chasing the old SWR tale. The performance will tell me what is really going on at 'MOP radio

Fred

[Warren]

hi Dave
The readings are at the antenna site. When I plug the MFJ  in the radio room the readings are the same, which says I have a transmission line, not a transformer. The balun is isolating the radio from the antenna field. When the balun is removed all readings go to hell !!
I think I have been chasing the old SWR tale. The performance will tell me what is really going on at 'MOP radio

Fred

Fred,
   If the load and the transmission line have different impedances then the transmission line will have an effect on the impedance you measure - that's why the Smith chart was invented !  The'balun' (actually a common mode choke) has NOTHING to do with what the impedance is that you measure!
In a transmission line the impedance repeats every electrical  1/2 wavelength of transmssion line. The electrical wavelength is just the wavelength x velocity factor, the velocity factor for foam dielectric coax is about 0.8 and for solid dielectric the velocity factor is 0.66 as Bacon pointed out.
 The 'balun' in your case is a common mode choke that presents a high resistance to prevent currents from flowing on the outside of the coax. It prevents the coax from becomming part of the antenna (or ground).
 If you are measuring the same impedance in the shack and at the antenna then you likely have a 1/2 wave (or multiple of 1/2 wave) of coax between the antenna and the shack.  For a good primer on how feedlines work I recommend L.B. Cebik's excellent web page:

http://www.cebik.com/fdim/fdim3.html

73 Warren K2ORS/WD2XGJ

[W2VW]

I think I have been chasing the old SWR tale. The performance will tell me what is really going on at 'MOP radio

Fred

Probably right on the money. At these frequencies with RG-8 the 2:1 loss will mean very little. You use a toob final so as long as you can dip to rated current and have no R.F. in the shack the system will work. 1:1 SWR is nice as your station metering relies on 50 Ohms resistive. Put duct tape over the SWR meter!

[Warren]

You guys on some big things.
The radials are just laying on the ground and the lengths were what Cuschcraft recommended. 7ea 66 feet long and 7ea 33 feet long. They warn that adding more may lower the impedance and raise the SWR of the antenna. The MFJ says that the impedance is 67 ohms presently. It was 78 ohms a day ago. Ground conditions are changing due to the WX.
The Cushcraft seems like it is better this morning. The ground is drying or maybe freezing and the SWR is getting lower down to 1.7:1 now. I remember reading an installation manual for an earlier attempt for a vertical back in the 90's that said, "If the SWR gets better when the ground is wet, then you need to lay more radials out under the antenna." I'm thinking reverse for the Cushcraft. It could be that the ground system is too good and the SWR will rise when wet. The antenna site is 100 feet from my house and it sits in a moist area of my property where a small stream runs through. I'm thinking I might have the ultimate antenna site for some DX.
I'll keep you posted. I think I have learned a lot more about these aluminum friends of ours. Especially the magic antennas.HA!
FRed

  Fred,
     If the real part of the  impedance is 67 ohms and the swr is 1.7:1 then you have some reactance in the system, otherwise your swr would be 67/50 = 1.34 instead of 1.7. You have some work to do to get the antenna resonant on your desired frequency.
     The fact that they tell you not to add too many radials means that they are relying on ground losses to make the antenna impedance go up to 50 ohms. Kinda like adding a resistor in the circuit, it will improve the match and broaden the antenna bandwidth at the expense of disspiating some power as heat in the ground !

73 Warren K2ORS/WD2XGJ

[K1JJ]

Hi Fred,

You've gotten good advice.  Warren, your last comments are right on...

Fred, continue working on it and you'll get it right!

As it is now, a 1.7:1 SWR is actually decent for 75M. The difference between 1:1 and  1.7:1 is not noticable in performance on the air.  But, despite what Cushcraft says, more than seven radials are needed. Ground losses.  It's all about collecting and returning the RF back to the base. Shoot for at least 60.  Too few radials is also the reason you are seeing big fluctuations in day to day readings due to WX conditions. The vertical's base impedance should go down as you add radials until in hits a point of diminishing returns for your soil. Also, as a clue, the narrower [higher Q] the swr curve, the more efficient the system is performing. ie, if the SWR is broad across the whole band, that is NG, like a dummy load. More radials needed to lower the ground system resistance to single digits, if possible..

Normally, by tapping a base coil or adjusting the tip length, you can zero the swr into what you need. I have usually had to add a variable cap at the base too. I sometimes use a parallel resonant circuit and tap the radiator off the the low end of the coil against ground. Whatever it takes.

What you should do now is make some on air comparisons between the vertical and your horizontal dipole. You may be very surprised.... Switching A/B back and forth quickly on the receiver is all that's needed. Use both close and far away stations.  Over two days write down the signal readings and get a baseline so that you can see if your future efforts are producing positive results.

My guess is that the vertical will be down at least 15db from the horizontal, even with stations farther out. Possiibly, it will be down 30db locally because of the high angle hole they exhibit. 30db is the difference between someone running 1000W and 1W.

Getting EVEN a full sized 75M vertical that is in the clear with a full blown 120 radial system  to work as well as a dipole is not easy. Even on the higher bands, like 10-40M, a short vertical  next to a house is going to get beat by your simple dipole that is matched for each band with your super tuner..

I'd be curious two months from now how often you use it over the dipole and what you think of its performance.  Let us know how the received NOISE differs from the horiz dipole to the vertical too.  [ie, Don't pull your hair out if it never meets your expectations - it's not you, OM   ]

73,
T

[W2VW]

And don't forget to ground the vertrical when using other antenna during receive. It can re-radiate noise into your other.

[K1JJ]

And don't forget to ground the vertrical when using other antenna during receive. It can re-radiate noise into your other.

Vely goo point, Dave.

The vertical could easily couple into the dipole's vertical coax or open wire run.   They become like one  antenna.  Same thing with a vertical fed tower coupling into the coax run up the tower for a dipole. Makes a capacity hat.

I had that happen with a 160M dipole and a vertical ' T'.   They were only about 60' apart and I could see no difference between them in performance. When I finally moved the dipole 300' away, the difference between the two's performance and patterns was tremendous.

If a vertical is within 60' of the dipole's feedline on 75M , then watch out.  Even 120' away can  produce distortion of the pattern and interaction.  A real fooler making one think the vertical is performing better than it really is. 

T

[Warren]

Fred,
     I did some quick modelling of the antenna with EZ-NEC. A 6.7 meter high vertical will have a radiation resistance of about 7 ohms at 3.8 MHz. If you are measuring 67 ohms then 60 ohms represents resistive loss in the loading coil and the ground and 7 ohms represents the part that does the radiating. Doing the math 7/67 = 10% efficiency.  Be careful about the eham or other reviews - some of those guys have no other antenna to compare it to! And even if they have another antenna, there may be coupling between antennas as Tom and Dave have pointed out.
 Remember you can make some contacts with a 100 watt light bulb, and the dx reports these guys often reference in the reviews are usually on 40 meters.I have even made contacts with a  Hustler vertical when the conditions are good.
73 Warren

[flintstone mop]

That's a good point Warren. I would think the standard for comparison could be the Hy-tower. When I hear about Hustlers and Butternuts I get concerned.
And Dave, the vertical is about 100 feet away from the utility pole and the Vee. At these low freqs, even 1 acre is not enough space for our aerials, but it beats a 60X100 foot lot, with your neighbor looking in your windows.
I'll lay more radials, today and tomorrow are supposed to be 'warm'. 45 F is so nice!!
Fred

[Warren]

Fred,
   There is a nice online transmission line calculator at:
http://fermi.la.asu.edu/w9cf/tran/

You select the type of line from a drop down menu, the frequency, load impedance and length of the line. The calculator shows the impedance at the other end of the line, swr at end of the line and how much additional loss you get from having a mis-matched load. The only drawback is you need the complex impedance and I believe that your MFJ is a scalar instrument that shows only the magnitude of the impedance.
The program does produce a nice graph that shows how the current and voltage vary along the length of the transmission line.
73 Warren K2ORS/WD2XGJ

[k4kyv]

The radials are just laying on the ground and the lengths were what Cuschcraft recommended. 7ea 66 feet long and 7ea 33 feet long. They warn that adding more may lower the impedance and raise the SWR of the antenna.

Incredible! They WARN you that adding more radials may raise the SWR?  Like that's supposed to be a bad thing?  I suppose that was designed to appeal to the typical appliance operator's obsession with the great SWR god.

If adding more radials lowered the impedance, that means you were adding ground loss to the resistive component of the antenna impedance.  Power was being wasted in the ground, warming the earthworms.  Put down as many radials as possible, until the impedance no longer significantly decreases, indicating you have reached the point of diminishing returns.  Any feedline loss due to standing waves (unless you have incredibly lossy coax cable) will be  much less than the loss due to ground resistance.  You  might just as well put a resistor in series with the antenna to bring the SWR to 1:1.

For example, If you have a short vertical, with say, 5 ohms resistive (not atypical of short loaded verticals) fed with 50-ohm cable, your swr would be 10:1 (ignoring any other resistive losses and any reactive component).  You could bring the SWR to 1:1 by adding a 45 ohms resistor in series with the antenna, or by using a matching network, such as a simple L-network, to transform the the impedance.  You would likely get much more rf to the antenna by coupling the 50-ohm coax directly to the vertical and running 10:1 SWR, than by matching it to 1:1 with a 45-ohm resistor, which converts 90% of your rf to heat.  This may be an extreme example, but that's essentially what Cushcraft is advising you to do.

It is not unusual for the SWR to increase once the losses have been reduced in an antenna system.  Always go with the lowest loss, not the lowest SWR.