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Author Topic: To modify or not to modify my existing antenna system That is the question!  (Read 6493 times)
W5WSS
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« on: May 02, 2013, 01:00:27 PM »

I do OK with a Hustler Mo-1 54" shaft with the super resonators installed in the center of the roof.
And I consider the existing antenna system to be optimised.

I am curious though.

How much improvement In field strength would the Hot Rodz provide when optimized.

I understand that when the Hot Rodz system is working properly that the feed point drops to a lower non inductive driving impedance requiring another purchase the step up transformer.

I have some preconceived estimates but thought I would ask here.

73
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N6AJR
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« Reply #1 on: May 02, 2013, 01:47:26 PM »

Not familiar with the hot rodz brand of antenna.  If they look like a hustler they are a center loaded (or top loaded) coil on a vert mast.  if the coil is in the bottom that are base loaded antennas, if they have a coil of wire  going all the way up ( like a ham stick) they are still different yet.  if they are tunable they either use jumpers, or and electric motor, like a screwdriver antenna.. 

pleas do a google on home brew antennas for the band you are intrested in and see what you can build, ( much cheaper too)
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AC4RD
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« Reply #2 on: May 02, 2013, 03:05:57 PM »

First off, this is the second time I've posted this link recently, and I apologize for doing it so often.  :-/

I've been using modified Hustler antennas with various homebrewed elements for a few years now--to me, they're a great way to get more performance than a hamstick while still keeping the antennas fairly cheap and small.

Here's the link: 
  http://people.duke.edu/~kuzen001/ac4rdmobile.htm

As I say on that page, capacity hats clearly DO help performance.  But I build my own, with brass rod from the local hardware store.  They're cheap to make and easy to fiddle with, to get a particular resonator/whip combination just where you want it.

I sometimes use either a capacitor or an inductor at the base of the antenna for matching, but it's rarely actually necessary.  What I do is get that antenna resonant (X=0) at the desired point in the band it's for, and the SWR is usually good enough that it doesn't need any further fussing.

Hope this helps; feel free to email if you want to discuss it further.  73!  -ken
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K7RBW
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« Reply #3 on: May 03, 2013, 06:54:53 AM »

To modify...
That's not even a question, but an assumption!

After all, ham radio is all about experimenting (IMO).
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W5WSS
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« Reply #4 on: May 03, 2013, 08:41:58 AM »

Yes Thanks of course modify vroom. Yes AC4RD I have previously looked over your site thanks for doing the work.

I like to speak technical English rather than mathematical formulas anyway...

Therefore the carbody,rather than efficient elevated tuned radials, is representing some of this loss which in turn raises the feed point sum total Rr. masking the fact that the system is suffering an inefficiency of radiating it's full load of power provided by the transmitter.

The swr looks great but the actual transfer of energy to the environment is partially lost in the loss resistance leaving only in part the difference to be radiated.

That is unquestionably possible and I have been aware of it for many years.

That is what the Hot Rodz  capacity hat add on kit is supposed to combat.

By truthfully eliminating a large percentage of loss resistance and exchanging it for a range of real surge driven Rr between 12.5 to 25 ohm range. But one must tune the system in conjunction with using a step up transformer... that is not to much of an issue just money

But.

I am however confident that my Hustler Mo-1 with Super resonator in conjunction with my particular vehicle is not suffering to the extent that the Hot Rodz would satisfy a justifiable increase in field strength relative to when either is optimised and then compared to each other.

Not knowing how much improvement is there by the Hot Rodz capacity hat add on kit relative to my existing system.

I guess what I am asking is more about whether my system  benefits any measurable or detectable radiation field strength increase by the Hot Rodz capacity hat kit enough to justify the extra work involved with using it IE. the added time and complexity of band hopping as relative to having a bunch of rodz and extra resonators some how all fitting in the car trunk pickup bed etc.

It is more a tally of inconveniences vs justifiable performance enhancement relative to the system In place.

In the end I would have to reconstruct the hat for band changing vs adding a pre tuned resonator and whip that I have ready to do in less than a minute of time.

No easy answers?

Listening to all opinions

73

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KB4QAA
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« Reply #5 on: May 03, 2013, 09:33:11 PM »

http://www.k0bg.com/caphats.html

Yes, capacity hats will definitely improve the efficiency and performance of shortened antennas like mobile antennas.

They work by distributing the RF current more evenly across the antenna.  And RF current is what creates our EM field.

They should be placed at the END of antennas.

They should be placed as far away from loading coils as possible.

You will need to retune the antenna after adding a cap hat.

See Alans website above for more info.

p.s.  "Hot Rodz" Capacity Hats are sold by DX Engineering.
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AC4RD
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« Reply #6 on: May 05, 2013, 05:07:42 AM »

W5WSS--OK, sorry, I wasn't following your question at first.  I'm using the cap hats as KB4QAA says, to increase the efficiency of a mobile HF antenna for a given length.  And I really never worry about SWR as long as it's below 3:1.  I get the antenna resonant at the desired frequency, and the SWR usually isn't any worse than 2:1, more or less.

I haven't used the step-up transformer you mention; as long as my SWR is low enough that the transmitter isn't folding back the power, I don't worry about it.  I have experimentally tested the cap hats change in a given antenna's resonance. Now, I've never actually tested this, but it appears to me from my fiddling around, that getting an antenna resonant at a given frequency is more important to performance than getting the SWR to exactly 1:1.  I have no data to support it, but that's what it feels like to me.

I am, as you say, doing the bit where I have a pre-tuned resonator/whip/hat for each band, each on a quick-disconnect.  To change bands: I pop off one resonator and pop on the one for the band I want.  In the past I've had separate ones: one for the phone portion and one for the CW portion of the band.  And special long ones for trips, where I didn't need to worry about fitting into my parking garage.

Anyway, sorry my rambling replies didn't really help.  Maybe you could use antenna modeling software to try to give you some answers.  Either way, good luck with it, and we'd be glad if you tell us what you wound up doing and how it worked for you!  GL 73!  --ken
« Last Edit: May 05, 2013, 05:10:33 AM by AC4RD » Logged
K0BG
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« Reply #7 on: May 06, 2013, 07:54:21 AM »

One often missed aspect is what the SWR looks like at resonance. Unless the antenna exhibits a 50Ω load, the lowest SWR is not at resonance. Whether the transceiver may like the match is moot for several reasons.

The higher the SWR, especially when the load is reactive, the worse the third order IMD is. Although no one seemingly worries about such things, they should!

Any antenna (save for a full length 1/4 wave) with an unmatched, on resonance, SWR of less than about 2:1, is losing a lot of power heating up the coil. This is especially true of antennas which have large end caps, like the high power (?) Hustlers have.

Cap hats work because they tend to even out the current flow along the length of the antenna. They only do this if they're properly designed and mounted at the top of the antenna. Folks who mount them just over the coil are actually reducing their overall efficiency. Yet, if you go to a big hamfest, you'll notice this is exactly where most people put them—right on top of the coil. I guess they just don't get the picture!
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W5WSS
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« Reply #8 on: May 07, 2013, 10:12:30 AM »

OK thanks, OK my use of the word optimised is meant to imply that the two similar antennas   One being the Hustler mo-1 long shaft and resonator and the other being again a Hustler with the Hot Rodz capacity hat. After when both are determined to be working properly and exhibiting their maximum contributions relative to the common vehicle then is the proper time for a field strength comparison between each downrange.

Yes perhaps some miss it but not me. And is why I stated a 50 ohm Non inductive surge impedance as driven at the antenna expressed as a 50 ohm Rr.

Anyway guess I will need to try to emulate the test controls for a fair field strength comparison between the two techniques with similar mobile antennas and look for any worthwhile advantages relative to out the door detectable signal increase to be utilized for my Radio pursuits.

Ac4rd OK I do like the quick change disconnect idea and use one with my Hustler mobile antenna, and seems like your adaptation relative to having them ready made and readyfor service is "modified" if one has the room available.

Thanks to all that were willing to respond and

73 Smiley

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KB4QAA
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« Reply #9 on: May 08, 2013, 01:41:59 PM »

Quote
And is why I stated a 50 ohm Non inductive surge impedance as driven at the antenna expressed as a 50 ohm Rr.

Surge Impedance?  We aren't dealing with HV power transmission lines.
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K0BG
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« Reply #10 on: May 08, 2013, 03:10:58 PM »

Surge Impedance indeed!

It is easy to look at an HF mobile antenna and say to one's self, it is just a mast, a loading coil, and a whip. That's like saying an automobile is nothing more than four wheels, an engine, and a transmission! There is so much more that goes on, that literally thousands of books have been written about both subjects!

If you really want to test someone's knowledge of an HF mobile antenna, just ask them to draw a schematic of same. I did this test at the last talk I gave. To my surprise, five folks tried, but only one got close. What he forgot wasn't significant (conductor resistive losses), so he's to be forgiven.

So the question remains, can you draw said schematic?
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G7DIE
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« Reply #11 on: May 09, 2013, 01:11:33 PM »

I'd end up drawing what I call the Moxon car, and it's equivalent electrical circuit:

The Moxon Car

In one of the posts above mention was made of a step up transformer, I'm guessing it's this:

http://www.dxengineering.com/parts/dxe-mm-1

Hell fire that's a lot of money for a shunt inductor Shocked
« Last Edit: May 09, 2013, 01:28:01 PM by G7DIE » Logged
K0BG
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« Reply #12 on: May 09, 2013, 03:01:40 PM »

Yes it is! Heck, a chunk of #12 bare copper building wire works better than the box, and if you take time to adjust it correctly, no further adjustments are necessary!
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W5WSS
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« Reply #13 on: May 10, 2013, 08:17:42 AM »

Hello kb4qaa, No we are not just dealing with high power transmission lines.The surge impedance formula is also useful and used to work out varying vertical diameters with height and for included loading coils and capacity hat systems and is accurate down to 0.64"D and is also useful for these types of antenna system calculations.

So yes surge impedance indeed.

My original question is not how to tune a vertical mobile antenna. That was an attempt to answer to k0bg retort and sorry I was sidetracked lol Smiley

It is about whether or not the cap hat addition to an exacting system is feasible to consider as a worthy addition to when one already has in operation an optimised Hustler mobile antenna.

To g7die: Dx Engineering MM-1? I did not mention a brand, hmm I was Not referring to it but since you mentioned it yeah might be a perfect solution for my installation slap it in ya know? Rather than building a shunt inductor at the feed point.
Geeze those young and lazy newbies "well when I was your age!"
Anything you can do I can do better. Grin

73









« Last Edit: May 10, 2013, 08:21:33 AM by W5WSS » Logged
K0BG
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« Reply #14 on: May 10, 2013, 09:08:55 AM »

Hum. It must take a lot more work than I even imagined loosely winding 8 or so turns around a 1 inch dowel.
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