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Author Topic: Long mast on trailer hitch or short mast on roof?  (Read 1802 times)
KB6OMN
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« on: November 23, 2005, 03:06:11 PM »

I am re-evaluating my mobile HF antenna options.  So far it has been a Hustler 20m on a Diamond trunk mount for my Subaru Forester.  I am now considering going home brew for greater efficiency - like winding my own coil, using either copper pipe or some commercially available longer mast for the lower and a whip for the upper.  The goal is to have more radiation resistance and a lower loss coil.  I'm concerned about having that additional weight on a trunk lip mount, so I am looking at where else I could mount it.  

One option is some kind of trailer hitch mount (either right off the hitch or have an extension bar made that goes to the side/corner of the car).  If I remember correctly, the section below the coil radiates more than the section above.  Also, getting the coil away from/above the sheet metal of the car is a good thing.  So....I'm thinking a long mast that will put the coil above the roof line (the Forester is kind of blend of a wagon and a SUV, so the lower mast would probably be between 4' - 5').  I know that trailer hitch mounts typically incur greater ground losses, and having the lower section of the antenna near the hatch back of the car is going to eat up a chunk of the emitted power.

The other option is to mount a shorter mast on the roof rack (assuming I can ground it...the rack is insulated by rubber pads).  I would have a much shorter mast (maybe 24" or so) and have the antenna over the roof of the car.  I would probably just repurpose my existing Huslter lower mast for this.  I'm thinking I should get a better ground plane (assuming the grounding is good), and all of the antenna will be free to radiate.  I am thinking to ground it via a braided strap somehow - either to a screw or as two strips running in-between the rubber feet and the roof.  I assume the later will be less efficient...probably like a mag mount.

Which do you think is better....greater radiation resistance on a lesser mount, or less radiation resistance on a better mount?  Are there issues with grounding insulated roof racks that makes that option a non-starter?

Thanks...
- Robert
KB6OMN
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N1QKH
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« Reply #1 on: November 23, 2005, 04:57:23 PM »

I had the same problem on a Toyota Camry. I put trailer hitch sockets (hitch receivers)on both sides of the car so, I still have easy access to the trunk. I put my two antennas up on 30" pipes to get away from the ground. I drilled and tapped holes for extra bolts so, I could get a good ground on the trailer hitch. This has been in place for two years with no problems.
 
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N1QKH
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« Reply #2 on: November 23, 2005, 04:58:17 PM »

I had the same problem on a Toyota Camry. I put trailer hitch sockets (hitch receivers)on both sides of the car so, I still have easy access to the trunk. I put my two antennas up on 30" pipes to get away from the ground. I drilled and tapped holes for extra bolts so, I could get a good ground on the trailer hitch. This has been in place for two years with no problems.
 
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K0BG
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« Reply #3 on: November 24, 2005, 10:19:27 AM »

Wow! A lot of poor decisions here.

First, putting the antenna on the roof does indeed reduce the ground losses, due mainly to less ground coupling. However, shortening the antenna greatly effects its radiation resistance. For example, shortening the antenna by half, reduces its Rr to one fourth. This fact more than offsets any gain derived by mounting it on the roof.

There is a VAST difference between DC ground, and ground plane. The fact the roof rack is isolated from the body isn't the issue. Adding a strap has little effect of the extra ground losses such an installation garners.

Placing the antenna atop a long pole also adds greatly to the ground losses, and all of the ground straps you can add will not help the situation, unless they are long enough to act as radials. In a mobile scenario, this is a ridiculous notion.

While bumper, trailer hitch, and any other low body mounting positions increases ground losses, it is preferred in leau of the above. The major consideration is keep the coil as clear as possible from surrounding objects, metal or otherwise.

Reducing ground losses is the single most important issue when addressing mobile antenna efficiency simply because it is the largest loss we have to deal with. This is why proper bonding is so important.

The effect of a high Q coil, even a loss-less one, won't do very much for your radiation efficiency. Cut the ground losses by the same amount, and the efficiency difference is easily measured with an MFJ 259B or similar antenna analyzer.

Winding your own coil is an admirable idea, but unless you have access to some sophisticated machine tools, you'll end up worse off than purchasing pre-made coil stock.

If you want more information, visit my web site. There are links and references to all manner of technical information, as well as some well-founded personal experience information. It should prove enlightening.

Alan, KØBG
www.k0bg.com
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N3ZKP
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« Reply #4 on: November 24, 2005, 12:18:54 PM »

In addition to what Alan said, any of the _full_sized_ motorized antennas will run rings around anything you can homebrew.

My High Sierra 1800 is mounted off the left rear corner of my Windstar and I am more than satisfied with it's performance there. Example - this morning I was working into the Gulf Coast from Connecticut on 40m with an S9+ signal and last night on 75m with the same result. 20m into the Maritime Net is a daily occurance, as is Europe. All of this with 100w.
73,

Lon - N3ZKP
Baltimore, Maryland
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AA4PB
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« Reply #5 on: November 24, 2005, 04:23:28 PM »

In addition to what Alan said, any of the _full_sized_ motorized antennas will run rings around anything you can homebrew.
-----------------------------------------------------
Doesn't that kind of depend on your knowledge, skill level, and what machine tools you have available :-) It's probably a true statement however for most of us, especially if we are looking to home brew a motorized band changing mechanism.
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Bob  AA4PB
Garrisonville, VA
KB6OMN
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« Reply #6 on: November 24, 2005, 08:39:00 PM »

Alan - I've been to your web site and have the 259B analyzer.  For the hitch mounting option, I didn't mean have the base of the antenna on some other kind of mast that isn't part of the antenna; I meant have the antenna section below the coil longer in order to get the coil above the roof.  I know the better solution is to have the longer antenna on the roof; I'm just thinking in terms of mounting safety and practicality - I don't want a dismount under hard driving or accident, I don't want to have to have a second support arm or such going up to the mid section of the antenna, and I don't want to scrape on overpasses.

Is winding your own coil really a no-go?  I remember there being at least one freeware program out there to help you with this(?)  I thought there is some DOS freeware with the ARRL antenna handbook that covers that...I'll have to check the CD that came with my copy.  It seems one could get close and then with a little trial and error with the 259B eventually get it dialed in while testing the whole antenna mounted....like alligator clips on the coil while testing, then make it permanent.

So to revisit my question after your response:  The goal is to have a better 20m antenna than the stock Hustler that is still not too expensive (so no screwdrivers and no SGC-237 and mega whip).  So let's say I use a Hustler coil - how about the 17m coil figuring I'll have a longer lower and upper section to get down to 20m.  If that won't work then the 20m coil.  Let's say I could go with a 36" lower if on the roof (longer than the 24" in the first post).  Would it be better to go the route of a 36" lower on the roof or a 4' lower on the trailer hitch?  Assume the same whip length for both.  Of course I could just do both and use the 259B to pick the best one, but if it is clear that one direction will be better than the other, then I'd like to go down that route first.  I guess one other issue is that with the 4' lower, the coil is less effective since it is higher up from the mid-point than with the 3' lower, right?  I know that reducing the antenna length will have an effect on efficiency, but with the hitch option having most of the lower next to the back of the tail gate also reduces efficiency, not to mention that the trailer hitch will have much higher ground losses.  With a longer lower (3') than in the first post in order to be able to dial in the 17m coil to 20 m (along with a longer than stock whip), it sounds like the decision should be to go with the roof rack mount.
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KB6OMN
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« Reply #7 on: November 26, 2005, 10:13:44 AM »

I did some research yesterday and I think I have the answers to my questions.  From looking around on the internet, I saw some pics of a guy who mounted a HF antenna on the same roof rack that I have in a similar way I was thinking to mount mine.  I also re-read most of  "Everything You Forgot to Ask About HF Mobiling" by DK Johnson (W6AAQ).  Here's what I learned (re-learned) from that:
 - The coil position becomes more effective as its position moves toward the center of the antenna (moving up from the bottom). Its effectiveness declines rapidly after it passes the center.

 - DK presents some findings that the section of the antenna above the coil radiates a lot more than originally thought - he did a field strength test with having the lower section in the trunk of his car...the field strength was not affected.  

 - DK advocates winding your own coil.  He warns of some gotchas he went though in using various coil forms.

 - The coil needs to be a at least a foot above the roof of the car, unless the rear of the car slopes forward so the end of the roof is farther towards the front than the bumper; in that case you could experiment with less than a foot.

 - Shortening the lower section of the antenna does not make nearly as much difference as shortening the upper section.

So this tells me that the roof rack mount should work.  I need to go back and re-read some other books and online sources I've used before to get a better idea of what the design of the antenna will be and what parts I'll use.  Maybe I'll repurpose a Hustler 17m coil as phase 1 of the project so I can at least get the new mount up and running.  I know I'll need a much longer whip for that - I experimented with the 17m coil and a 4' whip and 2' lower before...I couldn't get it down to 20m.  I'm thinking to use the 17m instead of the 20m since there would be less resistance loss from it, and I can have a longer antenna.  Maybe the resonant frequency is higher than the 20m coil as well.  Hopefully with the 259B analyzer and the Un-Un I have from DX Engineering I'll be able to get something dialed in.  I might look at cannibalizing the Un-Un in order to get it even closer to the base of the antenna.

This also tells me that in looking at a hitch mount; I need to make sure the whip length is at least as long if not a little longer than the lower section in order to keep the coil at or below the middle of the antenna.  I would need to make sure the lower is long enough to get the bottom of the coil one foot above the roof of the car.  The problem with the hitch mount will be the ground losses, so the hitch option will be plan B.
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KB6OMN
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« Reply #8 on: November 26, 2005, 10:15:46 AM »

PS - I remember not thinking much of DK's book when I first got it, but now after going through it again I think it is a great resource for anyone considering going mobile HF.  
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K0BG
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« Reply #9 on: November 26, 2005, 07:16:43 PM »

It is and it isn't. Unfortunately, there are a lot of technical errors in the book, and why they haven't been corrected by World Radio (the publishers) I'll never know.

The position of the coil for optimum radiation efficiency depends a great deal on the amount of ground loss. If the ground losses were zero, you'd be better off base loading the antenna as the coil would become the lossiest portion, and minimizing it would be paramount. Conversely, if the ground losses are high which they are particularly on the lower bands, placing the coil past the center can be advantageous especially if you can maintain a high Q. This is rather hard to do when inductances are high. One way around this is to incorporate a cap hat.

Don is correct to some extent about winding your own coils. As I explained to you in my e-mail, you have to shy away from some forms of PVC. Lexan and Delrin are very good alternatives, and so it Teflon. Problem is, they are rather expensive.

Your comment about shortening the base makes less difference than shortening the whip is a false premise without knowing other factors such as ground loss, coil Q, stray losses and a few other esoteric data. In other words, you cannot make a pat answer as he does in his book.

Like a lot other things in life, length matters. The closer to a 1/4 wave an antenna is, the more efficient it will be. Unfortunately, we can't drive around with antennas much longer than 13 feet or so, and even less in most urban areas. While you can effectively shorten an antenna and maintain a certain level of efficiency, doing so arbitrarily will net you additional lose.

One rule of thumb to keep in mind is, with in reason, doubling the length of an antenna will increase its radiation efficiency by four, all else being equal. Conversely, cutting it in half will decrease it to one fourth. There is no free lunch when it comes to mobile antennas.

I have a suggestion. Go to Tom Rauch's (W8JI) web site and look over the Antennas section. There is a lot to wade through, but once you get it all read, you'll have a better appreciation of what's good and what's bad. And you'll learn to take about 50% of what you just read in Don's book with a grain of salt.

Alan, KØBG
www.k0bg.com
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