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high swr with co-phased 102" steel whips
- Thread starter kamikaze701st
- Start date
i would like him to get a 18 foot of 50 ohm coax and try just one. and report back the differences...
Kamikaze, I have an Eznec model of a basic Bronco sent to me by Henry Hpsd. This mobile truck model does not show the good low match that a single 1/4 wave whip should be able to produce, so I can not be sure how accurate this basic model might be representing a real Bronco.
Some would say this bad match is what should be expected mounted on the real bumper of a P/U truck, but I've run several such setups and I never had a matching problem. So, I don't buy the idea this type of mount won't work for a single antenna or a co-phased setup if installed correctly.
Below I have a Bronco model with two 102" whips on the rear bumper as well as a model with only one 102" whip.
I did these two models in order to allow me to compare a co-phased setup vs. a single antenna on a mobile/automobile.
I did not model the physical co-phase harness to connect the two whips, nor did I use the matching feature in Eznec that provides for matching antennas. So, the match indicated in the models shows a bad SWR match as noted on the attached Source Data reports in the images. This indicates the SWR at the feed point for these models.
To try and answer your question, if you don't have the proper harness that is installed correctly, this may account for the problem you are having to deal with in your installation...a very high SWR match.
BTW, these two models seem to indicate that co-phasing 102" whips on the real bumper of this Bronco should show a nice gain and pattern over the single antenna if both setups were installed correctly.
I also included a second image of the overlays for the patterns of these two models...so we can more easily compare the patterns, gain, and angle details.
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20 plus years tells modeling isnt the same as real world results. i got a book written by lou franklin . under antennas running duals you DO have more gain but only in the front and back not the sides, which he went onto say this is why truckers used them way back when.but te gain is small and only in the front and rear. my tests years ago pretty much had similar results base to mobile couldnt tell much/if any difference. but out on the interstate using both dual and single i could see a slight difference. this test was performed over 20 yrs ago using stock cobra 19 and single skipshooter with 50 ohm coax then switched to duals and 75 ohm coax.my 75 ohm coax Y off so didnt need a adapter. the right coax to use
Well, yeah, you don't get any gain (directivity) with out losing something else.
add a 1/4 wl section to one side of the feedline and see what happens
My 20 plus years of experience disagrees with yours on this topic.
Unfortunately, neither of the above lines have anything useful on the topic at hand. They are little more than a waste of space. I'm sorry, but when I see someone using "X years of experience" instead of an actual explanation I see it as a cop out.
/rant
Hotrod, I know it is the second time in this thread, and I'm honestly not trying to pick on you, but your experience with models has been proven wrong time and time again with real world comparison tests. The reality is while the patterns said programs make look nice, you almost have to be a modeler to understand what to look for and how to properly read a model. You can't, for instance, make one model for an antenna and treat it like all possible installations of said antenna will have the same resulting pattern. There is far more to modeling than most people realize...
Speaking of models, I have some models of single and dual antennas done on a small car. I'll see if I can't dig those up...
The DB
i never daid i did any modeling. but ive tested MANY antennas of various types . with real world results. i feel those modeling programs show under perfect conditions whats gonna happen. and i know your smart enough to know those perfect conditions just dont exist. which is why till this day i still test them myself. and yes on different vehicles setups you probaly see different results..
First off, I appreciate the calm response to what, as I reread this morning what I posted last night, could have easily been taken another way. I apologize for that, I should have worded what I was trying to say differently.
I've only been modeling a short time compared to the time I have been working with antennas, and I've learned much because of it. It was a rather bumpy road to get where I am now with it as well, so I can see how it wouldn't be for everyone.
Modeling can model perfect conditions, i.e. freespace and perfect grounds, neither of which exist here on earth. However, they go far beyond that. There are many other ground types, including the Sommerfield-Norton ground, which is a modern and extremely accurate ground. You can choose from a list of general ground types, and if you have the capability to actually measure the ground quality in your area, you can manually enter that data in. That is hardly a representation of "perfect conditions".
Further, you can have the antenna elements made out of various different conductors, which sometimes makes a difference. Your antenna is made out of T6 aluminum? We can model the effects of that material. You have a tapered 102 inch stainless steel whip with a one inch diameter stainless steel riser? We can account for the material, the tapering, and the riser that is wider than the rest of the antenna. These are hardly representations of "perfect conditions that just don't exist". I can go on here.
When it comes to modeling mobile antennas, that gets a bit more difficult as you have to model the entire vehicle as well. How many models have you seen that actually take into account the actual shape of the vehicle? I can tell you that I have seen very few, namely the bronco model Eddie posted above, and one of a car I did a while back. This is, admittedly, an area that I will need to do more experimentation with. At some point I would love to have a built up database of vehicles to show the differences with, unfortunately, that is going to require a lot of work...
One question for you, if modeling is so inaccurate, why would the government accept them as proof that a given system is operating within ERP (Effective Radiated Power) regulations from not only companies, but ham radio operators operating on certain ham bands?
The DB
i dont know why our goverment would do that. but then they alot of questionable things.im getting off topic. my opinion is id much rather experiment and see for myself what works and what doesnt..if funds allow.and much like eddies model that showed a high swr ive done similar installs with acceptable swr...not perfect but good enough. on the other hand i tried on a van [i believe] and did have a little higher than 2.1.1 swr so yes i have mixed feelings. in my opinion i think mounting them on the bumper is too low and get too much refect. even though some have had good luck with it there including myself ,i feel its a less than ideal location
Here are some results from the car model I used a while back. I included a 1/4 wavelength ground plane antenna with 4 radials at the same height as the car roof for reference...
I know, I called it collinear, but it was actually cophased as the picture below shows. That was an error on my part when I made the models a while back.
And a picture of the vehicle model I used...
Yes, I know it is a small car, but its what I found that I could make work at the time.
Resonant lengths for these antennas came out different for each of these models as well... For the 1/4 wavelength groundplane, the vertical and the radials all came out to 9 feet. The single antenna in the center of the roof came out to 7 feet and 8 inches. The cophased antenna lengths came out to be 6 feet and 10 inches. Note, these numbers are from being modeled over a very small car, smaller than most people use, most vehicles would require longer antennas to be resonant...
At some point I intend to make/find other vehicle models to use when modeling. Modeling mobile antennas is almost never actually done partially because of the difficulty of making said models...
The DB
I know, I called it collinear, but it was actually cophased as the picture below shows. That was an error on my part when I made the models a while back.
And a picture of the vehicle model I used...
Yes, I know it is a small car, but its what I found that I could make work at the time.
Resonant lengths for these antennas came out different for each of these models as well... For the 1/4 wavelength groundplane, the vertical and the radials all came out to 9 feet. The single antenna in the center of the roof came out to 7 feet and 8 inches. The cophased antenna lengths came out to be 6 feet and 10 inches. Note, these numbers are from being modeled over a very small car, smaller than most people use, most vehicles would require longer antennas to be resonant...
At some point I intend to make/find other vehicle models to use when modeling. Modeling mobile antennas is almost never actually done partially because of the difficulty of making said models...
The DB
I've always tuned one antenna at a time then hooked up both and fine tuned, I've seen where they were usually long before fine tuning so my experience agrees with this. However these numbers are larger than I would have expected even considering the size of the car, very interesting.
It doesn't seem intuitive that resonant lengths would change like this, is the reason for this happening known?
PS:
Be sure to make a model of a Suburban with a PVC pipe stuffed up its nose.
In my experience with real co-phased setups, the way you describe your work is correct. Both loads should be as close to equal as possible, but I never saw the need to shorten my whips.
Most guys would suggest that a 6" spring is needed to be added in order for a 102" whip to work at 11 meters, so that doesn't jive...even though these mobile models posted here indicate what you claim.
No sure what particular numbers you are questioning, but I question them all for now?
I agree 100% here, and I'm not sure why either.
Like DB, I suspect it is the basic car models that we are using are not accurately representing an auto. I don't know if it is a lack of wires for these models or are they too big or too little. However, I can't add many more wires to my models due to Eznec's segment limiations. DB, have you tried adding a lot more wires to maybe check this out?
I just modified my model smaller, more like DB's, and I cut the trunk area cut off. The problem got worse...requiring the radiator to be even shorter. That surprised me.
Not really questioning anything, it just seems as though the difference in lengths between the different antenna configurations is much larger than I would have expected. I would have thought that the length of the duals would be quite close to the length of a single. It is more the percentage of difference than the actual numbers themselves.
Just to clarify I've never tried to run whips in phase, my experience is mostly with shorter fiberglass antennas on truck mirrors. I'm talking about typical trucker highway use setups here. After adjusting each antenna individually I would hook up the dual antenna harness and find both antennas would be slightly long. I don't know how the amount of fine tuning I've done on these types of antennas would correlate to length on whips though.
Keep in mind I'm far from an expert so don't take anything I say here as a challenge to what has already been posted, I'm just curious as to what would cause this effect.
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Resonance is based on more than just the length of the vertical, but also the size and shape of the groundplane, or vehicle in this case, underneath. It also takes into account environmental factors, such as ground quality. Something as simple as changing the height of an antenna at low heights over an earth is enough to have some effect on the length it needs to achieve resonance. Having a second element not to far away will also have an effect...
Still, While I did expect that there would be some difference in length, I didn't expect that much of a difference in length just to maintain resonance.
Be sure to make a model of a Suburban with a PVC pipe stuffed up its nose.
I don't know about a Suburban specifically, but I own a Ford Explorer that will likely be my first attempt...
That particular vehicle model has 203 wires. I think that should be sufficient for an object that size.
That is a good question...
The DB
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