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Why are VHF larger than UHF antennas? or Why aren't VHF available in sizes as small as the UHF?

mwb9122 wrote:Why aren't VHF available in sizes as small as the UHF?

There are short/stubby VHF antennas available from Moto..


-Marc

mwb9122 wrote:Why are VHF larger than UHF antennas?

Physics and mathematics!

The length of the radio wave directs the size of the antenna. A full wavelength at 150 MHz will be approximately 77 inches, thus a quarter of that wave is approximately 19.5 inches. As you may have heard, most antennas are quarterwave. That means they have to find a way to fit 19.5 inches of conductor in that puppy! If you are using a 5/8 wave antenna, that means approximately 49 inches for VHF.

At 450 MHz, a full wave is approximately 26 inches, so a quarterwave antenna is going to be very close to 6 inches.

Antennas are designed for their respective frequencies, so the higher you go in frequency, the smaller the antenna becomes, and as a result of
the rise in frequency, the 'skin' effect plays a larger role in antenna design as well.

We all know portable antennas are not efficient, but are designed to couple the most RF energy, in a height, width and spectrum bandwidth in a rather small form factor.

Radiation efficiency will always be a tradeoff for small size, and bandwidth also accounts for some of that size difference too.

VHF antennas are tyically helical in nature, to provide modest low angle of radiation, instead of vertically, so the helical can TX and RX moderately well in many different orientations instead of up or down as it were.

UHF portable antennas are pretty much identical, but due to the frequency range these operate, they do not require a helical design, as this would cause
too much loss in the antennas itself, and the skin effect would provide far too many losses for such an antenna to be useful, so the standard 1/4 wave is
simply a small diameter element made up (usually) of several smaller wires twisted together to form the main antenna radiator, with the benefit of being fairly
flexible in its design.

There is the 'stubby' for UHF, which is constructed identically to the standard VHF helical, but because the UHF stubby IS designed to be used as a sapce-restricted antenna, it can be made similar to the VHF helical, only far smaller, while still having a radiation pattern that closely resembles its cousin, the 1/4 wave whip.

The VHF stubby is a less efficient design of its big brother, but in both cases, there are tradeoffs in their use.

With the main one being range, both TX and RX.

Take the whip antennas for 800/900 mHz. Both of these antennas are also harmonically usable in the 450-470(900-blue) as well as the 440-450(800-red) range, and if you view them through a return loss bridge on your analyzer, you can see the resonance points along the usable spectrum they are able to operate within.
Pretty sneaky, marking the whips as 800 or 900, and having people believe they are only usable within those frequenccy ranges....

I use the 800 whips on my UHF portables for the ham bands, and the 900 whips on my commercial frequencies...and have good results, and never any problems due to their use.

The only antennas you can NOT do this with, are the dipole antennas for these same frequencies, as they are tuned for their respective bands.

So...........

I can use my 800 red 6.5 inch whip on my APX between 440 and 450 and the RL is acceptable to the radio? It would seem that the antenna would be a half wave at 800 and present around 200 ohm impedance at the base at that frequency. Whatever compensation they use must be invisible at UHF, right? . The baseball bat that comes with the radio has UHF, 7-800 and GPS. GPS isn't provisioned and not using any 700 MHz yet so the flex would be a great thing for me.

I could set up to sweep the flex I guess but I would need to make something to compensate for the radio chassis as a GP. How did you do this?

TIA

Dave

I used my my Eagle RF bridge and swept the UHF bands of interest using both antennas, and then I used a dummy trick and mounted a panel SMA connector on a sheet of aluminum and attached each antenna, and ran them through the telewave and found both to be below 2:1, with the 800 being the best match at
447xxx mHz.

Neither the 800 or the 900 antennas used in my testing have shown negative effects from being used 'out of band' and work very well, even from a distance.
The 'ball' at the top is just a plastic retainer for the antenna to keep the braids from separating, there is little affect caused by these, and both antennas
are quite usable in the bands I stated, and for the antenna's color coding as well.

On air tests, Rcvr. sensitivity tests and SWR bridge tests confirmend they are very usable and cause no harm to the transmitter section of the radio.

I've been doing this for about 3 years now, and not once, have I detected a problem with alignment, or degradation of the receivers with the use of these whips.

I don't have a TDR or high end test gear, but I do have an R2001D/HS and IFR 7550, and both give almost identical responses, with identical tests.

Maybe some are a little afraid to try this, but those that will, may find the results to be quite satisfactory.

I do not have an APX, nor do I intend to own one, so i can't comment on that model radio's antenna design, but with GPS and the fact the radio is built using such disparate frequencies, I can share your concerns.

That's quite a bandwidth to cover with a single antenna(I presume).
I would probably have the same reservations as do you, with using this 'method' I described.

the coil in the base of the antenna does not introduce any negative effects I can measure, and the thin metal(maybe copper) cover acts like a shield to limit radiation from the base, coupling it to the whip instead of loss at the base, and maintaining a match for the antenna.
The base material is clear plasticand the 3 turn coil is easily visible also.

Harmonic relationships come into play here, just like using a 1/4 wave on 150, and 450 as well.

As I stated, I don't have the test equipment to get down to the nuts and bolts of perfect matching, radiation resistance losses and such, I only use what empirical data I have at my disposal, and take it from there.

This must be what you are worried about.....

http://img715.imageshack.us/img715/8849 ... 132900.jpg

Cellphone shot, so it sux, but shows the matching (loading)coil clearly, with the foil around the base.

have to keep a 50 Ohm impedance at 800/900.

Does anyone have a part number or link to one of these VHF stubbys? I would like to see/order one!

mwb9122 wrote:Does anyone have a part number or link to one of these VHF stubbys? I would like to see/order one!

http://batboard.batlabs.com/viewtopic.p ... stubby+vhf


-Marc

Here's a list for your records.. ll listed below are SMA antennas for Jedi/Astro series radios.

NAD6566: HELICAL, 136-151
NAD6567: HELICAL, 151-162
NAD6563: HELICAL WIDEBAND, 136-174
NAE6546: HELICAL, 403-435
NAE6547: HELICAL, 435-470
NAE6548: HELICAL, 470-512
NAE6549: WHIP, 403-512
NAF5037: WHIP, 800
NAF5038: WHIP, 900
NAF5039: DIPOLE, 800
NAF5040: DIPOLE, 900
NAF5042: 1/4 WAVE STUBBY: 800, 900

My experience has been that using antennas made for one band on a "harmonically related" band works when the two frequencies are related by a factor of three and not two. This is why, for example, some VHF antennas work on UHF.

Using an antenna at its second harmonic usually makes for a bad impedance. A good example is a 1/4 wave antenna on 450 MHz. Here the impedance is low (higher current and lower voltage). However, at 2x450 MHz or 900 MHz, the impedance is high (lower current and higher voltage).

Thank You guys! Just looked some up on MOL and they a little pricey but I guess I'll have to eat the price a bit!

W8RW wrote:My experience has been that using antennas made for one band on a "harmonically related" band works when the two frequencies are related by a factor of three and not two. This is why, for example, some VHF antennas work on UHF.

Using an antenna at its second harmonic usually makes for a bad impedance. A good example is a 1/4 wave antenna on 450 MHz. Here the impedance is low (higher current and lower voltage). However, at 2x450 MHz or 900 MHz, the impedance is high (lower current and higher voltage).

I am looking at at this strictly as an effective means to utilize two portable antennas in a band they were not meant for.
The impedance mismatch is well known, and aside from testing for a match, including return loss, forward power coupling, the antennas exhibit
very good results, with quite acceptable SWR response across the band.

A portable is already at a disadvantage, and the mismatch of impedance/return loss will probably never affect the radio to such a degree, as to make it
a problem which may cause irrepairable harm.
With people hanging keys from the antennas, transmitting without the antennas present, and using coathangers in place of real antennas, I doubt a minor mismatch would ever be noticed by a portable.
They have a poor groundplane to begin with, and are designed to work under such conditions.

I wish I had an RF current meter to use inline with the portables; I'd love to see where the current lags or leads with a change in voltage, and at what point.

Then transfer that information to a smith chart showing resonance points within those test points.

I still doubt there would be negative consequences. but I will digress and say I may not have enough empirical data to fully back up my claim, but what I have to date, suggests that I can safely use 800/900 portable antennas on a UHF radio, with no worries about excessive losses, forward or return.

I realize second harmonics provide a better current/voltage match, but aside from my use, these whips will work very well in emergency situations where the loss of an antenna may occur in bad weather, SAR activities, or anywhere a portable may be used for extended periods outdoors.

A 12oz pop can (or beer can - your choice and favorite brand) are great dual band UHF/800 antennas. Not very stout though (pardon the beer pun) above 60mph. Loggers and beverage distributors love them though. Coffee cans make much sturdier UHF antennas. Cool factor is totally gone. Haven't found any good VHF cans yet, not even a Fosters. But, I keep wondering about those olive oil cans behind the deli. Some things are best left unknown I think.

Do they just cut the top off and drop the radio in or?

Making soda cans into antennas is interesting but it's also quite a bit above and beyond the level of current discussion here. I suggest that you start with the basics and learn from some of the good texts that are available on the subject. One of those is the "ARRL Antenna Book" which is available by mail from http://www.ARRL.org. Every person on this Board was new at one time and we all learned the ins and out of this techie "stuff" one page at a time.

And just to answer your last question the answer is no. The HT would be connected to the can via a length of coaxial cable and an appropriate radio frequency plug and connector. You may even find this type of antenna in the ARRL book under the title of "Beer Can Vertical Antenna."

I understand Tom. The discussion started to veer into which portable antenna is better, and I wanted to point out that antenna shapes, and lengths can be deceiving. After you do enough covert work, you learn the fine art of matching side mirrors to the radio, or do rude things to stuffed animals in a confiscation vehicle. Sometimes vswr is over-rated. Smith charts make my head hurt.

AEC wrote:Here's a list for your records.. ll listed below are SMA antennas for Jedi/Astro series radios.

NAD6566: HELICAL, 136-151
NAD6567: HELICAL, 151-162
NAD6563: HELICAL WIDEBAND, 136-174
NAE6546: HELICAL, 403-435
NAE6547: HELICAL, 435-470
NAE6548: HELICAL, 470-512
NAE6549: WHIP, 403-512
NAF5037: WHIP, 800
NAF5038: WHIP, 900
NAF5039: DIPOLE, 800
NAF5040: DIPOLE, 900
NAF5042: 1/4 WAVE STUBBY: 800, 900

Can anyone tell me what the difference is between the NAF5037 800 Whip and the 8505241U03 800 whip? They're the exact same price...

Some of the antenna part numbers are 85xxxxx and are the same as the NAx numbered antennas.