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Author Topic: Ground Breaking Mobile Antenna Articles in QEX  (Read 9828 times)

Posts: 3639


« Reply #30 on: April 11, 2014, 01:26:54 PM »

I can see how that works for two sections (coil and whip). What do you do for three sections, e.g. your stub made of 50/450/50 sections?

The principle is the same with one interesting difference. While the phase shift is positive at the 450-50 ohm impedance junction at the top of the 450 ohm section, the phase shift at the 50-450 ohm impedance discontinuity at the bottom of the 450 ohm section is negative and subtracts from the 15 degrees of 50 ohm line. In the previous example, the negative phase shift subtracts about 13 degrees from the 15 degree 50 ohm section tied to the feedpoint.

Have you ever noticed how little effect adding length to the base section under the coil has on the resonance frequency vs how much effect adding length to the stinger changes the resonant frequency? The negative phase shift at the bottom of the coil and the positive phase shift at the top of the coil are responsible for that phenomenon.

It is well known that for a fixed length mobile antenna, moving the loading coil from the base to the center of the antenna requires considerable increase in the inductance of the loading coil. That is because we are making the stinger shorter.

If we keep the same long stinger and simply add a base section under the loading coil, the resonant frequency doesn't change much because the negative phase shift at the bottom of the loading coil cancels out a large number of the added degrees. Adding 15 degrees of bottom section to the physical length of a resonant base-loaded antenna only adds about two degrees to the electrical length of the antenna.

Hoping that people would see the similarity between shortened mobile antennas and dual Z0 shortened stubs, I wrote the following article about five years ago but nobody seems to have had an epiphany because of it.Smiley 

I know there's nothing new under the sun concerning antennas but as far as I know, the above information is original, i.e. I've never seen it explained anywhere else.

Posts: 3639


« Reply #31 on: April 14, 2014, 06:43:30 AM »

Here's an interesting graph produced by EZNEC/AutoEZ that displays the amplitude and phase of the current through a 75m Bugcatcher mobile antenna using the helix function of EZNEC to simulate a 75m Texas Bugcatcher mobile loading coil. The antenna uses mininec ground and is resonant on 4 MHz.

The current through the loading coil is about 33 degrees of a standing wave current waveform with a sky high SWR on the standing wave antenna. The very small phase shift through the coil happens because of the standing waves and is not indicative of the ~21.5 ns traveling wave propagation delay through the coil.

Note that the stinger is 16 degrees long but has only a 0.04 degree phase shift through it. Again, that is a characteristic of standing wave current and certainly not indicative of the traveling wave propagation delay through the stinger. 0.04 degrees of propagation delay in 16 degrees of stinger would be 400 times the speed of light.
« Last Edit: April 14, 2014, 06:57:19 AM by W5DXP » Logged

Posts: 815

« Reply #32 on: April 19, 2014, 02:33:33 PM »

What amazes me is that it took three pages to realize that High-Q and Low-Q are
useless phrases.

Without knowing what is considered high and low in terms of Q there is no meaning
to any statement around that.  This is made worse in that Q is ratiometric number and
going from 50 to 100 is a larger change than going from 100 to 300.

I'd expect low means something south of 100 and high is something north of 400.  Most
coils tend to fall in that range so which means the difference form one to another is
measurable but not a large quantitiy.   Therein the 80/20 rule applies, where getting a
3db improvement is likely much easier and noticeable than getting that last db or fraction
there of.  When your in the realm of antennas especially in the 160 to 30M range the
initial problem is that the antenna is electrically small and the other half (car/truck
capacitive coupling to earth) represents  lot of lossy things that physics says just are
what they are. 

The usual loss area is not the antenna besides some awful constructions out there its
the radio to feed point interface.

The best example of this is a Workman antenna for 40M that's about seven or so feet tall.
When it failed from street salt I pulled the wire off to find it was enameled copper plated aluminum.
Rebuilt it exactly with same size enameled copper wire trying to duplicate it exactly netted
a noticeable increase in signal, half the swr bandwidth, and a much lower base impedance
measured.  A matching transformer corrected the SWR.   Since i had two of the antenna
one "fixed" and the other not it was easy to compare using a remoted spec analyser as RX
and was found to be nearly 3db, part of it being resistive loss, the other part being the coil was
also made with the same high resistance wire greatly lowering the measured Q.  The resistive
loss make the antenna match but it was a lossy match so lit looked good while doing nothing
to help radiation. 

So yes Q counts, its the total effective Q of the antenna including the coil and I cant see how a
antenna of stainless or aluminum with a really cute coil of fancy plated metal is that much better
that one using mostly copper with a simpler and still good quality coil. 

The law of diminishing returns prevails.



Posts: 437

« Reply #33 on: April 20, 2014, 06:00:31 PM »

See Table 2 (80m) and 102" antenna with coil at top.

                                         Hi-Q  Lo-Q
Resistance at resonance        43.6     43.5
Field strength                        8.5       8.6
2:1 Bandwidth                      12       25

Wider bandwidth should go with higher resistance or lower efficiency. But both are same. How do we get a wider bandwidth for nothing? Is it real or a mistake? Perhaps I should ask Barry...

Ignacy, NO9E   

Posts: 3639


« Reply #34 on: April 22, 2014, 03:21:22 PM »

How do we get a wider bandwidth for nothing?

Seems to me the question is: How do we get equally strong radiated signals from a "low-Q" and high-Q loading coil? It seems obvious that Barry's "low-Q" coils were not really low-Q, just lower Q than the high-Q coils. A 75m hamstick has a truly low-Q coil that is much lower Q than Barry's "low-Q" coils.

I suspect that the shorter, large diameter high-Q coils and the longer small diameter low-Q coils had the same loading reactance and when the longer small diameter low-Q coils were in place the antenna was physically longer resulting in a larger radiation resistance that compensates for the lower-Q.

I will try to contact Barry on this subject.
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