2
$\begingroup$

I have an assortment of whip antennas for various RX & HT devices such as a 2M/70cm FT740, a Uniden race scanner 29-54Mhz/108-174Mhz/406-470Mhz/806-956Mhz, a similar Realistic Pro-32 scanner and couple of CB stubs. All the antennas look similar and are easy to mix up. Only the FT470 stub is marked as a dual-band YHA-28. I have a Rigol DSA815TG and a Mini-circuits ZMDC-10-1 500Mhz directional coupler for doing VSWR measurements. I have the DC attached to a 12x12” Al plate to act as a counterpoise. The DSA is attached to the DC with 10” RG174 and the antenna is attached directly to the DC IN port.

The YHA-28 has an obvious loading coil in the base. In free air it resonates sharply at 142Mhz 1.5-2.5SWR and broadly from 350-450Mhz 4-8SWR. But the 144Mhz resonance can easily be shifted 10-15Mhz lower with my hand in proximity. How can this work effectively as a 2M TX antenna.

Another spiral stub antenna sharply resonates at 154/196/358/465 all with SWR in 1.2-3 range. Clearly not for the 2M HT and likely for the Uniden race scanner. Though not matching all the available RX bands.

The OEM Uniden stub sharply resonates at 149 & 438Mhz 1.2-1.8SWR and broadly from ~350-400 2.2SWR, a better match to the mid-bands anyway.

I get it that most single or double conversion receivers are rather unaffected by SWR, but how can and amateur HT work with a 1/4 wave antenna that shifts out of band so easily? Maybe I am missing something here. The current ARRL handbook doesn’t really address HT whip antennas.

$\endgroup$
3
$\begingroup$

I think you've just described the awful truth about handheld antennas.

I've always worked on a figure of -3 to -6 dB in ideal conditions, just because the whip is short and lossy. Then many more dB loss in actual use, because of detuning and losses in the meat counterpoise, holding it near your body, etc.

As for the resonance shift - this is expected and unavoidable. I think it might be why the stock antenna is made quite lossy, lower gain but wider bandwidth. The higher gain, longer antennas that one can buy, are badly affected by detuning in real world use.

For receiving, antennas can be made more broadband with strategically placed resistors, but achieving good broadband performance is impossible when the antenna is under a quarter wavelength.

$\endgroup$
4
  • $\begingroup$ I think this is a fine answer, but let me add that ham HF transceivers are well known for reducing power to protect themselves when transmitting into an SWR > 2.0. Given the "awful truth" about HT antennas, I think it's likely that HT designers design the final amplifier stage to be more robust than the finals of HF transceivers and tolerate more of a mismatch. (I'm still chuckling about "meat counterpoise", haha!) $\endgroup$
    – rclocher3
    Jan 4 at 1:31
  • $\begingroup$ Yes I'm sure they are more robust. The real constraints on handheld are DC power and heat dissipation. Given the low price of the actual transistor, it's also possible they just work fine into infinite SWR. Might be cheaper than measuring SWR and cutting back. Some time ago I asked how much ham transmitters cut back, and at what power/SWR levels, but no answers. $\endgroup$
    – tomnexus
    Jan 4 at 19:13
  • 1
    $\begingroup$ I've never seen an HT that measures SWR, so I doubt that HTs have circuits to monitor voltage or current to the final transistor. (I have seen HTs that reduce power when they get too hot. Thermistors are cheap.) I've transmitted to broken rubber ducky antennas (or no antenna) several times, and each time the HT was fine when I connected it to a good antenna afterwards, so I doubt transmitting into an open circuit hurts an HT. (I haven't tried transmitting into a short circuit.) To summarize, I think you're probably right: HT manufacturers likely use MOSFET finals that can handle any load. $\endgroup$
    – rclocher3
    Jan 4 at 19:40
  • 2
    $\begingroup$ My old T81's manual has detailed diagrams! "The APC circuit protects the power amplifier from a mismatched output load and stabilizes the output power. The APC sensor detects driving current from the on R911 (between VCC and the power amplifiers). The detected current is converted into DC and applied to the APC control circuit. Compared with a voltage from the CPU, and the APC control circuit outputs "VGGC" voltage to control the [gain of] drive and power amplifiers.". From the circuit diagram this is just measuring amplifier DC input current, so it protects against short circuits only. $\endgroup$
    – tomnexus
    Jan 4 at 21:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.